ABS Information

ABS Diagnostics: Pre-Amble

Given that the bespoke VAG1710 tester is only likely to be found at certified Audi workshops, this section shall focus on the OBD capability that is supported by VAG-COM.

The OBD functionality is concerned with the electrical components of the ABS. As such it can only detect faults affecting the system's electrical signals. Some system malfunctions may not be detected by OBD. However in the event of system malfunction, the first step is to read the diagnostic trouble code (DTC) memory. This can only be performed with a VAG1551 tester, VAG-COM or similarly equipped scan tool. The ABS controller does NOT provide blink code outputs like the ECU.

When switching the ignition key ON, the switching of relays and start-up self check noise of the return pump being tested should be audible. If the ABS controller detects a malfunction at this time, the ABS will be turned off and the ABS lamp (K47) in the cluster will remain illuminated. Normal braking remains fully operational whenever ABS is turned off. On a healthy system at startup, the ABS warning lamp will only be illuminated for a two seconds before it is turned off at the end of the self-test.

The ABS controller continually checks the health of the system, so if the ABS warning lamp comes on while driving then there is most certainly a fault which needs to be rectified ASAP.
ABS Test Requirements

The OBD dialog with the scan-tool (VAG-COM in my case) can only be commenced when the vehicle is stationary (below 2.5km/h) for safety reasons. Once the OBD dialog has started, the ABS is switched off. OBD communications will continue up to a maximum vehicle speed of 20km/h after which OBD is cessated and the ABS resorts back to normal operation.

Other items to check before using the ABS OBD are as follows :
  • Same size wheels/tyres all round, correctly inflated
  • Conventional brakes fully operational including brake light switch
  • Ensure there are no hydraulic leaks
  • Wheel bearings all OK
  • Good ground strap from the electro-hydrualic unit
  • Relay modules properly connected into electro-hydraulic unit
  • ABS controller properly engaged with multipin connector
  • Rear diff lock must NOT be engaged
  • All fuses OK
  • Good supply voltage (~12V)

ABS Diagnostic Trouble Codes (DTCs)

The ABS controller recognises thirteen system malfunctions which can be retrieved by reading the fault memory with VAG-COM. The table linked below shows all the known DTCs along with possible fault sources and corrective action.

ABS Measuring Blocks

The ABS controller has eight parameters which it can be display in the measuring blocks of Group 0. Note this will NOT work in shareware versions of VAG-COM - only fully activated versions. The table below describes the purpose of each channel.


Channel #
Designation
Test Requirement
Display Expected
01
Front Left wheel speed
-
1-19 (km/h)
02
Front Right wheel speed
-
1-19 (km/h)
03
Rear Left wheel speed
-
1-19 (km/h)
04
Rear Right wheel speed
-
1-19 (km/h)
05
Brake Light Switch (F)
Pedal NOT pressed
0
Pedal pressed
1
06
Test ABS Return Pump
-
0

Note: If result is 1 then perform
functional test with VAG1710
07
Test ABS Solenoid Valve Relay
ABS ON
1

Note: If result is 1 then perform
functional test with VAG1710
08
Rear Differential Lock Switch
Diff. lock NOT engaged
0
Diff. lock engaged
1

Remember that OBD only operates up to speeds of 20 km/h. Channels 1 - 4 indicate wheel speed in km/h which is calculated by the ABS controller as it counts the pulses from the wheel speed sensors. If the car is moved evenly on a dry surface, a deviation of +/- 1 km/h is permissible. If the difference is greater than that, the wheel speed sensors should be checked.

To check that the speed sensors are correctly connected, it is possible to turn each wheel by hand (with others chocked) and verify that movement registers in the correct channel.

Last Updated 8th October 2002