P38 ABS Fault and Sporadic Speedo Reading

Just taken the p38 out after doing a heap of work on it. Basically all the half shafts have been out and just about everything between those and the wheel has been replaced, including the CV joints and ABS rings. Everything to do with the brakes after the hard brake lines has been replaced.

Just gone for a spin in it this morning and I notice the ABS light is stuck on and the speedo is giving a sporadic reading, and I have not got anything here to read codes.

Before I start making assumptions about ABS sensor faults does anyone else have any ideas? The car drives nicely. Brakes work fine. No steering wobbles or anything like that.

The ABS sensors provide the speed info, I would guess that one is not pushed fully home.

Datatek said:

The ABS sensors provide the speed info, I would guess that one is not pushed fully home.

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+1. Sensor not in or a wire damaged. If a tooth on the reluctor ring is rusty or broken that would cause odd readings too.

The speedo is based on the average of the 4 reaadings from the ABS sensors.

Thanks, ill go and have a fiddle with 'em.

You guys knowledge is astonishing sometimes. Its all as right as rain now, thank you.

It is showing an airbag fault though, which I am about to google. Unless anyone would care to further share their knowledge.

Also, if your alternator is about to turn its toes up the speedo will work/not-work and give silly readings.
Happened to me on my T-reg 4.6 HSE.
Alternator then failed totally and fitting a new one fixed all the problems.

Just a thought for you.

jimllshiftit said:

You guys knowledge is astonishing sometimes. Its all as right as rain now, thank you.

It is showing an airbag fault though, which I am about to google. Unless anyone would care to further share their knowledge.

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If you have airbags in the seat, it could be the connector under the seat (the yellow cable), Don't unplug it with ignition on though.

jimllshiftit said:

You guys knowledge is astonishing sometimes. Its all as right as rain now, thank you.

It is showing an airbag fault though, which I am about to google. Unless anyone would care to further share their knowledge.

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If the car has side airbags, the under seat connector is likely to be the problem. I got rid of the side airbags on mine and fitted a resistor to fool the ECU.

Sadly its not as simple as the yellow plug under the seats. Originally the car had manual cloth interior with a low line control module, but some years ago I fitted a 2002 electric leather interior and a high line body control module but the yellow SRS plugs arent there to plug in. Never had an issue with SRS faults until now, and that was probably 10 years ago I fitted those seats. Where else could the trouble be?

jimllshiftit said:

Sadly its not as simple as the yellow plug under the seats. Originally the car had manual cloth interior with a low line control module, but some years ago I fitted a 2002 electric leather interior and a high line body control module but the yellow SRS plugs arent there to plug in. Never had an issue with SRS faults until now, and that was probably 10 years ago I fitted those seats. Where else could the trouble be?

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The rotary coupler is another possibility.

Are we talking srs, could also be a dead bulb in the dash!
Have your battery capacity tested.
Also as said à failing alternator can give abs and airbag ghost issues.
Check the output from alternator.
Back of the alt to engine current the alt to chassis and then battery they should all be with very similar readings.
If not there's an issue between live supply and or earthing.
Diagnostics will show if there's an srs fault warning.

Thanks. The alternator was fitted a few months ago as a brand new part. Not that i'm doubting what you say but that must be something to consider. The SRS light staying on has also been an intermittent fault over the last few years. Often the SRS light comes on after the car has been standing for a little while and that was both on the old alternator and the new one. I wonder if the wiring to the alternator is OK.

jimllshiftit said:

Thanks. The alternator was fitted a few months ago as a brand new part. Not that i'm doubting what you say but that must be something to consider. The SRS light staying on has also been an intermittent fault over the last few years. Often the SRS light comes on after the car has been standing for a little while and that was both on the old alternator and the new one. I wonder if the wiring to the alternator is OK.

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A bad earth from engine to chassis is a common problem. What voltage do you have at the battery with the engine idling? Your new alternator may be only producing 13.8 volts which is not enough for a lead calcium battery, you need to see around 14.7 volts.
You can check the engine to chassis earth by putting a jump lead from the engine to the battery negative and see if you get more volts.

Mark Piercy said:

Are we talking srs, could also be a dead bulb in the dash!
Have your battery capacity tested.
Also as said à failing alternator can give abs and airbag ghost issues.
Check the output from alternator.
Back of the alt to engine current the alt to chassis and then battery they should all be with very similar readings.
If not there's an issue between live supply and or earthing.
Diagnostics will show if there's an srs fault warning.

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+1. 2 bulbs in the dash and if either the main or backup gets a bad connection to the circuit board or blows it can cause trouble. Sometimes needs clearing with diag.

Well. On Mark and DTs advice, Ive just gone out armed with my multimeter only to find that the battery is stone dead despite a 10 hour charge only yesterday and having the car running perfectly after that. Does anyone else thing the SRS issue and a battery parasite might be linked?

jimllshiftit said:

Well. On Mark and DTs advice, Ive just gone out armed with my multimeter only to find that the battery is stone dead despite a 10 hour charge only yesterday and having the car running perfectly after that. Does anyone else thing the SRS issue and a battery parasite might be linked?

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Could well be but you need to do a drain test. What RF setup have you got?

Get the battery tested, it could simply be that

Forget the battery drain idea, its not an issue. I was rushing last night, tired and drained after a very bad day and just hadnt noticed the car wasn't in "park" when I was turning the key. So the battery is fine. This morning, after reading 14.5 volts at the battery with the engine idling and wiggling the wires around the battery and starter motor the SRS light has gone out, so now its MOT time.

No doubt the light will come back on in future but at least i'm beginning to home in on the problem now. Will check DTs jump lead suggestion later. Thanks.

Check both dash bulbs are working, failing that there's a connector under the covers by your right knee that can go wonky

Phantom SRS faults occur with low battery volts.