A number of calls to the telephone technical support team have highlighted a common problem regarding drivability of the vehicle. This problem is so severe that the vehicle will start and idle OK, but the engine does not seem to have sufficient fuelling to enable the vehicle to be driven.

On closer inspection, involving retrieving self diagnostic fault codes from the vehicle, a common fault has been identified regarding the electrical brake circuit. On various versions of the Bosch EDC engine management system, if a fault is identified with the electrical brake circuit, the diesel engine management system enters into a “limp home” mode of operation. In this mode, the fuelling is reduced to “minimum” values to enable the vehicle to be driven (in reduced power mode) for repair.

Note. On some versions of the Bosch EDC system, a more basic method of sensing the electrical brake circuit is used, which may not allow the Bosch EDC system to recognize a fault, and therefore a fault code for the electrical brake system is not registered.

On inspecting the Bosch EDC wiring diagram for the applicable vehicle it can be seen that the Bosch EDC ECM has one or more terminals connected to a brake pedal switch. The Bosch EDC ECM uses the brake pedal switch circuit to indicate when the vehicle is being braked, and at this time engine performance and therefore fuelling can be reduced to help reduce emission levels from the vehicle’s exhaust.

If a fault exists in the brake pedal switch circuit or related electrical brake circuits, the Bosch EDC ECM (on some versions) may interpret this fault as the brake pedal being operated constantly and therefore the fuelling for the vehicle may remain at minimum values (fuel shut off), thereby preventing the engine from having sufficient fuelling for acceleration.

It has also been apparent through conversations on the help line that it is not just faults with the brake pedal switch which can cause drivability problems with the vehicle. A number of calls to the help line have identified that incorrect or faulty stop/tail light bulbs can also be responsible for this type of fault. Therefore before carrying out checks on the brake pedal switch circuit, it is advisable to first check the operation and fitment of the stop/tail light bulbs.

Typical Brake pedal switch circuits

According to the vehicle manufacturer & year, there have been different versions of the brake pedal switch which can be fitted to the Bosch EDC engine management systems.

Single two terminal brake pedal switch

The most basic form of brake pedal switch fitted to the vehicle can be a single switch unit using two terminals on the switch to distribute the signal through the brake light circuits and to supply a “Brake ON” signal to the Bosch EDC ECM.

Note: on some versions of the Bosch EDC engine management system, two separate single switch units may be fitted. One switch is used for the brake light circuit, whilst the second switch is used to supply the “Brake ON” signal to the Bosch EDC ECM.

Dual brake pedal switch

The most advanced form of the brake pedal switch fitted to the Bosch EDC engine management system is the dual switch unit. The dual switch unit can use either three or four terminals to distribute the electrical signal through the brake light circuits and to supply a “Brake ON” signal to the Bosch EDC engine management ECM. The two contacts inside the brake pedal switch are designed to make and break their respective circuits in opposition to each other. i.e. one set of switch contacts are closed, whilst the second set are open with the brake pedal released; and  when the brake pedal is depressed, the set of switch contacts that were closed are now open, and the set of switch contacts that were open are now closed.

Using the dual three or four terminal brake pedal switch enables the Bosch EDC engine management ECM to diagnose correctly the operation of the brake light circuit. If both switch circuits do not operate correctly, in opposition to each other, the Bosch EDC engine management ECM will diagnose a fault and register a diagnostic fault code.

Note. On some Bosch EDC engine management systems, a diagnostic fault code can also be registered if the brake pedal is pressed at the same time as the accelerator pedal is operated. If this occurs the warning light for the Bosch EDC engine management system can be turned off by using diagnostic equipment to clear the fault codes from the Bosch EDC engine management ECM and road test the vehicle to ensure no additional faults exist.