"A" -for "ABS System" by Ian Hopley
The 164 ABS system fitted to the pre-1992 cars is probably similar (if not identical) to all other 164s but this particular story relates to a 1990 3 litre 12 valve V6.
As each of the wheels turn, a sensor on each hub generates a small AC voltage as the raised portions of the big "spline" round the outside of each stub axle pass the sensor located on the hub. There is only a very small gap between the sensor and the teeth on the spline - quoted as being between 0.25mm and 0.75mm in the manual, although the closer it is, the better the signal.
The ABS computer
checks to see that it is receiving a signal from each sensor as the car moves.
If one or more of the sensors are NOT sending a signal AND the brake pedal is
depressed, those wheels are thought to have locked and the ABS pump cuts in
and releases the pressure on that (those) wheels and then re-applies it very
rapidly so as to simulate the "cadence" braking technique that ABS cars mimic.
If it stops seeing a signal from a sensor and the brake pedal is NOT depressed, it knows there is a fault and puts the light on. This isn't the only thing that can go wrong with an ABS system - far from it, in fact, but it is probably the most likely thing because the wheel sensors live in appalling conditions getting water, mud, salt and stones thrown up at them all the time.
A few days before the MOT was due, the ABS light came on once the car was moving at more than (say) 5 MPH. Starting the engine gave all the usual signals, ABS light stayed on until the alternator light went out, whereupon it too went out, but as soon as the car started moving and the computer realised that one of the wheel sensors wasn't working, the light came back on and stayed on.
What follows is an outline of the tests I carried out to pinpoint the fault. Obviously, there's plenty of other things that can go wrong with an ABS system and it is a highly safety critical part of the car so if you are in any doubt as to what you are doing, please go to a dealer or at least someone who know what they are doing! That said, taking an elderly 164 to a dealer for ABS work can also have adverse effects on your health and well being - usually when you get the bill so if you want to have a go, read on.
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by Ian Hopley.
© Ian Hopley / alfisti.co.uk 2002