Antilock Brake System On Board Diagnostic
Beginning with the 1994 model year, the ABS system with On Board Diagnostic (On Board Diagnostic) is available on vehicles with front wheel drive. With this change, the ABS combination relay -J156- is eliminated.
The ABS control module is equipped with Diagnostic Trouble Code (DTC) memory. This section of the repair information describes the On Board Diagnostic procedure using Scan Tool :VAG1551 (ST).
Function
The On Board Diagnostic (On Board Diagnostic) function is concerned with the electrical and electronic components of the anti-lock brake system; i.e. the diagnostic program only detects malfunctions that affect the electrical signals.
The first step is always to check Diagnostic Trouble Code (DTC) memory.
The ABS Control Module (w/EDL) -J104- is equipped with DTC memory. If any malfunctions occur in the sensors or other components monitored by the system, these are stored in memory as diagnostic trouble codes.
When switching the ignition on and driving at 5-6 km/h (3-4 mph), the system conducts a self-test. The switching of the relay and the start-up noise of the hydraulic unit return pump may be audible. Do not replace the hydraulic unit due to complaints about this noise. Another noticeable part of the self-test is a slight vibration at the brake pedal.
If a malfunction is detected, the ABS switches off automatically and the ABS warning light -K47- comes on in the instrument cluster. The normal brake system remains fully operational. ABS remains switched off for the duration of the trip with the exception of a malfunction in the voltage supply. (In this case, the ABS switches on again as soon as the battery positive voltage has returned to a normal level).
The following malfunctions may be recorded in DTC memory when the battery has been fully discharged (e.g. after a jump-start):
Inlet/outlet valve -N137-, -N138-, -N139- or -N140- faulty
ABS solenoid valve relay -J106- faulty
In this case, the ABS warning light will come on for the corresponding period. The malfunction is stored in DTC memory as a "sporadic" malfunction. As long as no other malfunctions are detected when testing the ABS hydraulic unit, these malfunctions alone do not justify replacing the ABS hydraulic unit.
If a malfunction (which may have been detected and stored in DTC memory during the last single trip) no longer exists after the ignition has been switched off and on again, or if it cannot be detected with the vehicle stationary (certain malfunctions can only be detected when the vehicle exceeds a road speed of 12 km/h (8 mph)), ABS warning light -K47- goes out 2 seconds after the ignition is switched on (as soon as the ABS self-test routine has been completed by the ABS Control Module (w/EDL) -J104-), and if no malfunction is detected.
If no further malfunctions are present, the static malfunction stored in DTC memory is converted to a sporadic malfunction after switching the ignition off and on.
A sporadic malfunction is automatically erased if it does not occur again for a certain number of single trips (ignition switched on and off again).
Sporadic malfunctions are also specially identified as such by "/SP" displayed on :VAG1551 (right side of display).
After processing the input signals, the ABS Control Module (w/EDL) -J104- distinguishes between 15 different types of malfunctions (DTC table, DTC TABLE ) and stores them in DTC memory until the memory is erased (after checking) using the scan tool :VAG1551 .
The On Board Diagnostic program is available only by using the scan tool :VAG1551 and "Rapid data transfer" operating mode 1.
The On Board Diagnostic program sequence for ABS can be used not only for storing, checking and erasing malfunctions in DTC memory but, together with the "Check Control Module Version" and "Read Measuring Value Block" functions, offers additional capabilities. The "Blink Code Output" operating mode 2 is not operational with ABS control module -J104-. Any other operating modes possible with :VAG1551 are described in the corresponding operating instructions.
On Board Diagnostic data transfer using the scan tool :VAG1551 is possible only when the ignition is switched on or when the engine is running.
The On Board Diagnostic program sequence can be started only when the vehicle is stationary. If the vehicle is travelling at more than 2.5 km/h (1.5 mph) the ABS Control Module (w/EDL) -J104- does not respond.
Checking and erasing DTC memory, identifying the control module version, and reading the measuring value block are only possible up to a speed of 20 km/h (12 mph) (the On Board Diagnostic program sequence is stopped if this speed is exceeded).