Introduction | System Description | Schematics | Cancel Switch | Diagnostics ABS Information |
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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. |
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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 :
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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.
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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.
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