Volkswagen Workshop Service and Repair Manuals

As the control units are interconnected with a data bus wire, always start fault finding by interrogating fault memories of all the control units in the vehicle.

This occurs in "Automatic test sequence" and is activated with key function 00.

When doing this, check to see if there are faults stored which may influence the ABS.

The self-diagnosis relates to the electrical/electronic part of the ABS, in other words only faults via the electrical connection to the control unit are recognised (e.g. speed sensor open circuit).

The 47-pin ABS control unit -J104- forms with the hydraulic unit a compact unit. The unit is located on the left of the engine compartment. The control unit is equipped with a fault memory. The self-diagnosis connection is located in centre console below the ashtray insert.

The control unit recognises faults and malfunctions during vehicle operation and stores them in a permanent memory, the contents of which remain despite the lack of battery voltage.

Sporadic (irregular) faults will also be recognised. But if these faults do not occur again within the next 15 vehicle starts and driving off sequences for faults which affect the data bus system, or 50 vehicle starts and driving off sequences for other faults, they will be erased from fault memory except for the fault "Control unit defective" (forget fault counter).

After the ignition is switched on and/or the engine is started, the ABS warning lamp -K47- and the ESP warning lamp -K155- will light up for approx. 2 seconds.

During this period a test sequence (self-check) is run in the control unit for the following functions:

• ◆ To check if the supply voltage is at least 10.0 Volt,
• ◆ To check control unit including the valve windings,
• ◆ To check the coding of the control unit,
• ◆ A static check of the speed sensor (no speed signal),
• ◆ If after driving off and exceeding a speed of approx. 20 km/h (approx. 13 mph) the speed signal is not OK. the ABS warning lamp -K47- will light up again.

Fault reader V.A.G 1551 or tester VAS 5051

The self-diagnosis must be initiated always at the commencement of fault finding. Electrical faults which influence the braking characteristics will be stored. They can be interrogated with the fault reader V.A.G 1551 or with the new tester VAS 5051.

The fault information displayed is used in conjunction with a fault table which has information on the possible fault causes for pin-pointed repair measures.