Brakes - ABS Activates Upon Light Braking
45 08 042010096/3
November 19, 2008. Supersedes Technical Service Bulletin Group 45 number 05-01 dated March 1, 2005 for reasons listed below.
Affected Vehicles
45 ABS activates during light brake application
Condition
ABS activates when braking lightly or control noises are heard from the ABS unit at low speed without brake application.
Technical Background
May be caused if an ABS control takes place without the flashing of the ESP lamp and thus without ESP operation.
- This is purely an ABS action.
- The ESP sensors have no influence on this.
The following may be the cause for incorrect wheel speed values:
- Damaged wires from the wheel speed sensor to the ABS control unit.
- Tires of unequal size or greatly unevenly worn tires.
- Slight cracks in the ABS rotors (especially on older vehicles).
- Dirty or incorrectly adjusted wheel speed sensors.
Production Solution
Service
Do not replace any components until the following has been performed.
Tip:
DTCs may not be stored in the ABS control module, and On-Board Diagnostic (OBD) Read measuring value block 1 may not show any deviation in the wheel speed value.
1. Thoroughly check the ABS for any cracks, debris or damage to ensure proper operation.
NOTE:
Since slight cracks may be hard to detect with a naked eye, use an oscilloscope.
2. Using an oscilloscope (VAS 5051), check the wheel speed values.
- Curve and frequency must be smooth and uninterrupted as shown in Figure 1.
- A short disruption (such as a scratch or a notch) in the curve of the DSO picture that is repeated periodically may indicate a damaged ABS rotor. If a short disruption is observed, inspect the ABS rotor for damage. The ABS rotor may be located on the outer CV joint, the wheel hub, or the inside of the brake disc. (See Repair manual >> Group 45 - Anti-lock brake system).
Warranty
This TSB is informational only and not applicable to any Audi warranty.
Additional Information
All parts and service references provided in this TSB are subject to change and/or removal. Always check with your Parts Department and service manuals for the latest information.