Power Supply

Fig. 1 ABS Control Unit Connector Terminal Identification:

1. Check battery for full charge.
2. Check that fuses 3, 12 and anti-lock brake power supply relay fuses are satisfactory. The anti-lock brake power supply relay fuse is located on top of the power supply relay.
3. Check for continuity between ABS control unit connector terminal 13 and ground, at battery ground cable. Fig. 1.
4. Turn ignition switch to the Off position.
5. Check for continuity between ABS control unit connector terminal 10 and ground.
6. Check for battery voltage between ABS control unit plug terminals 1 and 10. Approximate battery voltage should be read.
7. If approximate battery voltage is not read, remove anti-lock power supply relay.

Fig. 2 ABS Power Supply Relay Socket, Front View:

Fig. 3 ABS Power Supply Relay Socket, Rear View:

8. Check for battery voltage between contacts 2 and 4, then 4 and 5. Figs. 2 and 3.
9. If approximate battery voltage is not read, repair circuit for open or short.
10. Connect voltmeter between contacts 8 and 4, then press and hold anti-lock switch.
11. If approximate battery voltage is not read, check wiring. If wiring is satisfactory, replace ABS switch, then recheck.
12. On Quattro models only, connect voltmeter between contacts 1 and 5 of power supply relay socket.
13. Lock rear differential. Approximate battery voltage should be read.
14. Unlock rear differential, voltage reading should be 0.0.
15. If readings are not satisfactory, repair open or short in wiring, or replace differential lock switch.
16. Install ABS power supply relay.
17. Check for voltage between contacts 4 and 6, on back of relay socket. Reading should be approximate battery voltage.
18. If reading is not approximate battery voltage, replace ABS power supply relay, then recheck.