TideLog Archive for the “Volvo” Category

Rikku’s Volvo B7TL double decker has recently hit over 1 million miles, (yes 1 MILLION), and just today threw a Brake ECU message with yellow CHECK lamp. Her B7TL was the first double decker we bought in 2009 from West Midlands Travel, owned by National Express. Today I actually had time to do some pictures of the diagnostic menu.

It’s the most common “Check diagnostics for xxxxx ECU at next stop” message you’ll see, and as the brake system has wear level sensors, if you have drivers that don’t realise there’s a RETARDER (engine brake between the driveshaft and gearbox), they use the brakes too much and the lining wears out, triggering this fault message. It isn’t really a fault, but a friendly message to say, “Hey, your brakes are wearing thin!”

If you get this too much, you need to take your drivers into a meeting and tell them to use the brake pedal only half way for just the retarder! Pressing it all the way activates the service brakes as well, and can make the braking too bitey and uncomfortable for passengers. Retarders are great, they’re like using the gears to slow down in a manual car, saving the brakes.

The drivers always send me photos of the display at the roadside once they’re empty and pulled over safely, and I keep them as part of the service history, and they’re good for TideLog too, here you can see the message, plus the yellow CHECK lamp, so the vehicle systems are telling us gently there’s something needs replacing that isn’t critical, such as brake light LED clusters, or brake discs/pads:

The yellow CHECK lamp means a non-critical message or error. If the red STOP! exclamation lamp EVER illuminates, don’t do what First Manchester do and continue driving, you MUST STOP IMMEDIATELY.

So, I have trained Rikku and the drivers to be able to do basic diagnostics as they’re on the radio to us at Comms when out on the road. On the Volvos (B7TL, B9TL double deckers and B7RLE single decker) it’s really easy, you can do it using the steering column stalks without the IMPACT software on a laptop, that’s usually my job if I need to access freeze frame data from the systems when the error occurred, a bit like the flight data recorder of an aeroplane. Here the bus tells us what’s wrong, and as we’ve entered the diagnostic menu the yellow CHECK lamp goes out:

“Sensor lining left forward value too low. Active. Number of events 34”

This error basically translates to “Left front wheel brake wear sensor has detected brake lining worn past limit, error has occurred 34 times (ignition cycles) and it is still an active error.”

Replacing the brake pads and discs (I usually do all of them for safety in one go) recalibrating the sensors, and clearing the faults solves the issue!


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I was on the 471 on Saturday on the way to Greg’s from Jenny’s, one of First’s Wright Urban Gemini bodied Volvo B9TL double deckers and as soon as I sat at the back, and the driver set off I knew straight away something was wrong. It juddered badly upon acceleration, and the engine was making a machine gun type noise whilst being sluggish. This told me straight away the engine had a problem, possibly a misfire.

The cylinders in Volvo engines can be shut off electronically by the Engine ECU if there’s a fault, so it could have been a result of this shut off caused by a misfire. The bus wasn’t in limp-home mode, it was still quite responsive. As soon as the driver let off the accelerator the shuddering stopped, so I knew it wasn’t a powertrain problem, so this excluded the gearbox, propshaft and differential.

It also couldn’t have been crankshaft or conrod big end bearing shells either. Anyway, being a respecting mechanic, as I got off I had this conversation with the driver:

Me: “I think you should call Comms and get this bus straight into the garage, your engine sounds like it has a misfire or internal problem.”

Driver: “A misfire, are you sure?”

Me: “Can you not feel how it judders when you accelerate, and stops juddering when you don’t? The engine sounds like a machine gun back there!”

I glanced down at his dashboard infodisplay, and sure enough the CHECK indicator was lit. There was no message displayed as he had it set to INFO rather than MESSAGES.

Me: “Even your CHECK light is lit. I’d call it in to be safe. If it gets any worse the bus computer will put the engine in limp home mode, you’ll notice it go really weak. The STOP exclamation indicator will come on your dash too.”

Driver: “Thanks for letting me know! How do you know all that?”

Me: “I work with buses like this nearly every day, for a friend in the Midlands”.

Driver: “Ah, it’s good to have guys like you to notify us of issues, us drivers can’t always tell there’s a problem when we’re in the cab at the front here. I’ll let someone know!”

Anyhow, I went on the 471 again yesterday to get to Manchester through Bury to get the tram to Manchester to catch a train to go back up to Birmingham, on the SAME BUS, with the SAME problem! It either hadn’t been called in, or it hadn’t been fixed! If that’d been Rikky’s bus, it would have been out of service within ten minutes of the problem being noticed, and wouldn’t be out again until fixed!

Just goes to show First don’t give a shit about a machine in their fleet that costs DOUBLE the cost of your average semi-detached HOUSE!! The single decker Volvo B7RLE’s are £130,000, and the doubles can be up to £200,000! It shows just how disposable they treat their equipment!

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I recently had to overhaul the steering system on Rikku’s B7R coach after it hit a very bad pothole, shattering the track rod & link rod balljoint cups. Here’s an article on my most common steering problems, causes and remedies. This article covers all Volvo coaches and buses with recent power steering systems, such as the B7R, B7RLE, B9TL, etc.

NOTE: If balljoints, joint cups or sealing rings are damaged the system should have parts replaced. NEVER repair damaged balljoint systems, as they will simply fracture again. Never re-use or repair gaskets, they are single use only. When inspecting, ideally you should steam clean or wash the steering system completely and use an infrared detector and magnifier to detect hairline cracks in the WHOLE system, including hubs and hub bearings as not all damage is obvious to the naked eye. Seizure of the hub shafts can result in a wheel coming off and a serious accident occuring, I have seen hubs sheared straight off due to fatigue at the hub shaft and the bus/coach has lost a wheel at speed when it finally gives way.

