ABS help - Odd issue

[Chrise30999]

Hey all,

wondering if anyone has had this ABS Issue before. Turn the ignition on and ABS light is on like normal, start the car and it goes out like normal. Rev the engine up in neutral and the ABS light will come on and stay on. Turn ignition off and start engine ABS light goes off again, rev it up again in neural and it will come on - and repeat.

As above if I start the engine the light goes out, if I start rolling with the clutch ABS light won’t come on at all and system works normally, works normally for the duration of the drive no issues. It’s only if I rev it up at idle.

has anyone experience this before?

I know the abs is simple - light comes on when moving then ABS sensor likely, light comes on and stays on then likely control unit/relay/wiring but it’s none of those.

1988 convert 325i

[Chrise30999]

Bump anyone. Here is some more information....

Did some more investigation and put a power supply/charger unit that I use for programming on the blue wire out of the back of the alternator, set to 13.8v, i disconnected the wire from the alternator and attached the power supply revved it up and its still the same, the abs light came on. I totally disconnected the alternator in the end inc large red wire and the abs light still came on when revved. So I think the alternator is ruled out.

So back to the same question why would revving the engine cause the ABS light to come on, the alternator blue wire is the only connection to the ABS regarding the engine so how else would it know its revving?

Again scenario is...

1) turn on ignition, ABS light is on as normal
2) start engine and ABS light goes out as normal, stays out if you leave engine running.
3) dont put it gear and rev the engine on the spot and the ABS light comes on and stays on till key is turned off.

The above can be repeated at will every time.

At step 3 if instead of revving you put in gear and pull off with the clutch/no revs and get the car moving the light wont come on, if you drive 20ft and then stop and put it in neutral and just rev the engine like in step 3 the light still does not come on so system operates as normal till car is turned off.

if you turn the car off and repeat 1 to 3 again the light comes on.

Any suggestions on what to check next? or what would cause it? The fact it can be repeated at will and manipulate to fault or not fault makes me think its not wiring.

Edit - I also checked the alternator output with an oscilloscope and all looked normal, no big dips or spikes, voltage was high at some points at around 14.8v. i also scoped the blue output wire and it looked normal although there was quite a bit of noise on that line.

[Chrise30999]

Bump

[rahhusseini@hotmail.com]

look to the wiring of right passenger abs sensor if is very close to main coil wire then it be under magnetic field . u need to push away the abs wire from coil (hight tension) wire . beware of magnetic field near all abs wire on any location. chek the abs motor wiring inside im the connector.