ABS - Wheel Speed Sensor Testing
GROUP: 45NUMBER: 91-01
DATE: 12/20/91
SUBJECT:
ABS wheel Speed Sensor Testing
80/90/100/200 Front Wheel Drive
80/90/Coupe/100/200 Quattro
5000 Quattro and Turbo Quattro, 5000S and 5000 Turbo
MODEL(S):
V8 Quattro
CONDITION:
Most ABS problems (especially intermittent problems) are the result of an incorrect reading from one (or more) wheel speed sensor(s).
POSSIBLE CAUSES:
^ Bad Sensor - The resistance value of the sensor should be approximately 1,000 ohms.
^ The speed sensor is not correctly adjusted (not pushed-in all the way) or may require new tips (if applicable).
^ Rust build-up on the toothed rotor (the rusted area has a different magnetic property than the non-rusted area).
^ Excessive resistance in the wiring from the ABS control unit/to the speed sensor.
FUNCTIONAL CHECKS:
Note:
These additional tests should only be done AFTER the normal tests are done with the LED II tester, and no problem areas are detected with those tests.
1. Verify that all four tires are of same brand (manufacturer), size, inflation and of equal wear. Check power supply fuse.
2. Check the wheel speed sensors as follows:
Connect the VAG 1598 and 1598/3 adapter harness to the ABS wiring harness. Leave the ABS control unit disconnected.
Have someone else drive the vehicle on a straight and level road. Using a known good voltmeter set on A/C volts, take the following voltage readings using the illustrated data entry sheets.
ALTERNATIVE TEST METHODS
1. Voltage readings can be taken directly from the wiring harness (not using the 1598).
2. Connect the ABS LED II Tester to the ABS wiring harness, leave the ABS control unit disconnected.
Move the tester selector to position 5 (Wheel Speed Sensor Test).
Have someone else drive the vehicle on a straight and level road at approximately 10 mph while watching the needle and the red LED. The problem sensor will be harder to spot than in the previously described test, but you should still be able to determine the problem sensor. The LED may not flicker consistently or the voltage needle may not read as high.