WARNING: This article is for EXPERIENCED mechanics familiar with bus steering systems and Volvo Impact software. You MUST ALWAYS follow Volvo’s guidelines for repair, bolt tightening torques, and always use GENUINE parts. Vehicle and passenger/driver safety can be compromised otherwise as the steering is a critical function of the bus that should have its safety and reliability maintained.

The steering and suspension of a bus comes under EXTREME stress under load at normal use due to bumps and uneven surfaces, NEVER overlook things that don’t feel right in terms of handling. This article only gives general guidelines, it is up to your judgement and professionalism as a mechanic to carry out the work.

Steering stiff in both directions

Cause Remedial actions
Oil level too low, or drops continuously when filled. Check and find any leakage in system and hoses/attaching bolts.Fill with oil and check the level of the reservoir. If the oil level continues dropping after being filled this is certainly a leak.
Air in hydraulic system. Check the reason and bleed the system.
Oil pump control valve sticks or oil drilling clogged. Disassemble and clean the valve and replace if necessary.
The filter is clogged. Ducts are obstructed. Change the oil filter and clean the ducts.
Universal joint hard to move. Check for interference.
Valve plunger stays open or leaks. Remove the plunger from the valve. Replace the plunger set.
The sealing ring at bottom of plunger not sealing. Replace. DO NOT attempt re-use or repair!
Plunger sealing ring damaged. Replace. DO NOT attempt re-use or repair!

Steering stiff in one direction

Cause Remedial action
Pressure limiting valves leaking. Replace the valves. For safety DON’T attempt repair, they are designed to be replaced.
Pressure limiting valves open too early. Check adjustment and adjust as necessary. Refer to Volvo Impact software for procedure.
Plunger sealing rings damaged. Replace. DO NOT attempt re-use or repair!
Cylinder lower chamber without pressure. Replace all sealing rings. DO NOT attempt re-use or repair!
Cylinder upper chamber without pressure. Replace all sealing rings. DO NOT attempt re-use or repair!

Steering heavy with rapid steering wheel turns

Cause Remedial action
Air in the hydraulic system. Check the reason and bleed the system.
Oil pump control valve sticks or has the oil drilling clogged. Disassemble, clean and make sure the valve works freely.
Servo pump oil flow low. Repair or replace the pump. Replace any sealing rings or gaskets. DO NOT attempt to repair rings or gaskets, they are single use only!

Stiff steering return

Cause Remedial action
Steering column shaft not lubricated or damaged. Lubricate, and if damaged with scratches or pits, replace the column shaft assembly.
Steering column not fixed properly. Check attachment and fixings, tightening as necessary.
Interference between spindles on fixing sleeve. Adjust.
Pressure point set too hard Check adjustment

Difficult to hold straight course

Cause Remedial action
Oil level too low. Check for leakage, top up the reservoir with oil.
Air in hydraulic system. Check for leakage and bleed the system.
Looseness in steering system Tighten the loose parts to correct torque settings given in Impact software.
Ball joint with excessive play. Replace. DO NOT REPAIR OR RE-USE DAMAGED BALLJOINTS, this is CRUCIAL for safety!!
Loosened connecting sleeve. Tighten the sleeve.

Impact felt in steering wheel

Cause Remedial action
Oil level too low. Check for leakage, top up the reservoir with oil.
Air in hydraulic system. Check for leakage and bleed the system.
Excessive play between worm shaft and piston. Check and adjust the worm shaft. If damaged with scratches, replace it.
Excessive play between sector shaft and piston Check and adjust pressure point.

Steering wheel vibration (Shimmy)

Cause Remedial action
Wheels unbalanced. Balance the wheels according to type of tyre and wheel manufacturer’s guidelines. This MUST be done correctly, as an unbalanced wheel puts stress on the wheel hub and shaft, possibly resulting in it shearing clean off with the wheel on prolonged use.
Steering out of alignment. Disassemble, clean and reassemble the steering using Impact software guidelines. Check it works freely and test extensively.

Steering wheel independently goes to either side (pulls)

Cause Remedial action
Oil level too low. Check for leakages, top up the reservoir with oil.
Air in hydraulic system Check the reason and bleed the system.

Oil leakage

Cause Remedial action
Reservoir cap loose Fix the cap or replace if worn/damaged.
Servo pump shaft sealing ring leaking. Replace the sealing ring. DO NOT attempt re-use or repair!
External leakage in power steering Replace sealing rings. DO NOT attempt re-use or repair!

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The following functions are handled by the chassis control unit (CIM) on Volvo’s B7R coach, B7RLE low entry city bus, and B9TL double decker bus. Any faults usually result in a CHECK lamp illumination or a warning icon illumination, accompanied with a “Check diagnostics for Chassis Control Unit at next stop” message in the driver infodisplay.

Brake lights

The control unit controls the body’s brake lamp with the help of a retarder signal and a brake lamp signal from the bus Vehicle Electronic Control Unit (VECU).


The chassis control unit’s idling function signals to the body when the accelerator pedal is in idling position.


The kneeling function works together with the electronically controlled suspension (ECS) and activates a lamp on the instrument panel when the bus kneels. The function also activates a CHECK warning when the ECS indicates that pressure is too low.


The buzzer function controls the external buzzer in the dashboard which is connected to the chassis control unit (CIM). The buzzer is activated on request from the body and from the internal CIM functions. The buzzer sounds when CHECK messages or other info is displayed in the instrument panel.

