Troubleshooting procedure

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 detailed under REQUIREMENTS FOR TEST .
Connect the VAG 1551 scan tool and select "Brake electronics" with address word 03. See VAG 1551 SCAN TOOL, CONNECTING AND ENTERING ADDRESS WORD .
Check the control module version
Check DTC memory. If a code is stored, make a note of it and erase DTC memory. See ERASING DTC MEMORY .
End data transfer via the "End Output" function. See ENDING OUTPUT .
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. See TROUBLESHOOTING ABS AND ESP, SAFETY PRECAUTIONS AND GENERAL NOTES . 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)

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.
See BLEEDING BRAKE SYSTEM .
Welding work with electric welding equipment can affect the ABS/EDL or ABS/EDL/ASR system.