Help Please - Electrical gremlin?

Hi Folks.

Can anyone help? After driving for a while - time depends on temperature - the ABS light comes on and when I touch the brakes the taco and speedo both drop to zero. Release brakes and they return to normal position for the speed I'm doing.

Clearly it's a load on the circuitry somewhere. When it first happened I had all the symptoms of a failing battery. Dash lights dimmed and she wouldn't crank over. Even a starter pack didn't have enough energy to start the engine. As the battery was several years old - and a calcium one which in my experience fail without much warning - I have replaced it with a new unit. Checked all the wiring and couldn't see any apparent problems. Fuses are all intact also.

I believe it's temperature related as it happens after driving for a while. Switch off and leave the car to cool down and it will start and run ok until it gets - or should I say something gets - warm. Then it's ABS light on etc.

Has anyone experienced this?

Regards John M.

Hi, it does sound like you have a fault in the ABS system somewhere. I would first get a spare ABS sensor and just plug it in to each wheel in turn to eliminate the sensors first. No need to fit to the wheel, just buy a secondhand spare and plug in to the front and rear one by one.
The connectors are easy to find, they are each close to the top nut of the shock absorbers.. You can get a second hand sensor for less than a tenner.
If this fails, then it could be the ABS modulator, something you cannot swap easily as it would need a full bleed of the brakes.
Shorts usually come by the way of damaged cables, but as you say the fault does not occur until the system has warmed up, then this may not be the case, still may be worth looking for damaged wiring especially close to parts of the car that warm up, such as the engine, exhaust and radiator.
The ABS self checks itself on start up before cancelling the light when the system is happy. If you turn off the ignition and then restart the car after the fault has occurred, does the light stay on, or does it extinguish for a while?
Looking at the wiring diagram (which can be downloaded from the community section) the ABS is pretty much a self contained circuit, with the only external connections a wire connection from the ECU and a signal wire going to the instrument cluster (for the warning lamp).
I would download the diagram and check the component wiring carefully, especially the brake pedal switch wire (green and blue).
But again, I think I still would be testing the wheel sensors one at a time, then probably I would be looking at the modulator unit which is under the bonnet, opposite to the battery.
One final check is a diagnostic plug in which may offer a pointer, as the ABS is connected to the diagnostic socket, but a cheap code reader may not help here, you may need a more dedicated diagnostic unit, such as a Pscan. Some of theguys on here who have one may be better informed than myself to advise on that one!
Good luck with the fault and keep us all posted.
Oh, I get my spare second hand ABS sensors from MGFnTFbitz. They only charge a fiver for them, but if it is one of them that is at fault, there are two types, depending on which wheel they come off, so may be worth getting each one in. I have one of each as a spare for what they cost.

Sundance

You don't say which version you have, there are differences between the mk1 F and the mk2 F/TF.

The fault sounds like it is around the ABS ECU and the way it receives and process the speed signal.

A www.pscan.eu will read the ABS ECU, I have one in West Sussex and could do a quick scan for you if you can get to me.

I'd be looking for a bad earth, too many "other things" are affected.
The speedo and tacho could be earthing through the tail light circuit, this is then disturbed when the brake lights are activated.
A bad earth could also confuse the ABS.
The earth could fail due to thermal expansion, coming good again as things cool.
This could also be a problem on a printed circuit of a control module, perhaps a "dry" soldered component.

I have this problem on my Ford Falcon ute, there is a fault in the printed circuit of the body module, that sometimes prevents the starter motor from doing its thing.
It only plays up on hot days or when the heater is set to blow into the footwells, the module is a pain to get at, so I have a switch wired in to bypass it when trouble lurks.

Hi Folks

Sorry, it’s a 2005 TF. Have now definitely established its heat related. When cool everything behaves as it should. ABS light goes out after “self-test” and car runs normally. Depending on ambient temperature and length of run, then the gremlins appear 🤨

Thanks for the Pscan offer but I’m in France at the mo. Yes, we’ve had some very hot days of 40C+ but last year she went around the Grand Tour doing 11 countries through Europe in temps up to 46C without any issues at all.

All earths checked and are confirmed solid. I’ve re-done the boot lid wiring in the past because that does separate due to the loom being clipped too tight to the boot lid, so I know that’s good.

I’ll get some spare wheel sensors sent over and try that next.

Local Auto garage tells me it’s related to the head gasket. “Didn’t I know these K series are prone to failure”?

Guess who isn’t getting my car in his garage😧

If its definately ok when cold, I would not expect the ABS sensors to be at fault. They either work or dont...I would be looking at possible electronic failure in maybe the ABS modulator control circuit or maybe in the ECU. Tricky to put your finger exactly on this one without swapping parts bit by bit until you find out the fault.
As an industrial electrician, I have come across circuit boards that have failed under heat, although the excessive heat required would not be prevalent in your TF. Sometimes power transitor junctions can break down under heat, the other favourite component can be the electrolytic capacitor which do fail in time.
I think the best way to go is to get a Pscan on the car when the fault is present, this may shine a light on where the fault may be.
The only other thing I can think of is to have a run to a local MGF or TF second hand parts supplier and see if they can help by swapping over parts from their stock until you find the gremlin.

Sundance

Thanks Sundance for your thoughts. Sadly, I don’t have a MG garage/parts anywhere near me. Can the ABS control unit be swapped without changing the whole ABS?

I presume if I change the ECU then all the locks and keys have to be changed too.

Have managed to borrow an OBD 2 scanner. It’s not a Pscan, but is a rather expensive German unit. So, today it’s drive around until I run over a gremlin and see what happens. With the car cold, all readings are “normal” and there are no stored errors. This scanner does allow for monitoring whilst driving so hopefully I might know more in an hour or so.

Cheers. John M

Hi Folks. Many thanks to all those who gave suggestions and ideas. It turned out there were two problems. Firstly, the battery - a modern “calcium” crap one - had a duff cell. The other main problem was a failing alternator. Now before everyone goes “what!” How can that cause the ABS light to come on? Can I quote that excellent manual by Roger Parker on the F and TF ..... page 184 ... he says the ignition warning light is usually very reliable but,,,, certain conditions will cause the ABS warning light to come on, the instrument lights to dim and the engine to cut out when the indicators are operated, etc., etc..

Only wish I’d RTFM’d it would’ve saved hours of diagnostics 😩

The cause - alternator failure! 😱. Turned out this was because the previous owner had removed the protective cover that stopped the sh1t getting in and wrecking the alternator. As for the battery, for what it’s worth I’ve had trouble with these calcium crap units before. Unlike good old lead acid - shock horror from the green brigade - when they fail they just fall over. No warning, no sluggish turning over, just dead and you are stranded.

One final point. If you are experiencing electrical issues and are more than a few miles from home, remove the 40A EPAS fuse. Ok the steering will be a bit heavier but you’ll get home. Otherwise the PAS will flatten what is left of your battery in no time.

My grateful thanks to Rimmer Bros, who discussed the problem with me - they’d never come across it either - supplied me with an updated replacement alternator plus belts, fixings etc., in just two days delivery to France for only £12-50p carriage. Brilliant service.

Cheers. John M