TideLog Archive for the “Volvo” Category

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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