Check of battery voltage level

The function senses when the bus’s batteries (2x12v batteries, each with an alternator) voltage is less than 22 Volts and then shuts down all voltage users that have low priority. When battery voltage hits less than 16 Volts an internal signal is generated for low battery level, and with voltages exceeding 36 Volts an internal signal is generated for high battery level. These internal signals are used for opening and closing certain inputs and outputs. The signal for low battery level also closes the normal signal transmission on the J1587/J1708 link. Low battery voltage will result in erratic engine starting and other electrical failures.

Retarder control

In normal operation the retarder (engine brake, known as JAK, JaKob’s engine brake) will only accept a number of preset braking torques. In those cases when the vehicle control unit (VECU) wants the retarder to brake with a torque which is not preset, this limiting function must be disconnected. This is achieved by activating an earth signal on the retarder control unit’s inputs.

Brake pressure

The brake pressure function reads off the air pressure on the front and rear brake tanks respectively, and controls the corresponding gauge on the instrument panel via data link J1587/J1708. The function also activates an alarm on the instrument panel when insufficient pressure is shown.

Vehicle speed

The function gives three output signals when the vehicle’s speed exceeds 5 km/h, 20 km/h and 95 km/h. The function also controls a frequency-modified signal with constant pulse width which represents the vehicle’s speed. (This signal is known as the C3 signal.)

Starter motor

The chassis control unit checks that the requirements for starting are fulfilled and then activates the starter motor’s relay.


This function reproduces the rheostat level which is given from the instrument panel to be used in the body instrument’s lamps.

Parking brake

The parking brake function gives a signal indicating the parking brake’s status to the body system when the parking brake is engaged. The body system uses the status signal to activate an alarm if the door to the driver’s cab is open when the parking brake is disengaged. The chassis control unit (CIM) also activates the parking brake lamp on the driver’s instrument panel.

Fire alarm

If a fire is discovered in the engine bay or in the bus body, a fire alarm is activated on the instrument panel. The fire alarm also activates the instrument cluster buzzer.

Main switch

The vehicle’s main electrical circuit cannot be permitted to close down in an uncontrolled way, i.e immediately. After the ignition is switched off certain functions must be provided with power for a certain period of time. The gearbox takes ten seconds to close down, which is the longest closing-down time. For this reason, the chassis control unit’s holding circuit function releases the holding circuit ten seconds after the ignition has been switched off and leaves the switching off of the main circuit to the main circuit’s switch.

Steering wheel adjustment

This function checks that the parking brake is applied and that the ignition is on before it permits adjustment of the steering wheel and instrument panel.

Fuel level

The fuel level function receives signals from the fuel level sensors and controls the fuel level gauges on the driver’s instrument panel. The function also sends a warning to the instrument panel when insufficient fuel level is recorded.
The calculation of the fuel level is determined by how the jumpers are set, (so that the function can be adapted to different fuel tanks). In those cases in which there are two fuel tanks, the jumpers decide the relationship between the sizes of the tanks. If there is only one tank, the jumpers give the setting the shape of the tank. (The jumpers can only be set to 15 V, earth or not be connected at all).

Fuel cut-off

This function controls the fuel cut-off’s solenoid valve and retains its position until the valve needs closing. A pulse for closing the valve is sent when the emergency switch is activated or when the main switch is switched off. The valve is opened when the emergency switch is switched off and the main switch is active or if the main switch is activated while the emergency switch is switched off. If both are activated at the same time, the emergency circuit breaker overrides.


The bogie function checks the air pressure in the bogie axle’s air suspension bellows and activates a lamp on the driver’s instrument panel when the air pressure is released. This funtion only applies to tri-axle B7R coaches with a double rear axle (bogie). Most B7RLE and B9TL doble deckers have one front and one rear axle. The inner-most axle of a bogie can be raised or lowered using the air suspension.


The chassis control unit (CIM) senses the status of the alternators, illuminating icons on the instrument panel if any of the alternators are not charging. This warning is shown before the engine is started in order to ensure that the function works. If the alternator lights do not go out once the engine is running, check the alternator(s) drivebelt, or the regulators on the alternators for any faults. If the battery voltage in the infodisplay is less than 22v but the batteries are fine, there is a fault in one or more alternator regulators.

Engine start/stop

The chassis control unit (CIM) handles the start signal to the vehicle control unit (VECU) and checks that established requirements for starting are fulfilled.


The temperature function reads from an inside sensor and an outside sensor and sends the signals to the instrument panel where the temperatures are shown on the infodisplay.


In those cases where the vehicle is equipped with MGS or EGS, the gear position function signals to the instrument panel if the gearbox is in high gear. The function also informs the instrument panel if the EGS gives a warning signal.

Open door

The open door function senses whether a door is open and, if so, lights up a symbol on the instrument panel, sounding the instrument cluster buzzer all the time the door is open. Once the open door is closed, the buzzer turns off. The open-door function is used with a conventional electrical system in the body. It monitors doors such as the emergency exits, cab door (not all bodies support this) and the bonnet. On some vehicles the passenger entry door is linked to this so the bus cannot be moved if it is open or has a faulty potentiometer.

On the B7RLE and B9TL, a faulty potentiometer or potentiometer track switch on the passenger door usually results in the STOPPING sign not extinguishing once the doors open. It should go out when the doors open, not when they close.

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As I mentioned in THIS PREVIOUS POST, the electronic accelerator pedal position sensor on Volvo’s B7R coach, B7RLE low entry city bus version and their B9TL double decker can go faulty. This will result in the Engine ECU reducing the engine speed to safe mode, or limp home mode. The sensor is actually a big potentiometer that is adjusted. The more you press the pedal, the resistance increases, so the engine ECU knows how much to increase engine speed, and as you’ll know from those radios and other goods that use pots for volume controls, they go bad after so long, and start to become intermittent.

