944 turbo ABS Questions
 

For some unknown reason, my ABS system quit. The master warning light on the dash is lit, and so is the ABS orange light on the dash.

I purchased a used ABS controller (assuming it's good), and no change. ABS still "lit" and no abs.

I checked the relay in the fuse box, opened it up and tested it, and it seems to be fine.

According to the manuals, there are relays on the ABS Controller in the wheel well.

Can you get to these relays w/o removing the ABS controller (and the associated brake lines)?

Would a wheel sensor cause this?

What else could cause this?

Thanks,

Tony Garcia

Tony, the ABS brain needs inputs from the wheel speed sensors to compare to vehicle speed.

The fronts are more likely to cause a problem, dust and debris collect behind the rotor and can affect the reliability of the signal due to a cruddy sensor, toothed wheel, or metal worn from rotors that attracted to the magnet.
Clean everything there with compressed air and give that a try first, then insert one "known good" WSS into the system and move it around 'til you find the culprit.
Don't know the electrical test procedure for a static test, but each sensor should have an uninterupted voltage output that's speed variable.

Good luck.......

I understand about the wheel sensors... but the ABS light should go out when you first start the car, therefore, the sensors aren't detecting any speed let alone a speed difference.

Would a bad connection to a speed sensor on a wheel cause the ABS light not to go out when you start the car?

The ABS system runs self diagnostic when you start your car.So if you have a bad connection to a speed sensor it can cause this problem.

Olli
'88 951

The ABS idiot light will stay lit if the unit fails the self-test at start up. It will also stay lit if you interrupt power to the electrical system in various ways I don't quite understand but have managed to do several times.

If you have a non-968 model you can reset the light. Check the archives ... I wrote up the reset procedure. Not too hard.

Thanks for the info..

I've searched the archives but have been unable to find the abs reset procedure, although I've found numerious air bag reset procedures (that just doesn't help here though :-)

If you could dig it up and repost that would be much appreciated.

Thanks in advance,

TonyG

Oh Boy, do I feel stoopit! I was talking about the airbags rather than your ABS. Sorry.

I don't think the ABS self tests until you roll the car so it can check relative wheel speeds. If it doesn't pass the system shuts down. You can actually hear the test - sort of a whirring sound coming from the unit.

As others noted, start at the wheel sensors. I assume you didn't make a radical change in wheels/tires. You will have a test failure if the rolling diameters are too different from each other.

The self test of the 944 ABS system is done at drive off speed (10 - 15km/h). I can start the engine and my ABS sets up great - no lights. Drive off and ABS fails - light comes on. I have set up a reset switch connected to the ABS fuse, and can turn the ABS off - I need to accelerate, and switch it back on. The ABS light goes out and I have ABS again. All wheel sensors and wire to the computer test good. So I can only believe it is a relay on the ABS pump or a bad earth somewhere.

In my race car, I've "cooked" the rear sensors so they fail to give the correct speed to the brain, but the system passed the self test at ignition on.
The relays on the top of the hydraulic unit are accessible if you remove the fender liner and wheel: it's tight but there is a plastic cover on the top held on by a plastic nut, two large (expensive) square relays right there. Check your ground by the brain, and check the ground on the mount for the hydraulic unit. Also, check your brake lights and switch--the signal to the ABS the brakes are in use is from the light circuit and I recall someone posting about chasing their tail (so to speak) only to find they had a brake light problem. Hope this helps.