Diagnosing ABS problem without diagnostics

[jeffbebe 1559]

The ABS has given up on my 1991 850i however the dash check light is not lit (bulb is not blown as it lights at start up). Unfortunately, I have not been able to (actually Josh hasn't been able to) scan the car and nor have a number of experienced BMW service agents locally. Josh is determined to solve the diagnostics issue but I'm wondering what manual ways I can easily test what's wrong with the ABS. If it was the pump or control unit I'd expect the dash light to trigger... Would I be right?

I can probably get a working control unit so can test that easily but don't want to start switch pumps unless I know it's the root problem.

Thanks in advance internet people.

[will 169]

What pump unit and ecu (part numbers) does your car have Jules? Car have ASC or ASC+T?

[jeffbebe 1559]

Hmmmnnn... Realoem isn't very clear, Will, as it lists both options for my VIN (though normal ASC seems to have been superseded by ASC+T), so I'll have to have a look and try and get the actual part numbers.

[will 169]

Do a basic ecu check, unplug one of the wheel sensors and see if it faults when you start driving. If it does, then I would think it is ok and the fault likely is with the hydraulic unit. There are 2 relays on the unit, one a standard 85-86-87-30 and the other a more specialised one, but which is found in most hydraulic units of that era eg e32/34, so sourcing one should not be a problem. The relays are easily tested to see if they are functioning properly.

The hydraulic unit (asc) is the same as found in the e32 740i but the ecu is different. 750i e32 asc/ecu is compatible. IIRC, Darren (SRBMW) wrecked a 750i (e32/e38?) some time ago, he may still have the ABS/ASC bits around as all he wanted it for was the motor for one of his projects..

[jeffbebe 1559]

Thanks Will. I'll see if I can suss it out over the weekend. Have been trying to listen out when I start the car to see if the pump is even engaging but can't hear it over the bloody exhaust noise! hah