Rust dust / ABS warning light

Mark Elder · Apr 22, 2015

My car has not been used for about 6 weeks and last time I was driving in snow, so disks were a bit damp when put away. I was conscious of quite a bit of rust on the disk surfaces yesterday when I set off on a 200 mile trip and indeed the brakes were extremely weak until I'd worn the rust off them with a few firm stops.

I stopped at a garage after a few miles to top up a couple of the tyre pressures and immediately saw the wheels were coated in a fine red powder!

As I moved off from the garage the ABS/ABD warning lights came on. I guess the dust has got into the teeth that spin past the sensors so the sensor can no longer tell the position of the teeth. This is definitely not the fault that causes the warning light but the ABS still works - it is very easy to create a cloud of blue smoke from the rear tyres now!

Anyone else seen this problem? Is it easy to get at the teeth to clean them? I had hoped a good blast for a couple of 100 miles would clear them out but today the light is still on.

clyde · Apr 22, 2015

I'm not sure I understand what you're saying here Mark. Does the abs still works but the abd doesn't?

Mark Elder · Apr 22, 2015

Hi Clyde, The ABS is definitely not working (press brake hard and the wheels lock up). I was only commenting that there's a thread I found on here about the ABS where the warning light comes on but the ABS does still work. In my case I have the light and non-working ABS.

(Not sure about the ABD - a bit tough to test without a bit of rain!)

clyde · Apr 22, 2015

The only time I've had the abs light come on it was due to a sensor failure. I would have thought if it was due to the rust dust it's more likely that the surface of the sensor is fouled up. You could probably manage to wash most of this off with a hose set at fine spray. Not sure I would want to use a pressure washer on it though.

38F3E1 · Apr 22, 2015

Most likely candidate would be a dirty speed sensor. Light on and no ABS usually indicates the ECU thinks it cannot sense the speed of one of the wheels and therefore cannot apply ABS across just three wheels (rather unstabalising). Plugging the car into a code reader like a Bosch Hammer will likely show a fault identifying which wheel or whether it is a more central upstream problem. Shame the club recently got rid of theirs.

Ian.

nfearn · Apr 23, 2015

Another vote for cleaning the ABS sensors. Location is as shown about a quarter of the way down the page here.

As I understand it, if the light comes on as soon as the car is started, and doesn't extinguish, it's likely to be the ABS pump at fault. If the light extinguishes after start-up but then comes on as you reach fast walking speed, then it's a sensor. Apparently if you hook up a multimeter set to the lowest (mosr sensitive) AC voltage scale across the terminals of the sensor and spin that wheel, you can see a small AC voltage being generated if the sensor is sensing the wheel rotation. If three do and one doesn't, that's clearly your culprit.

Googling "944 ABS sensor fault" or similar (944 sensors are largely the same as 993 ones for this purpose) will bring back loads of discussion threads on cleaning them, diagnosis and so on.