Porsche Enthusiasts Club Forum

My 'antilock' warning light came on yesterday. It does not come on when you start the engine, only shortly after the car starts moving. I understand that there are speed sensors fitted to each wheel, that it is all set up for standard size wheels / tyres, and that if the difference in rolling radius exceeds 6% the ABS de-activates and the light comes on (clearly copied from the handbook).

I haven't had a look yet, but is it likely that a sensor could have fallen off / come loose when the new tyres were fitted last week? What do they look like? Where are they? Also, it may be relevant that I washed the engine / engine compartment on Sunday. I usually leave the engine running while I do this, and it's usually fine, but this time I had to dry out the distributor cap afterwards to get the car started again. Do you think it is likely that something else has been affected by the water to make the warning light come on?, and if so what?, or is that likely to be coincidence?

Any advice appreciated. Thank you.

Water could be a problem. If it's OK on startup but fails once moving, then it has passed self test & is failing once it tries to detect a signal from one of the hubs. The sensors are in the back of the hubs & I think detect off the wheel bearing or a toothed ring. Sensors fail and/or get dirty. A loose wheel bearing can cause problems as well. The only way to find sensor errors is with a DVM to measure the resistance of each one & if they're all OK then you'll need to scope the output of each sensor. The wiring from the hub can fail as well.

A quick way to check the sensors is disconnect the plug by the driver's right knee - this allows each corner to be checked using the DVM. I can't recall the pin numbers however a search on Rennlist will give you all the answers. This happened to me after I replaced my engine and it just 'went away' after about 50 miles and has not come back since...

I also get this problem intermittently. I have just put it down to either a faulty sensor or a broken wire somewhere. As it usually goes away again after a short time I'm living with it.