Teves ABS Fault Finding
The ABS warning light comes on and stays on. Often it starts as an intermittent fault, with the light coming on while you're driving and then going off (for a while) next time the car is used.
ABS ECU BehaviourThe ABS ECU does a self-test every time the ignition is switched on, but only tests the wheel sensors once the road speed reaches about 20 kph. If the self-test is OK, the warning light stays (On or Off) as it was when the engine was last switched off, so the light should go out. If the light stays on until the road speed reaches 20 kph, then you have an intermittent sensor fault. Typically this will cause the light to come on randomly, stay on until the ignition is turned off then often go out once the road speed reaches 20 kph.
When the warning light is on, ABS is disabled!
Ideally a diagnostic tool such as a Lexia is needed for diagnosis of an ABS fault. But 90% of the time, the fault can be accurately found by doing what too many workshops fail to do - check the simple things first:
- 1) ECU Multiplug connections
The ABS ECU is located just forward of the LHM reservoir. Make sure the connections in both plug and socket are clean and corrosion-free.
- 2) Test the ABS sensors
A faulty ABS sensor is perhaps the most common cause of ABS faults.
While the ECU connector is unplugged, measure the resistance of each wheel sensor with a digital multi-meter (DMM) set to its 2000 (2K) Ω range; each sensor should read about 1080Ω, usually rather more because of the mid-point connector. The diagrams below show the pin-outs of the ABS ECU multi-plug. Do not test the ECU socket! If you spin the corresponding road wheel fairly slowly, you should see pulses on a meter (set to AC Volts, 2V or 2000mV range). For some reason, the O/S/F sensor seems to be more prone to faults.Up to Jan 1998 (looking at the plug):
From Jan 1998 (plug):
- 3) Check the connector
Once you've identified the 'faulty' sensor as above, trace the cable back from the sensor. The front connectors are easy to get at. The rears are pigs – clipped to the top of the rear cross-member where you can’t see them. Even worse to put back in place!
Securely support the front of the car on axle stands. Remove the corresponding front road-wheel. Leave the suspension in Service HIGH.
- 1) Double-check
There is a connector just inside the engine bay (usually covered in oily filth). Trace the cable back from the sensor to find this connector - don't confuse it with the pad wear sensor cable! Wash/wipe the filth off with WD40 and separate the connector. Now test the resistance of the sensor from this connector. It will probably be open-circuit; if not, flex the sensor cable to make sure there isn't an intermittent fault. The fault is usually in the sensor or its integral cable (amounts to the same thing).
- 2) Removal
The front sensors are easy to remove. Use PlusGas on the (10mm) sensor retaining set-screw and the (13mm) set-screw that holds both the heat-shield and the hand-brake cable guide onto the hub. Remove the two bolts in that order. Gently pull the sensor cable out of the holding clips.
- 3) Replacement
Put the new sensor in place and do up its retaining set-screw finger-tight. Route the cable back through the retaining clips (WD40 will help ease the grommets into place). Make sure the loops of cable allow free movement of the cable when the steering moves from lock to lock - you may have to move one or more grommets along the cable. Replace heat shield, hand-brake cable guide and securing bolt.
Securely support the rear of the car on axle stands. Remove the corresponding rear road-wheel. It helps to improve access if the suspension is put in LOW (the front of the car drops, the rear rises). It’s also a good idea to disconnect the battery. Underneath the car, soak the disc shield's three retaining bolts with PlusGas or similar penetrating fluid (not WD40 or similar de-watering fluid!)
Disconnect the sensor from the loom. Access is poor from the wheel hub, the best way to do this is from beneath the car. For the N/S/R sensor, best access is next to the exhaust, using a mirror and a long nose pair of pliers. For the O/S/R it may be easier to remove the spare wheel and carrier. The plug is held on by a U shaped clip which needs to be taken off before splitting the sensor wire from the loom. Carefully remove the disc shield's three retaining bolts. Be patient with these and let the PlusGas work, if you shear any of them, it's a whole load of extra work to remove the broken stub and fit a new bolt.
You should now see a roughly oval shaped rusty piece of metal with an ABS sensor going through it. It should be relatively easy to remove the sensor holding bolt, but removing the actual sensor will be a problem as it corrodes itself into place. The best solution is to neatly hack-saw a radial slot through the thinnest part of the swinging arm so that the sensor housing (in the swinging arm) is converted from an 'O' shape to a 'C' shape similar to the front sensor housings. Don't worry about damaging the sensor (it's scrap anyway), and squirt in some PlusGas if the sensor is still stuck. With a sharp file, clean-up and open the slot so that you can get a big screw driver blade into it.
Now remove the sensor cable. Push the grommet out of its hole and feed the wire down around the trailing arm - easier said than done. Failing that just snip it, but remember the route of the wire.
- 2)Potential Problems
Sheared brake disc shield bolts: If you do break one, the best way to drill these out is to unscrew the brake calliper and remove the brake disc. There shouldn't be a need to disconnect the pipe unions if the weight of the calliper is taken up with string or wire to a secure point. Two of the sheared bolts can then be 'punched' and drilled out. One of the bolts cannot be drilled out as the hub is in the way, access is easy from underneath the car though. If you destroy the thread, you may have to clear drill and use nuts and bolts the right size from stock.
Rusty ABS shield: The shield performs the essential function of protecting the sensor and cable from red hot disc/pad debris off . While everything is off it would be wise to replace this with a new part from Citroen, if available. If you're handy with metalwork, its easy enough to make up a pattern part with the old one as a guide.
- 3)Refitting the new sensor
Make sure the sensor fitting hole is clean and smooth; otherwise fitting the new sensor will be difficult. Just use some wet-and-dry paper to remove any rust and clean it up. Put the heat shield on and loosely fit the new sensor in the hole; screw the securing bolt in a few turns to align the sensor and prevent it being damaged by the ABS cog teeth. It is a tight fit for such a delicate part so use grease and tap it in very gently, winding in the securing bolt as you go. Tighten the bolt to 8nm.
Clip and feed the sensor wire in around its path and feed a piece of string just above the trailing arm but below the grommet hole around the trailing arm and tie it to the connector plug on the new sensor. Pull on the string carefully to bring the connector plug up around the trailing arm and if you're lucky, through the grommet hole. Pull the connector as far as it will go with the string, then pull the sensor wire back a bit to ease the connector through. The hardest bit is getting the grommet back into place. Get the grommet above its hole and press it down. With the grommet above its hole, there is more wire to plug the connector into the loom. To actually get the grommet parallel to the hole is a challenge as the trailing arm is in the way. Jack up the trailing arm with a trolley jack, and manoeuvre the grommet as close as possible to the hole; the grommet will now probably be at an angle to the hole. Let the trailing arm down slowly and it should 'grab' the grommet and push it level to its hole. Push the grommet up using a long implement such as a long nose pair of pliers from underneath the trailing arm housing, if you can push the grommet into its hole, great. If not, push it right up through and use the extra length to reconnect the sensor to the loom. Again, its easier to re-plug it underneath the car, pull the connector towards you using the string and put the clip back on. You can then put the grommet into its hole. Then its just a case of refitting everything back together!