BX ABS problems

From 1987 an ATE ABS system was fitted to 16v and 1.9 GTi models, then also to Turbo D and TZI varients. Both the ABS ECU and wheel sensors were of ATE origin. As its difficult to buy ATE ABS components on the aftermarket, unfortunately this has meant that ABS sensors are "genuine Citroen". The good news is that its very rare for the ABS ECU to fail, unless its been shorted out, and that they are available as Exchange recon units from some aftermarket suppliers.
The main cause of problems causing the ABS warning light to illuminate are breaks in the cabling to the wheel sensors, dirty sensors and wheel hub rings. There is also a relay mounted in the dash (remove the glovebox and you'll see it above) but its fairly rare for this to give problems. Theres also an ATE valve block on the n/s inner wing that connects to the brake pipes, but I've never heard of these giving trouble.

If the ABS is lit, and does not go out, or it illuminates periodically when driving, the best plan is to make some simple tests that will cost nothing, and may put out the ABS light! Remember that if your BX has ABS, then the light must go out when the engine is started or its an MOT failure.

Jack up car, place on axle stands (2 under front subframe, 2 under rear beam) and remove wheels. At the front of the car, remove the ABS sensors (use 10mm square cut socket) and carefully clean the 2 contacts on the sensor end. Then, rotating the driveshaft clean the ABS rings with some brake cleaner and a wire brush. Refit the sensors, start the car and see if the ABS lamp goes out, you could be lucky. If not, you are going to need to test each sensor from the plug at the ABS ECU. Its located under the front passenger seat. You should remove the seat squab (2 metal clips, 2 rubber bands!) and there you will see the ABS ECU. With ignition off, carefully remove the multiplug. You will see that each pinout is numbered.

Using a multimeter, you need to test the resistance of each sensor, its should be a minimum of 800ohms.
To test the n/s/f sensor its pins 5 & 24, connect your meter between them. Then 7 & 25 for the o/s/f, 6 & 24 for the n/s rear and 4 & 22 for the o/s/r. Note any abnormally high or low readings for the suspected faulty sensor. If all appears OK, start the car up and have someone slowly turn the steering left to right whilst you test the front sensors again, there could be a break in one of the cables that only manifests itself when the steering is turned.

At the back of the car, forget trying to remove the sensors, they are always always corroded into the hub and have to be beaten out. All I can suggest here is that the ABS rings are cleaned and wire brushed, and copious amounts of brake cleaner are spayed as close to the sensor end as you can to clean it. Its also worth raising the rear seat squab and checking that the sensors are actually connected! (not uncommon for them to be undone for some reason).

If all the sensors check out OK (check and double check) you can also do some simple tests at the ECU plug. Pin 2 should give a 12v feed with the ignition on, if not, its likely that the relay behind the dash is faulty as this is the supply. Pin 1 is the ECU earth and pins 20 & 27 should go to the dash ABS warning lamp. Pin 8 (brown wire) is the Relay earth.

Final checks are that Fuse 14 in the main box is not blown/missing, and finally the feeds to the relay itself under the dash, the blue wire to the relay is the 12v feed from the fusebox.Terminal 4 (grey wire) is an earth.

Note: Full acknowledgement to David Woollard for part of the above, in particular the ECU pin numbers which I'd completely forgotten. Jon