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PSM ABS Lights Come on with Headlights and Cruise Inop

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The strangest thing is happening and I have searched but can't seem to find a post with the same symptoms. I have a 2002 C4S. The cruise has stopped working but more importantly the PSM and ABS fails when the headlights (or even the parking lights) are switched on. The brake lights work from the brake pedal when the headlights are off but when the headlights are turned on the brake lights come on (including the center light) and stay on. I'm thinking headlight switch but that doesn't explain why the cruise control doesn't work when the lights are off. When the headlights are off ABS and PSM are fine.

Thanks for any help you can offer.


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That is an odd one.... the ignition switch is known to causes weird things to happen, they have such a high failure rate that it might be worth just changing out to see if that solves any problems.

On a side note- has the state of the charging system been checked?

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That is an odd one.... the ignition switch is known to causes weird things to happen, they have such a high failure rate that it might be worth just changing out to see if that solves any problems.

On a side note- has the state of the charging system been checked?

The chrging system has not been specifically checked but the voltmeter indicates just less than 14 on start up. I have read allot about the ignition switch going bad. I may change it out just to limit the possibilites. A short is definately a possibility. I suppose I can check for continuity between the running and stop lights with the lights out to verify that. I was kind of hoping the brake lights ran through the headlamp switch. The problem seemed to start when I stopped for a minute in Beautiful Beaver Utah. Stopped with no problem - 5 minutes later I turn on the headlight switch and the PSM/ABS annunciater lights come on and the MFD is directing me to go to the shop. When I shut down the ABS/.PSM system is fine until the headlights are turned on again.


Edited by Seaquester
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FIXED! Sort of. Just in case someone else might run into this problem...The spring contact in the right rear stoplight assembly broke and shorted to the running lights. I pulled the broken contact out and everything is working again. Now I just need to find the socket for the bulb and all will be as it should. (Right now I'm running down one running light)


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  • 7 years later...

Even though this is an old thread, it certainly helped me find the exact same really strange problem on my 2004 C4S. After replacing the ignition switch and the battery (battery was on it's way out anyway), the problem remained - so further searching on the forums got me to this post. The key was using "parking lights" in my search, which of course I didn't do until later in the process  :cursing:


Of course hind sight is 20/20, and this all started when I took all of the rear lights out to fix a light that was improperly installed by the previous owner. Those metal tabs (especially on the dual filament connectors) get VERY brittle with age, and they just cracked odd off and shorted together like Seaquester's above. Now to find a replacement...


I didn't put 2 and 2 together since I didn't do any night driving for a few days, which meant I didn't use the headlights and therefore no warning lights on the dash until several days later. Since I had gone several days without any problems after the initial work on the tail lights, I was trouble-shooting this one as if it was totally unrelated. 


Now I know how to change the ignition switch, adjust the brake switch, and replace rear lights. 

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