The pedal potentiometer is connected to the Vehicle ECU (VECU). The VECU converts the resistance value from the potentiometer into a percentage value, and sends it to the Engine ECU, which in turn adjusts the engine speed by increasing air and fuel intake amongst other things, while sending data to the Transmission ECU (TECU) about the engine speed so it can adjust gears, or operate the retarder (engine brake) etc. As I mentioned in the other post, the accelerator pedal potentiometer is connected to the VECU via two hard-drawn signals, one digital (IVS) and one analogue. The analogue signal is generated by the pedal’s potentiometer and the digital by an engine idling switch. If a fault occurs in the potentiometer, the digital signal takes over only allowing the bus to reach a certain speed, so that the vehicle can be moved (known as Limp-home). The digital (IVS) signal enters via a port in the VECU, which in turn is connected to the body contact block (BB1:1).

Here I’ll show you how to replace the potentiometer, as I did it a few days ago on my favourite B7RLE of Rikku’s fleet! The CHECK light came on with the “Check diagnostics for Engine ECU at next stop” message, it wasn’t accelerating smoothly. The turbo gauge wasn’t moving either, it all felt like the engine was in a deep sleep and didn’t want to move the bus!

As I discussed in my last article about the sensor, both pedal sensor and turbo failure can generate the same symptoms, causing limp home mode (Rikku calls it sleepy snail mode!) with the CHECK light and “Check Diagnostics for Engine ECU” message in the info panel. Thereby make sure you fully investigate all possible causes by reading ECU fault codes using VCADS Pro, and checking wiring.

Once you have verified the fault, and determined the pedal sensor is definitely at fault, it’s time to replace the sensor.

1. Remove the drivers seat. This isn’t ultimately necessary, but I found it made more workroom than just adjusting the seat as far back as its rails would allow. The removal of the seat depends on the seat itself, as different B7R’s and B7RLE’s have different seats, depending on the body and option configuration. If you’re not sure how to remove it, just make it go back as far as possible.

2. Make sure the engine is switched fully off, and that the ignition switch is in position 0. Disconnect the bus electrical system.

3. Locate the pedals (duh!), don’t forget that the right pedal is the accelerator pedal, the one we need! Remove the floor mat, or move it aside:

4. Remove the 3 bolts holding the pedal assembly to the bus floor, placing them safely aside, below is the location of the fixing bolts:

5. Move the accelerator pedal to the side and remove the cable clamp (1). Remove the connector (2). Remove the accelerator pedal sensor potentiometer screws (3) as numbered and shown below. Remove the sensor.

You shouldn’t need to remove the second cable mounting clamp with screw seen in the image, unless the sensor supply cable needs to be replaced. This runs under the bus chassis up to the Vehicle ECU, and is joined in various places depending on the bus, wiring and body, as the signals go from the VECU to the EECU and TECU.

6. Fit the new sensor, and mount it, making sure it fits back exactly in the same position as the old one, with the wiring connector facing downwards. Refit the pedal with the 3 bolts, and perform a hand test. Press the pedal with your hand and make sure it is smooth. Refit the floor carpet, and driver’s seat, if you completely removed it for workspace. If not simply slide it back into driving position and make sure the seat adjustment rails lock in place when the adjustment lever is released.

7. Switch the bus electrical system back on, start the engine, and perform a rev test with the gearbox in Park or Neutral (the B7R coach is manual, the B7RLE city bus version is often auto), pressing and releasing the pedal. Watch your revs and listen to the engine. If the CHECK light illuminates and/or any Check Diagnostics messages appear in the infodisplay, perform testing on other areas. There could be signal loss between control units and wiring.

8. Perform the VCADS-test: 27102–3 Accelerator pedal, test with your diagnostic PC connected to the electrical system of the bus.

9. If the standstill rev test and VCADS tests were successful, take the bus for a test drive, varying your pressure on the pedal. When I’m on a test track I like to thrust the pedal up and down to make sure the system and engine reacts, being gentle then rough, but DO NOT do this on public roads!

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In this article I’ll show you what all the dials and indicators mean in the instrument cluster of Volvo’s EMS (Electronic Multiplex System) v2.0, the symbols, and what they mean. This dash is not just used on Volvo’s B7RLE/B7/9TL buses, it is also used on their range of coaches too, there may be differences, but the specifics are here. Their FH & FM range of trucks have the same indicators and dials, but the cluster layout is different.

A. Oil pressure gauge, engine

This gauge indicates the current engine oil pressure. The needle must not enter the red zone when the vehicle is being driven! This can sometimes be unavoidable on very hot days, but if it does, lower your revs as much as possible to lower the pressure and thus the temperature. The engine can be seriously damaged if oil pressure is lost due to an oil leak for example. This gauge gives a reading when the ignition key is in the drive position.

The LED in the lower left-hand corner lights when the engine control unit sends a low oil pressure signal, no matter what the gauge reads. The STOP exclamation indicator will light if the oil level becomes too low due to a leak, if it does, stop the bus and have it checked. If the bus is driven with a leak and the oil runs out, serious engine seizure and damage will result.

B. Temperature gauge, coolant

This gauge shows the engine coolant temperature. The same procedure above as for the engine oil should be followed.

C. Turbo pressure (boost) gauge

This needle shows how much boost the turbo is giving. Generally it will be higher under heavy acceleration and on hills, but you should not stress it too much otherwise the engine may go into limp home mode, shutting the turbocharger off to protect it from overheating.

