ABS woes

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Kitch
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ABS woes

Post by Kitch »

Never really had a problem with the ABS system before, so have never done any real diagnostics on it.
Bascially I changed a CV boot a while ago which was a cheaper ECP job and of course....it failed! The sensor & ring got covered in grease and the lamp came on. Been like that for a quite some time as I never got round to sorting it, possibly 8 months or so.
The other day, I finally took the shaft out, put a proper boot on it, cleaned the ring and sensor and refitted. Light then went out after doing its check.

But now the light comes on again after the first 5mins of driving. I can hear a fainted fluttering, like a relay coming from the glovebox area in the car, but it's very very faint. Then after a while, a louder click and the light comes back on.

I guess the ABS system has a relay somewhere, any ideas?
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Post by DLM »

Not very scientific, this, but I'll throw it in.

I heard similar "squeaky" noises from the central dash while travelling in Tom's car before he passed it on to me. We even had the light come on after a minor barrage of squawks/squeaks - he pulled the car over, switched off, restarted and the light went out for the rest of the (long-distance) journey (to France and back, as I remember - no further probs).

I had more problems on taking the car over - the ABS light stubbornly refused to go out on strart-up, and lots of tracing, subbing ABS ECU and checking all connections failed to cure it. Head-scratching and hair-pulling time as the MOT was very close. Eventually I dismantled to the point where I was looking at what the diags told me was the ABS diode - the source of that squeaky, chirpy noise, I think - which I found under the clock tray AFAIR. I disconnected it and reconnected it in desperation, then found that the light went out...

There does seem to be a relay s well - I think it's glove-box out to reach, and lives on the rhs of the cavity thus exposed.

However the real culprit was a faulty earth/connection on the ignition switch or its cable. After tie-wrapping the switch cable in place, I found I could get it into a stable position where the ABS light remained inert, so I left it tied-up there until I replaced the switch later. That switch/lead was also causing charging probs - the battery light glowed dimly at high speeds when travelling lights-on at night.
Back on two wheels and pedal power for the moment.
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Post by docchevron »

DLM wrote: Eventually I dismantled to the point where I was looking at what the diags told me was the ABS diode - the source of that squeaky, chirpy noise, I think - which I found under the clock tray AFAIR. I disconnected it and reconnected it in desperation, then found that the light went out...
I'd start with that.
BUT, the diode is not always under the clock tray!
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Post by Kitch »

I was worried it'd be something like that. Oh joy, electrics!

Thing is it has always behaved perfectly until the boot gave way and got the sensor greasy. Could running around with a greasy sensor and the light on for that period of time cause a bigger problem?
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Post by prm »

Found most ongoing ABS faults on 16v’s, eventually ended up with a faulty or intermittent wheel sensor, or coax wiring to these points. I wouldn’t have thought the grease from your boot failure caused the problem. Just coincidence, stretching the cables while jacked up?? Aggravated the problem. The sensors have magnetic heads; possibly grease on the tone wheels/sensors picking up crud??

Checking all the electrical multi-pin connections on the system/relays, helps to eliminate any possible causes. Especially the block connector on the main hydraulic valve block. Closing these female contacts and cable tying the harness to stop it flapping around.

Set the car to lowest ride height – flexes all the coax cables, check to see if the ABS light remains on, then switch off.
Track down the sensor connectors. Fronts, above the chassis box section where it joins the bulkhead. Rears, under the rear seats. Could be buried under the sound insulation. If you split the rears first, gives you an indication how these pull apart by depressing the two locking tabs.

Check the resistance from each sensor, should be in the region of 900-1250?.
If all is ok, reconnect sensors, remove ECU plug to check the continuity and resistance on each circuit with sensor connected.
RH front, pins 7 and 25, LH front, 5 and 23, RH rear, 4 and 22. LH rear 6 and 24. All should read around 900 - 1500?.
Looking at the plug, contacts facing you, longest row at the bottom with cable inlet to the right. Rows are 1-18 and 19-35
Pin 1, is bottom right. Pin 19 is top right.

I would go for ?? N/S/F sensor first as favourite then, at 2-1!!! rear coax cables.

Regards

Pete
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Post by Kitch »

Very informative there, thanks Pete. My only contribution to that is that the light came on the same day that I refitted the boot, having noticed that it was covered in CV grease. Having cleaned it, the light went out. It's only now that it's returned, so it's either a coincedence that something else has gone wrong, or something related to that sensor as I see it. I've never checked resistance before though, but I've got a multimeter in my garage. I assume you just unplug and hold it across the two pins?
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Post by prm »

When you separate the sensor connectors, they look like sophisticated TV ariel male and female plug and socket.
Set your test meter to a ? resistance setting, within a range of 0 to approx 2 or 20k?.

One test lead onto the centre, the other to the outer, which carries the screening cable, almost identical to TV cable configuration. The sensors are not polarity conscious, any way round, + or – test leads, will indicate the resistance measurement.
Same testing at the ECU plug. Apply any lead to the pair of numbered pins

The sensors are of similar construction to an audio magnetic microphone. This being the reason for the outer screening cable to avoid any signal interference from onboard electrics.

One pain in the Ass!! BX. Carried out all the checks at least three times. Fault would still crop up whilst driving – sometimes.
So plugged the sensors, one at a time, into an amplifier to monitor the audio output. The sound is similar to screeching radio interference.
Turned out to be an intermittent break in the O/S/R sensor centre wire.

If you got access to a cable tone tracer, which transmits a set audio signal along any cable run to a hand held receiver, to locate the exact point of any cable breaks.
You can repair cables by utilising flexible guitar type leads, rather than replacing complete units.

Regards
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Post by docchevron »

prm wrote:
I would go for ?? N/S/F sensor first as favourite then,
Thats interesting.
Every BX (and Xantia) ABS sensor fault I've ever come across (and it's aLOT) it's always the O/S/F sensor thats carcked it.
No idea why, but others have told me the same too!
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Post by DLM »

Tom gave me a set of BX ABS diagnostic sheets with Grolliffe, Kitch. Would they be any use to you? Easy to bring them round.
Back on two wheels and pedal power for the moment.
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Post by Jaba »

There is a comprehensive ABS fault finding guide in the BX DIY section that Jon put on in the club's early days. It was published originally in the CCC mag in 2002.
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