Unusual ABS fault

[martinwiseman2000]

Hi,

I have an ABS fault on my W reg Xantia which seems to be a bit different from the usual faulty sensor type problem - it is modulating the brakes at low speed under light braking on dry roads.

For a couple of years now the car has had a habit of putting the ABS light on when driven really fast (enthusiastic motorway speeds). The light would then stay on for the rest of the journey but go out after cycling the ignition off and then driving the car a few yards. The fault is temperature dependant in that it occurred at a lower speed in the summer when the ambient temperature was warmer than it did in the winter. Gradually over the two years it has started happening at lower speeds so that up until a week ago on a warm day it would trigger at 80ish. Other than this the ABS worked fine.

In the last week as the weather in the UK has turned hot a new fault has appeared - the ABS will kick in at speeds below 15mph under light breaking on dry roads. It now does this every time - as I brake down to below 15mph the ABS starts pulsing the brakes. The warning light does not come on. Giving that the car is lightly loaded and the braking is light it must be the front brakes which are being modulated as the effect is quite severe.

I have investigated the sensors and founds resistances of 1128 ohms, 1122 ohms, 1127 ohms and 1106 ohms - the low one is the front right. In addition the back left sensor has a slight resistance to chassis - around 10 megohms.

Has anyone every seen anything like this before and does anyone think those figures from the sensors at all significant. The low sensor is less than 2% lower than the highest - their resistance will vary by more than this from hot to cold climate, and 10 megohms should be far too high to affect any sensibly designed automotive electronics.

Any suggestions? I'd rather not go to the hassle of replacing two sensors only to discover that it's actually the ECU at fault.

Thank,
Martin.

[Peter.N.]

Hi Martin

Electrically that sounds fine, not even a high resistance plug connection With those sort of resistances a 10m leak is not going to have much effect. I would think that the fault is probably mechanical, I wonder if you have an accumulation of crud in the rings which is causing an erratic signal. It doesn't take much to upset the ECU, one of mine starts up with the light on about 50% of the time but if you switch it of and then on again it goes out, sometimes it will stay on if I reverse out instead of going forwards.

Peter

[DickieG]

I have an ABS fault on my W reg Xantia which seems to be a bit different from the usual faulty sensor type problem - it is modulating the brakes at low speed under light braking on dry roads.

In the last week as the weather in the UK has turned hot a new fault has appeared - the ABS will kick in at speeds below 15mph under light breaking on dry roads. It now does this every time - as I brake down to below 15mph the ABS starts pulsing the brakes. The warning light does not come on. Giving that the car is lightly loaded and the braking is light it must be the front brakes which are being modulated as the effect is quite severe.

Any suggestions?

I've had this on one of my Xantia's, it was down to the air-gap being too wide on a front sensor, the heatshield was placed below rather than on top top of the sensor. I traced the fault by using a code reader to observe the speed being recorded by each sensor as the car was moving, the faulty sensor failed to record a speed once the road speed dropped below 10 KPH (IIRC) which caused the ABS to kick in.

[wheeler]

Have you checked all the pickup rings are secure & the teeth are all in tact ?