ABS PSM Lights - intermitant

[pedders911]

2006 996 C4S with ~32,000 miles.

I had a radiator hose blow a while back (clip not connected properly - my fault!) after which I had ABS/PSM light come on once - cancelled by turning ignition off and re-starting.

Since then the warning light have come on almost once a journey - usually near the start, once cancelled they do not re-appear. It stopped happening in the run up to its service (don't they always!), but soince service (colder weather) has started to come on two or three times at the start of the journey, but again once it has doen so a few times it doesn't come on again.....

Porsche service picked up codes suggesting pressure sensor fault - but the fault wasn't present during the service - advised I would need new ABS/PSM module ~£700 plus fitting if it continued or became other than intermittent - any ideas??

Mark

Edited December 19, 2009 by pedders911

[Loren]

Check your brake light switch - that is the most common source of this fault.

[pedders911]

Check your brake light switch - that is the most common source of this fault.

So what is the problem, is it a sticky brakelight switch, poor connections?

I have just had a quick look, not so easy to get to and a bit chilly tonight to have a serious go - how is the switch fitted?

[Richard Hamilton]

The switch contacts often get pitted and don't make contact properly. There are two contacts in the switch - one normally open, and the other normally closed. Even though the brake lights might work, the problem could still be in the switch.

Another possible cause is an ABS sensor. Did the radiator hose blast coolant all over the brake area? If so that might be the culprit. It might be worth checking the plug/socket connection.

[Loren]

Yes, Richard's points are exactly was I was referring to.

[pedders911]

Thanks guys, this does seem to have done the trick.....

First took screw holding foot-well ventilation duct in place out - allowing that to be moved an inch or two to one side to make access to switch easier.

Second remove switch (quarter turn left or right and pull)

Third remove wiring loom. catches on two sides need to be gently levered outwards.

Finally disassemble switch - mark it so you know how it is assembled, then gently prise double clips on both sides and allow spring to separate it

The problem was as indicated - dirty switch surfaces - clean and reassemble (the right way around - match your markings)

Make sure the plunger is fully extended before putting it back in the car, check brake light functioning.

Edited December 20, 2009 by pedders911

[Richard Hamilton]

Result! And so much cheaper than the OPC "solution"!!

[pedders911]

Result! And so much cheaper than the OPC "solution"!!

Yes very much so. I remember now having a similar problem with my son's VW Lupo, which had engine management light coming on - which was traced, by computer diagnostics to a faulty brake light!! VW diagnostics 1 - Porsche 0.

I guess the 'spikes' that occur when arching happens causes the computer to 'fault' picking the first code that comes to hand! I reckon it happens at the beginning of the journey because that is when voltages are at their highest, as battery charging is taking place... well a theory anyway.

Thanks again for all the help and the swiftness of responses.

Mark