986 ABS / PSM lights

[telford_mike]

Mrs telford_mike has a 2004 986 with about 50k on the clock. Recently the ABS &PSM lights have been illuminating intermittently. They always go away when the car is turned off, but return after a few miles of driving. I found the following on one of the US forums:

- A wheel came off the ground, wheel hop from accelerating quickly at a low speed, or you hit a pothole (Solution: Turn off the car completely, then turn it back on. The ABS/PSM light will clear itself if this was the case.)
- Failed Brake Sensor (Solution: replace brake sensor approx $20.00)
- Failed or damaged wheel speed sensor (Solution: check datalog codes for left or right sensor failure, swap left and right sensors and see if the failure moves to the opposite side. Replace if code states failure moved to opposite side. Leave sensors alone if code doesn't change.)
- ABS Ring on Axle (Solution: clean the ring)
- Bad Wiring Harness (Testing: Run double 16ga wire from wheel speed sensor directly to ABS box and check for code to - clear itself. Solution: Order repair kit and fix harness.)
- Bad ABS Module (This is expensive and should be your last resort. Solution: Repair ABS Module.

Can anyone confirm whether this is a good list of possible causes? If so, how much of it can I diagnose myself, and which items will need to be looked at by a specialist?

Many thanks.

[monthefish]

It's a good start to a list, but there are plenty other things it could be. Do you know anyone with an OBD reader to get the fault codes?

[spyderman8]

Does it have the correct size wheels and tyres (the 986 was never intended to take 19" wheels, for example)? PSM/ABS is sensitive to rolling radius.

[cd1957]

Hi check that brake lights work all the time and check MAF sensor for any faults.

Chris

[telford_mike]

spyderman8 said:

Does it have the correct size wheels and tyres (the 986 was never intended to take 19" wheels, for example)? PSM/ABS is sensitive to rolling radius.

Completely factory - 17" wheels.

I don't have any diagnostic equipment beyond some spanners and my eyes

[thegoose]

Classic symptoms of MAF sensor failure - if you can get to it, unplug it and see if the problem goes away. If it does you need a new one (circa £150 and about 2 minutes to change with a Torx security bit - sorry, can't remember what size) - you could try cleaning and replacing it but it won't last for long before it fails again.

The unplug test works because when the car doesn't see a signal from the sensor it ignores it and works to baseline settings, so it's not working properly but it won't throw up the fault.