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Dan Babb

ABS/PCM error - brake light or headlight switch?

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I'm having a problem and think the brake switch is at fault...but not sure.I have the ABS/ PCM lights showing up on the dash.When the headlights are turned off, my brake lights function normally. I don't get the warning lights when the headlight switch is off.

When the headlights are turned on, the brake lights do not turn on and the ABS/ PCM warning lights come on.

Would a faulty brake switch be the cause of this or could it be something else?

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I think you are referring to the PSM (not PCM) light; and , yes, sometimes a faulty brake switch can cause this issue. It can also be voltage related (low charging rate/weak battery).

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Update....one correction to make. My brake lights are actually always ON when the headlights are turned on...but they operate normally when the headlights are off.

Battery checked out OK and the voltmeter in the dash reads 14 when the car is running, so I believe the charging system is good.

I replaced the headlight switch with a new one from the local Porsche dealer and no change. So I took it back out and returned it.

Now waiting for a brake switch to arrive in the mail so I can try that out.

Edited by Dan Babb

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I changed the headlight switch and brake light switch and neither helped.  So I put the original headlight switch back in (wasn't going to pay to keep a light switch that didn't need to be replaced).  I gave up and brought it to a shop.  

 

They spent hours trying to diagnose things and even called Porsche to get help.  They suggested checking out the tail light bulbs.  It turned out that a prior owner had installed the wrong tail lamps in back.  I guess they used single filament bulbs instead of double filament bulbs.  That was shorting out the system and throwing the error codes.  Once the right bulbs were put in the car, everything worked perfectly.

 

The headlight switch and brake light switch are common causes, but in my case the problem was harder to find but easier to fix.

 

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