D. Display (Infopanel)

This is where everything is displayed. Trip measurement, fuel economy, time/date. It also displays “Check diagnostics for xxx ECU” messages, along with the CHECK light (see J.). Failure icons and messages are also displayed, along with the STOP exclamation triangle (See I.). Info messages are also displayed, with the INFO indicator (See K.)

E. Tachometer

The tachometer displays the current engine speed in revolutions per minute (RPM) from 500 revs to 3,000 revolutions. Engine tickover (idle) speed is around 650rpm, and optimal gear upshift/downshift revs are between 1,000 and 2,000 revs. As always, observe the safety colur band in the rev ranges, and do not go over the yellow range maximum, otherwise engine damage will result.

F. Speedometer

The speedometer (speedo for short) displays the current total vehicle speed in km/h, some UK vehicles have it in mp/h, don’t get the two confused!

G. Fuel gauge

This gauge shows how much fuel you have left in the tank. This relates to single diesel tank fitted buses and trucks. For biodiesel and adblue vehicles, and Volvo vehicles fitted with dual diesel tanks there may be another separate dial. For adblue and biodiesel these are not normally Volvo fitted, but fitted by the body manufacturer or retrofit company to customer order (for buses) or the fleet company (for trucks). For information consult your vehicle documentation or builder.

H. Brake pressure gauges

These dials show the air pressure in the brake cylinders of the system. The pressure must not be allowed to get too low, otherwise brake failure will result. If it gets too low or high a Check Diagnostics warning is displayed in the Infopanel display, and a failure of the system will normally result in the STOP exclamation indicator coming on, in which case the vehicle MUST NOT be driven any further. Damage to the vehicle, other road users, passengers and yourself will result.

I. STOP exclamation indicator

This indicator will illuminate if a CRITICAL failure or other critical event happens, such as engine oil, coolant and/or fuel getting too low, an ECU or CAN/J1939 communications bus failure on the multiplex controller area network electrical system, brake failure, engine failure. If this indicator illuminates, STOP THE BUS IMMEDIATELY and have the cause rectified by an engineer. They, like myself, have the tools, knowledge and software to determine the cause by communicating with the vehicle electrical computer systems to find fault codes that tell us exactly where the fault is.

You MUST get the vehicle towed if this light comes on, for utmost safety. If you’re carrying passengers, make sure they are your first priority. Shut off the engine and use the emergency cutoff switch in the cab if you are worried about fuel or electrical fires or problems, it cuts both off safely. You’d be putting passengers, yourself and road users in extreme danger by driving a possibly seriously faulty vehicle that weighs over 18 – 30 tons by thinking the indicator is coming on for nothing, this is rarely the case. Volvo multiplex systems and control units are very very accurate. I can’t stress it enough.

J. CHECK warning indicator

This indicator comes on when there is a warning condition in one or more electronic control units (ECU’s) in the bus. They are not extreme failures, just friendly warnings that a service is needed. It will illuminate along with the following messages displayed in the Infopanel display:

1. “Check Diagnostics for Brake ECU at next stop”

This is the most common one you’ll see, and it means the brake wear sensors have detected the pads or discs are nearing their wear limit. It will also trigger when slight drops in pressure or leaks occur on the brake air system. Drops in pressure can be determined by ear. Listen to the discharge pattern when the system lets off steam regularly. The system discharges in two stages, two hisses about 2 seconds apart, every 5 or so minutes. If they are too long, too regular, the pressure may need to be looked at. Determine this by reading the pressure gauge in the dash as earlier in the article.

2. “Check Diagnostics for Engine ECU at next stop”

This message appears when a warning is triggered from the engine’s ECU. Situations include engine temperature sensor failure (look for the oil temp needle being cold when it shouldn’t), engine coolant sensor failure, tachograph sensor failure (for detecting RPM’s) and various others.

3. “Check Diagnostics for Light Control Unit at next stop”

The Light Control Unit controls all vehicle illumination and signalling equipment. Headlights, tail lights, indicators and brake lights. On buses it also controls gangway lighting, as they are wired into the main lighting system along with the destination board, normally wired in with the running lights (sidelights). The Light ECU monitors resistance across all lighting circuits, and any failing bulbs or LED brake/indicator clusters (used on Volvo buses with Wright Eclipse Urban/Gemini bodies) cause high resistance, so the ECU knows a light circuit has failed.

I am unaware if the Light ECU can trigger the STOP indicator.

K. INFO indicator (Can illuminate in conjunction with CHECK indicator)

This light comes on when the Infodisplay is set to Info messages in the display, such as fuel consumption etc, and a CHECK warning is displayed. Switch the display over using the control stalk on the steering column to view any CHECK messages and the INFO light will extinguish.

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If you’re driving along and suddenly the “Check Diagnostics for Engine ECU at next stop” message appears in the dash infopanel, along with the CHECK indicator and the temperature gauge reading cold or just above cold, the engine temperature sensor has failed or is dirty and will need to be replaced.

Units Involved

The Engine ECU (EECU) and the Vehicle ECU (VECU) are involved in this process. The Engine ECU periodically checks the temperature sensor. The resistance readings are sent as a changing signal to the dash temperature tacho needle as the temperature decreases or increases, the signal power varies the needle position. After 5 or 10 minutes from cold start the engine ECU knows that the engine should be warm, so if it starts to get a completely cold reading after this time, sends a signal via the CAN (Controller Area Network) to the VECU which displays a message in the dash infopanel.

Cause Elimination

This fault should not be deemed to be the temperature sensor if only the temp needle is too low. The CHECK light and message MUST also be displayed. Otherwise, the tacho needle must be checked, and/or the signal from the VECU.

