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.

Purpose

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.

Requirements

  • 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

Function

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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