How to Test Anti-lock Brake Systems on Audi, Volkswagen Vehicles
EDL is an acronym for electronic differential lock. The EDL system allows traction to be directed to the appropriate wheel by applying a brake to those wheels that are spinning.
The system uses the signals from the wheel speed sensors to determine wheel spin. It then activates a self-priming pump in the anti-lock brake system (ABS) hydraulic unit that pumps brake fluid from the brake fluid reservoir through a solenoid valve and to the wheel or wheels that have lost traction, slowing them and allowing traction to be regained. The system can calculate brake system temperatures based on the number of EDL applications that occur during a given time interval and will disable the system if excessive temperatures are present. This temperature calculation is based on a time interval management (TIM) signal that is produced in the instrument cluster and sent to the ABS control module. ABS Trouble Code 01203 indicates a problem with the TIM signal.
To test, proceed as follows:
1) Using a labscope, check for the presence of a "time stamp" signal from the instrument cluster (IC) T32b connector pin No. 11 to the ABS controller at pin No. 10 (typically a green wire). This signal will appear as a series of quick 11-volt to 0-volt square-wave pulses a moment or so after the ignition is turned to the "on" position. It is critical that the square-wave signal drops all the way to 0 volts.
2) If there is no signal present, or if a signal is present that does not drop all the way to 0 volts, turn off the ignition. Disconnect the ABS controller wiring harness connector and recheck for the correct signal on pin No. 10 vehicle harness side as the ignition switch is cycled on.
3) If the correct square-wave signal described above is present with the ABS control module unplugged, there is likely an internal problem with the ABS controller. The ABS controller circuit board can fail in such a way that it can either short the TIM signal from the instrument cluster to ground or it can backfeed voltage onto the TIM signal wiring and interfere with the 11-volt to 0-volt square-wave signal. This problem is much more likely to occur at high ambient and under-hood temperatures. It is possible to use a heat gun to carefully heat the ABS controller and cause the failure to occur.
4) After unplugging the ABS controller, if there is still no square-wave signal to pin No. 10, check the wiring from the ABS controller pin No. 10 to the instrument cluster T32b connector pin No. 11. Be sure to pay special attention to the orange connector at the driver's side kick panel connector station for corrosion or damage.
5) It is possible for the instrument cluster to fail in such a way that the TIM signal is not produced, but this is an extremely rare occurrence.
Recently, I took a call on a 1999 Volkswagen Passat with a 1.8L AEB engine and equipped with Bosch ABS/EDL 5.3 anti-lock brakes. The ABS light would illuminate intermittently, primarily after the vehicle was driven for more than an hour on hot days. Testing for trouble codes revealed the presence of a 01203 code stored in ABS memory. After performing the diagnostic procedures above, a faulty ABS control module was identified. Replacing and version coding the new module to the vehicle restored normal vehicle operation.
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