To check if the needle is faulty, turn off the bus engine and ignition completely, turning the starter to 0. Turn it to I to switch on the ignition and the needles should all do a full sweep of their dials to the right then return back to the left. Turning the starter to II will start the engine, all fuel and air dials should go to their operating positions, showing the correct fuel and air pressures. To check the turbocharger boost needle you will need to drive the bus on hard acceleration on a straight road. It will not move if the footbrake or airbrake is applied as the turbo only operates when the bus is moving in gear.

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The accelerator pedal on Volvo buses is electronic, and I’ve seen cases where the potentiometer in the pedal has gone faulty. This results in the bus going into limp-home mode, which is like a safe mode so you can get the bus safely back to the depot for repair. But, many drivers wrongly assume that the turbo has gone faulty, as this can cause the bus to go into safe mode like most other engine faults including fuel supply and injection.

In this article I’ll explain exactly what makes the pedal work, and all systems involved.


The electronic accelerator system allows the driver to provide manual control of the engine speed, and thereby the speed of the vehicle.

Variant differences

  • The Accelerator setting & fault indication is only present on Volvo buses equipped with automatic transmissions or I-shift.
  • The IVS, CAN and J1708 connections differ between Volvo EMS1 and EMS2. EMS stands for Electronic Management System, known in the industry as Multiplex.


  • The engine must be running.
  • The transmission is in gear (D, or 1, 2, 3 ratio settings if available depending on the make of transmission fitted. Some ZF and Voith auto boxes have just P, R, N, D.
  • Vehicle speed must be higher than > 5 km/h for the system to be fully active


The following control units are involved:

  • Vehicle electronic control unit (VECU)
  • Engine electronic control unit (EECU)
  • Gearbox control unit (TECU)
  • Brake control unit (EBS5)
  • Body builder module (BBM). Also known as Body Control Module or Unit

The accelerator pedal is connected to the VECU via two hard-drawn signals, one digital (IVS) and one analogue. The analogue signal is generated by the pedal’s potentiometer and the digital by an engine idling switch. If a fault occurs in the potentiometer, the digital signal takes over so that the vehicle can be moved (known as Limp-home function). The digital (IVS) signal enters via a port in the VECU, which in turn is connected to the body contact block (BB1:1).

The analogue signal is converted in the VECU to a percentage value that is sent to the EECU via the control link. As a safety measure, the accelerator pedal position is also sent via the information link. The EECU uses the message to set the engine speed.

Buses equipped with automatic transmissions have a kick-down (downshift) function. When the accelerator pedal is fully depressed, the VECU software registers this as a kick-down and sends a message via the control link to the EECU and the TECU. The TECU ensures that the automatic transmission drops down a gear to give the driver higher engine speed and faster acceleration. Kickdown is also initiated when the bus is driven up a hill and acceleration is slow in mid/high rev range. This is normal, not a fault!

Accelerator pedal, fault indication

Volvo buses equipped with automatic transmission or I-shift have a function to switch out the requested throttle setting if both the accelerator pedal and the brake pedal or parking brake are activated at the same time.

The function is activated when the accelerator pedal is pressed at the same time as the brake pedal is depressed and the speed exceeds 5km/h. When the function is activated, the driver is informed via a warning on the instrument because the engine brake cannot engage while acceleration is taking place, the accelerator must be released first.

The function is deactivated when the accelerator pedal is in its idling (fully released) position or the ignition is switched off. If the fault comes up in the cluster even though the pedal is released, have the pedal looked at. The bus should always go into limp home if the potentiometer is defective. If it doesn’t there may be a fault in one or more subsystem ECU’s.


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This is to be carried out when squealing can be heard, either with the engine started and running, and/or on kickdown (downshift) on hills. Belt squeal signals that the belt needs replacing and/or re-tensioning. This only applies to the alternator belt, on the B7RLE the fan belt is toothed, and lasts much longer.

If the belt is left to deteriorate, loss of battery charge will result as the alternators struggle to turn, thus not keeping the batteries topped up, resulting in complete power loss, or failure to start the engine due to inadequate start capacity in the batteries (around 50% in a twin battery & alternator vehicle). Note I now grayscale all my service pictures, people were complaining they were taking too long to load!

1. Switch off the bus engine, and allow to cool. Alternators, engine and belts will be HOT.

2. Open the bonnet, and locate the fan belt. This needs to be removed first to get at the alternator belt as they overlap each other.

3. Loosen the fan belt tensioner, the location of which is shown above, and gently remove the fan belt. If it requires replacement, now would be a good time to get out the replacement belt ready for fitting!

4. De-tension the alternator belt tensioner and lock it with a M5 bolt (1). This prevents it going out of alignment.

5. Remove the belt from around the alternators, guide pulleys and cam pulley, after removing the fan belt the same way. My picture shows the fan belt still on, because I took the pic before removing it!

6. Clean all pulleys to remove dirt and rubber debris using a clean rag. DO NOT use LIQUID or GREASE/OIL anywhere near the belts or pulleys! They are designed to run with friction in the tension of the belt, getting liquid or grease contaminants anywhere near will cause the belts to slip and compromise the effect, and will mean a ruined new belt, the pulleys may also need to be replaced!

7. Re-assembly is the reverse of removal. Check the belts are tight, routed correctly and that all bolts are secure.

8. Start the engine of the bus using the underbonnet starter panel, let it idle for 3 minutes, then rev it erratically on and off with varying pressure on the accelerator to check for squeal. DO NOT go over the redline rev zone (above 3,500rpm), engine damage will result.

