TideLog Posts Tagged “Volvo”

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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