TideLog Posts Tagged “accelerator pedal”

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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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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