NOTE: Failing belts may not always be the cause of sudden or gradual power loss, or loss of battery charge. This could indicate either A). Faulty batteries, or B). Faulty alternators. I recently serviced Rikku’s 08 plate B7RLE after we suddenly lost all engine and electrical power on the motorway. One of the alternators had failed completely causing overcurrent, tripping the control relay, shutting the alternators off from the battery charge system, so use your judgement, and a voltmeter!

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With today’s newer buses being almost fully computer controlled in a multiplex setup, it’s hard to determine if a fault is severe or not while out on the road. Sensors monitor everything, and each subsystem (gearbox, body, compressor, brakes, electrical etc) has its own ECU all linked to the main engine ECU (Hence the Multiplex). But, even the sensors get it wrong, sending incorrect triggers to the ECU. Dirt, water, moisture, it all causes incorrect readings and triggers.

On the Volvo B7RLE and B9/B7TL it’s normally the brake ECU that never seems to stop notifying you on the in-cab diagnostic HUD to check its diagnostics at the next stop. But as a driver, you don’t have the training or equipment, and the message is generic, so you don’t know if it’s a minor thing or severe until the technician (that’s guys like me!) comes along and hooks his laptop up to the heart of the bus and coaxes ECU error codes out of it. To the driver it could be anything. The brake discs? Pads? Air system? Valves? The sensors even monitor brake pad and disc temperature! It might also just be a dirty sensor as in my case!

I devised a test while on the racetrack, out roadtesting a gearbox repair on Rikky’s ’08 B7RLE, I’d repaired the shifter rack, solenoids, and renewed the clutch because gearchanges had become erratic. The brake message started coming up every time I braked, and I’d changed the discs & pads not so long before, meaning it wasn’t the pad and disc wear error. So, I accelerated to the end of the green rev zone in 3rd gear, and just before the gearbox changed, pressed the brake slightly, and held it, still on the gas, hard. The gearbox ECU then holds off changing while the brakes are on, even slightly. I pressed the brakes harder, to heat the discs, pads and sensors up, while still at max revs for 15 seconds, and the warning came up every few seconds even with the brake released, because the system was nice and hot, the overheat code was running in the ECU log (I later checked with the laptop hooked to the ECU and it had logged the overheat code 23 times!). It cooled and stopped beeping constantly, and whaddyou know, the error went away in the HUD.

I’d forgot to clean the pad sensor, and it was dirty. This test also checks the high RPM gearchange, because if you’ve held the brakes at high revs with the gas to the floor and the bus still changes gear (upshift), the brake sensor has failed.

WARNING: This test MUST NOT be done by ANYONE other than an experienced bus engineer, and ONLY on a non-public road, such as a racetrack. The wear on the BRAKES and ENGINE during this test is IMMENSE, they will get hotter than normal use, and the brakes will need REPLACING following the test for safety, as imminent failure of the discs and pads may result otherwise, depending on their condition beforehand. I am an experienced and responsible mechanic, and CANNOT be held personally responsible for third parties trying out my tests.

YOU MUST check all brake components for damage after the test, especially the air supply system hoses for the above reason, BEFORE taking the bus back on public roads. Taking a PSV on a public road with possibly damaged brakes is careless, and puts you and other road users at serious risk, due to vehicle size and weight. Have the bus towed back to a garage with an inspection pit, as a bus has too low an overhang to do it anywhere else.

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Rikky did OK Friday night, but I think it was because she knew we were testing her, either that or she’s got the patience of a saint, which I’ve known for a while. The cab doors on Volvo B9TL buses helps too, all Rikky’s have the acrylic panels, and that channels noise away from the driver.

Given all the noise, I still couldn’t help notice a problem with the auto clutch during Rikky’s loud crowd test. I could feel it was hesitating during gearchanging. It was just starting to show slipping symptoms. It’s the judder that gives it away, and the not-so-right-as-usual gearchange whine. Some B9T’s have ZF’s that whine, some have Voith boxes that don’t. The B7RLE’s are dead easy to diagnose, I know the whining from the ZF’s on those by heart. Most drivers won’t notice, as the engine is at the back, and in the cab you can’t always hear it, especially with noisy passengers, but as an engineer who loves engines and listens to them, I do quite easily, and can often notice the slightest gearbox problem.

The auto clutch will eventually deteriorate so badly you’ll end up stuck at traffic lights because the thing won’t engage, and it’ll end up stuck in Neutral usually (when the brake is pressed, listen. That whine? It’s what I call temp neutral. Let go of the brake it goes back into Drive. All auto boxes do it, you can rev it with the brake pressed), and the engine will just rev uselessly. Sometimes it’ll be the relays and solenoids of the changer rack in the gearbox. My advice is to get it sorted ASAP before it gets that bad and you need Recovery, and annoy other drivers.

I took Rikky’s 59 plate B7RLE for a quick run last night. I love the new facelift B7RLE. Since Wright gave the Eclipse Urban body an overhaul in 2008, and Volvo changed the old dash to the one used in the Gemini B7/B9 double deckers it feels so much better. The dials aren’t cramped, and there’s two stereo mountings! Rik wants me to put a CD player in for the driver, and a radio/announcement system for passengers, which is a piece of cake on these new ones, as the wiring and mountings are there, plus gangway speakers. Ours even has the D7E320 320HP engine, which was a surprise, Volvo (and Dennis) normally only fit these to their refuse and tipper/artic truck line. They’re limited to the usual 74.15MPH, but I’ve had the limit off, and tuned the intake and ignition timing slightly. They can go up to nearly 100mph, which I observed using our diag computer. On a racetrack. Never do this on public roads, especially if you’ve knocked the TCS and SCS off. I think it can go higher, as Volvo seem to have given guys like me headroom for tuning. DON’T ever do anything like this unless you understand engines, are in a safe place, and won’t injure yourself or passengers. NEVER have passengers while doing this, unless they’re engineers.

