944 T ABS self test failure

[928NZ]

Hi Guys

I have an on going problem with my 1990 Porsche 944 Turbo ABS system.

I am selling the car and would like to fix the problem before it goes to a new owner.

ABS sets up correctly on start up –

I move forward and it fails the self test at 4 mph.

I have rigged a switch to the fuse and can turn the ABS on or off.

I turn it off – accelerated hard and switch it back on – ABS sets up beautifully.
It will not reset at anything but hard acceleration.

Until the next time I stop where it fails the next self test when starting off.
It sounds like a bad earth problem which is overcome when I accelerate hard.

Checked and re cleaned every earth I could find

Perhaps it is caused by one of the two relays on the ABS pump?

I booked it into the Porsche dealer a couple of years ago and they said that it was the right rear ABS cable, so I ordered one but the ABS failed immediately on start up.

Funny thing was when I collected the car it setup beautifully and only failed again when I was a mile or two away.

Hoping someone has a clue.

[thomasryan]

have you cleaned up the sensors at each wheel? they get kinda nasty with brake dust...I have never seen a clean one...

[928NZ]

I have removed the sensors - Cleaned them and the contact points where they plug in.

[thomasryan]

the teeth on the stub axle may be dirty causing the signal to be erratic at low speed....be nice to see the individual waveforms.

[928NZ]

Cleaned the teeth as well.
Strange that it will only reset the ABS at fast acceleration, and will not reset up at even moderate to high revs.
Another strange fact - When the car is on a vehicle hoist with the wheels in the air I can put it into gear and the ABS will not fail at any speed?

[Jfrahm]

Have you checked the AC voltage from each wheel at the brain while moving? That will show which corner is bad.

[928NZ]

Good idea - I will do that this weekend - Just have to figure out how to spin the front wheels.
I have a car hoist at home.

[Jfrahm]

I just hooked up a multimeter and drove around at very low speeds.

[928NZ]

I will try to figure that out.
Wow!! a 94 968 Cabrio supercharged!

[924RACR]

No, it's not the sensors - those will fault at any speed, as the monitoring and plausibility algorithms are running continuously.

The "self test" is indeed what's catching you, as you've already isolated; this is where the ECU runs the pump and fires all the valves. So most likely yes, you either have a bad relay or pump motor.

Have you checked the relays on the bench?

[Jfrahm]

It can fail when it attempts to cycle the pump but can also fail if it does not receive a speed signal (AC voltage) on all 4 sensors after you start moving.

[924RACR]

But that relates to the speed of failure, not to mention the conductivity tests. Also, if it failed to get those signals, you'd have the same result (faulting out) when restarting at speed.

Conversely, the pump motor and relay tests are NOT run at speed, only on the one drive-off cycle.

So no, still doesn't sound like wheel speed sensor.

[944 Ecology]

Here is an interesting DIY for rebuilding the ABS Pump on 944 cars:

Porsche 968 ABS refurb.

[924RACR]

Cool, but good luck if you try interchanging the ECUs... just 'cause the hardware's the same, doesn't mean the software is...

[928NZ]

Great advice and I loved that ABS pump refurb link.
I may try and test those two relays on the pump or just order a couple of new ones as I have been having relay problems recently.

[928NZ]

Turned out to be a faulty ABS computer.
Sent in the ABS computer from my 1990 944 T and my wifes 1989 944 2.7
both did not work for different reasons. Purchased 2 computers from Larts on Ebay.
Problems solved.

[928NZ]

The ABS Failure on the 944 Turbo turned out to be the ABS controller / computer.
When I replaced the computer it fixed the problem.