Diagnostic Strategies

Troubleshooting ABS and ESP, safety precautions and general notes
^ The ABS/EDL/ASR and ESP are safety systems knowledge of the systems is essential for performing repair work.

Road testing
^ Certain malfunctions will only be detected when the car is moving, so the system can be checked by performing a road test. Maintain a speed above 60 km/h for a period of 30 seconds. After ensuring that it is safe to do so, apply brakes hard enough to activate ABS-controlled braking (this can be felt from the pulsation at the brake pedal).

WARNING:
If you drive the vehicle after a malfunction has been detected by the On Board Diagnostic (OBD), bear in mind that the function of the brake system is restricted. If a malfunction has been detected by the On Board Diagnostic (OBD), there is a risk of accident, since the brake pressure at the rear wheels is no longer controlled by the electronic brake force distribution function. This can result in excessive braking at the rear, and the tail of the vehicle may break loose unexpectedly.

If test and measuring instruments are required during a test drive, these must always be secured to the rear seat and operated by a second person from this position. If test and measuring instruments are operated from the front passenger's seat and the vehicle is involved in an accident, the occupant of the seat could be seriously injured if the airbag is triggered.

If there is a complaint on the vehicle, perform the troubleshooting procedure as follows:
- First switch on the ignition and observe the warning lights. The table in the chapter on "Warning light functions" will allow you to make an initial statement.
- Observe the requirements for test.
- Connect the VAG 1551 scan tool and select "Brake electronics" with address word 03.
- Check the control module version
- Check DTC memory. If a code is stored, make a note of it and erase DTC memory.
- End data transfer via the "End Output" function.
- Repair malfunction according to DTC table. Select "Brake electronics" once again using address word 03. Check and erase DTC memory.
- Conclude the transfer of data with the function "End Output". Disconnect VAG 1551 scan tool.
- Make sure that all plug connectors are properly connected. Perform a road test. The warning lights should not come on during the road test. If they do, repeat the troubleshooting steps described above.

Note: If the connector is not plugged into the control module when the vehicle is moving, this may result in a spurious (non-existent) malfunction being registered in the DTC memory of another control module. The operation of the brake system is impaired because the brake pressure at the rear wheels is no longer controlled by the electronic brake force distribution function. This can result in excessive braking at the rear, and the tail of the vehicle may break loose unexpectedly. This could cause an accident if the vehicle is driven in this condition.

- Switch off the ignition before removing and installing components. Disconnect the battery Ground (GND) strap (on vehicles with coded radio, first obtain the security coding).
- If a component is indicated as malfunctioning, only replace it if you are sure that the electrical wiring for that component is in proper condition. The wiring must be able to transmit the necessary signals without interference or resistance. Check the wiring for:
- Corrosion and loose contacts (intermittent open circuit) in the plug connectors and Ground (GND) connections. If any of the plug contacts are bent, broken or corroded, use the VAS 1978 wiring repair kit to install new contacts
- Open circuits
- Short circuits to Ground (GND) or to positive (live voltage)

Refer to Wiring Diagrams

Note: The instructions supplied with VAS 1978 wiring repair kit normally do not permit repairs to the wiring of the ABS/EDL and the associated systems. However, this only applies to the screened wiring in the systems.

^ When replacing the control module, be sure not to install an incorrect control module. Check control module version first. It appears on the display of VAG 1551 scan tool after the diagnostic link is established between VAG 1551 scan tool and the control module, or can be called up using function 01 "Check Control Module Versions". The vehicle features can also be identified by a combination of letters on the vehicle data sticker, which is located in the spare wheel well.
^ 1AC Anti-lock brake system (ABS)
^ 1AH Anti-lock brake system (ABS) with electronic differential lock (EDL)
^ 1AJ Anti-lock brake system (ABS) with electronic differential lock (EDL) and traction control system (ASR)
^ 1AT Anti-lock brake system (ABS) with electronic differential lock (EDL), traction control system (ASR) and electronic stabilization program (ESP)

^ Absolute cleanliness is required when working on the hydraulic unit. Carefully clean the hydraulic connections and adjacent parts. Do not use aggressive cleaning agents such as brake cleaner, gasoline, thinners or any products which contain mineral oil, such as oils or grease etc. Do not disconnect hydraulic lines until you have cleaned the connections.
^ Observe the appropriate precautions regarding the handling of brake fluid.
^ Place removed parts on a clean surface and cover them over. Do not use fluffy cloths or rags.
^ Only use genuine packaged parts, and only remove replacement parts from their packaging immediately prior to installation.
^ Bleed the brake system if connections in the system have to be opened
^ Welding work with electric welding equipment can affect the ABS/EDL or ABS/EDL/ASR system.

Requirements for test
^ The vehicle must be equipped with the approved wheels and tire sizes, which must be the same on all four wheels. Check inflation pressures all round.
^ Brake system mechanical and hydraulic parts must be in order: No leaks in hydraulic lines or connections (visual inspection of hydraulic unit, brake calipers, wheel cylinders, tandem master brake cylinder).
^ Wheel bearings and wheel bearing play OK.

Refer to Steering and Suspension
^ Wheel speed sensors must be installed correctly
^ Supply voltage must be OK (not less than 10.0 V).
^ Connector on ABS control module (w/EDL) J104- must be properly attached, with retainer catch engaged.
^ The diagnostic procedure can only be started with the vehicle stationary and the ignition switched on (or with engine running). The diagnosis cannot be started if wheel speed exceeds 2.75 km/h, and will be terminated if the speed exceeds 20 km/h.
^ When testing the ABS/EDL and ASR systems, ensure that the vehicle's electronics are not subjected to electromagnetic interference: keep the vehicle away from equipment with a high current draw, such as electric welding units.