I want one of these myself, given the chance! Volvo buses and Wright bodies are perfect!

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It rung true today, that’s for sure! I was driving down to Bolton in one of Rikku’s Volvos, that I’d just fixed an electrical problem on, down the M62, at about 10pm, taking it to a lockup in Heywood. Suddenly, I lost all lights on the bus. Headlights, dash lights, rear lights, top tail lights, and destination board. The passenger cabin lights didn’t work, either.

I pulled over on the hard shoulder, and used my electric warning triangle to warn of the hazard, as I had no hazard lights. Connecting the diagnostic laptop and ECU reader up to the ECU, the engine stalled. Checking the fusebox, a relay had tripped, and a fuse for the alternators. I got the engine running again, using the under bonnet starter panel, after changing the fuse when suddenly a police car pulled up behind me. I heard a familiar voice as the officer came close. It was my half sister Tammy, who has worked as a CID officer and traffic cop for 6 years.

I told her what was wrong. I needed to get the bus back to the depot, (obviously I couldn’t carry on to Bolton in it) but Recovery weren’t available, they’d gone home, and my radio wasn’t working. She radioed through to her station, while I borrowed her phone to call Riksy, to open the depot for me.

5 minutes later her backup arrived, and they escorted me back to our depot, me driving the bus with no lights in between their cars. It must have been a funny sight! Rikky was quite surprised, and joked, “You’ve not got yourself into more trouble for speeding, have you!”, with a wink! It is very scary in the hard shoulder with no signal equipment on your vehicle. I don’t yet know what caused the issue, nor have I had time to even fix it, but the original fault was a dodgy earth, which was causing the diagnostics to issue the Check Light Control Unit error, which I’d fixed.

I got down to Bolton OK, just a bit later, by borrowing another bus, our fun little 1997 Dennis Dart, which has never had any problems since we got it, except a squeaky accessory belt. Shows how finicky new computer controlled vehicles can be, sometimes! I love machinery, and wouldn’t stop doing this for Rikku even if you paid me! I’m good with vehicles, and engines, but all this computer controlled vehicle stuff is quite new to me, and I’m still learning myself!

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Is when they’re in vehicles, especially Volvo buses. Rikku cornered me this morning, telling me that her brand new B9TL executive double decker wasn’t changing gear correctly. This thing’s a brand spanking new 59 plate bus, and has only been used for 3 corporate runs. So, being the vehicle loving guy I am, I went round to the garage, jumped in, and took her for a run on the rolling road (going nowhere fast, literally!). It had decided it wanted to ignore 3rd and 4th gear entirely, on upshift and downshift, clanking as it did so.

Now I know there’s nothing wrong with the gearbox (it’s a Voith, rather than ZF like our B7RLE’s), so I looked to the ECU, both main computer, and gearbox. Somewhere, the code for gearchanges wasn’t being executed correctly. A quick re-flash of the original BIN dump I did when we got it solved the problem, she’s happy as Larry, until next time!

There’s times when I wish buses had MANUAL gearboxes! The last time I drove a manual was a big Leyland coach when I passed my PSV for Blue Bus, in 2000, and even that was a big 20 year old junker. It had a tiny gearstick, you changed gear, and 10 seconds later the gearbox actually changed, with a big jerk! I hated the thing!

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This is a totally new one to me! It came up today while I was checking one of the rear LED stoplight clusters with Rikku as part of an MOT pre-check on our 2006 B7RLE. It seems that the computer watches the resistance on the signaling circuit too. When a light is working, the resistance is low, because it’s functional. When it isn’t working, resistance across the circuit increases, telling the control unit something isn’t right when a light should be on.

I need to get on to Volvo, the Impact software doesn’t cover the in-cab diagnostic system, or ECU error codes. The in cab errors are just generic warnings.

What gets me is, why does the system tell the DRIVER to “check diagnostics for xxx ECU at next stop“? Normal drivers don’t have the skills, experience, nor training for checking ECU faults! Nor are they employed to!

Very strangely worded messages, but very useful. I hate buses with just dash indicators, the faults could be anything! Volvo’s system is very to-the-point, and market leading, like their buses, in my opinion.

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Volvo buses have another use, as well as buses, racing tools, and back massagers. They are good UPS systems! They even beat my old Merc Sprinter! We had a power cut at 7pm tonight, and we needed to keep the 2 main servers in our server room going when our big UPS ran out of juice. I’d brought our 2008 B7RLE home from Matsuki to do some diagnostics. There’s 2 batteries and two dynamos in these so it was ideal!

So, I wired an extension lead and plug board to the inverter powering the bus CCTV DVR, started the engine, and revved it at 1,500rpm using the diagnostic software (the throttle is electronic!). It lasted well over two hours before the batteries gave out. I then had to jumpstart the bus using Kassie’s Subaru, and Mika’s Mini, each car wired to a battery, reset the computer, and take it for a razz to get some juice from the dynamos before charging the batteries once the power came back on. I had a laugh with the girls, when the bus gave out, I said, “Right girls, give us a push!”

The puzzled look on their faces was a picture! I was expecting them to fall for it!

CAUTION: NEVER connect a battery charger to an inverter in ANY vehicle and try charging it while the engine is running, or connected to the battery. You’ll damage the electrical system. I wiped out an ECU on an old van of mine a few years back trying it!

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