Hello.
I have a problem with the ABS cutting in at low speed when only braking moderately. The car has had this problem since the two rear ABS sensors were replaced, after having been out of service for two months. I have tried to measure the output from each sensor, but they only give a reading of 0.1 to 0.3 mV at speeds between 10 and 30 mph, and the readings are inconsistent. This applies to both rears and the n/s front (I haven't tried to measure the o/s front).
The only possible fault I can see is that one of the new rear sensors isn't quite fitted flush with the face of the hub casting. It looks like there could have been some dirt there when it was fitted, and I can get a 0.015" feeler in between it and the hub casting. Unfortunately I can't get the sensor out. It's in really tight and I am frightened of breaking it. Before I go back (again) to the garage that fitted the new sensors, does anyone know if a small increase in the distance between the sensor and the toothed gear would cause the ABS to trigger at low speed?
Edited by Webmaster on 21/03/2008 at 21:29
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why were the sensors replaced in the first place?
My first guess would be an issue with the reluctor rings. Either a crack or rust building up.
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The air gap between the sensor and the reluctor pick up ring is indeed critical.An excessive gap would certainly give problems especially at low speeds.Other possible faults are worn/corroded or broken reluctor rings on the hub.It is possible that when the new sensors were fitted ,the location holes in the hubs were not properly cleaned or prepared.Removing the sensors can be a real pain and you do run the risk of damage as they are quite fragile.Btw when doing a voltage output test on these the meter needs to be set at a/c volts.hth
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Many thanks for the replies.
The sensors were changed because they were damaged by a garage fitting new rear springs for me. They ordered and fitted a new sensor, then found it was the other one that needed renewing, and with some complications that took two months. When I noticed the problem with the ABS the garage ran a check and said that the new sensors weren't the problem. So I'm trying to find out if they are.
I have been trying to get a look at the reluctor rings, but can't get the probes out, and to dismantle the hub on the 156 requires the outer part of the bearing to be removed with a chisel, and I really don't fancy doing that.
Thanks for the tip on setting the meter to ac. I wasn't sure, but assumed as there was no sign of a rectifier in the circuit that ac was the setting to use. I'm trying to get hold of some connector plugs from someone who's breaking a 156 to be able to make a better connection to test again with a meter. What sort of voltage should I be seeing? Or would it be better to get hold of a battery powered oscilloscope?
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I had a similar proiblem on my 1999 Clio that was traced to a cracked reluctor ring on the hub. Hope this helps. P.S. The voltage output from the srensors will be a pulsed DC waveform i.e. on and off or high and low, as the sensor passes over the teeth of the relutor ring. It cannot be AC as there is no reversal in polarity.
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Thanks again for the replies chaps.
I have carried out some more tests and still can't determine what's at fault. I've tried measuring the output from each sensor and have found that you have to have the sensors connected to the wiring loom plugs to get any output. I then connected an oscilloscope in parallel with each sensor, and turned the road wheel slowly. I got the same waveform from all four sensors, showing nearly 1V peak to peak, and around 60 events per wheel revolution. Having drawn a blank here I tried to find out which sensor was causing the ABS to cut-in by driving the car with different sensors connected to the near side front wheel's connection to the ABS unit. I tried braking hard with everything connected as normal, and the car pulled up perfectly straight with hands off the steering wheel. I then swapped over the ABS sensors for the front wheels and still the car braked perfectly straight, with the ABS pumping all of the time. Therefore I concluded that it couldn't be one of the front ABS sensors at fault, otherwise the car would have pulled to one side under heavy braking with the ABS cutting in on one wheel. I then tried connecting each of the rear brake sensors in turn to the near side front wheel's circuit, and still the car pulled up straight when braking. So there doesn't appear to be a problem with any individual sensor or reluctor ring.
Any ideas anyone please?
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