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How do I fix a rear lighting fault that occurs only when the running lights are on?

Tiny
JASON_ROSS
  • MEMBER
  • 1990 GMC C1500
  • 194,000 MILES
OK. When no exterior lights are turned on the brakes and turns signals work fine in the rear. However when I engage the parking lights (running lights) everything in the rear stops working (to include license plate lights). I looked to see if it was the fuse but all the fuses are good. The Hazards work with the lights on so I know it is not a bulb or circuit board issue either. I removed the trailer wiring harness from the circuit as well to ensure it was not the culprit.

Please help. I want to test the wiring harning that comes down the left frame rail but I am unsure of the color scheme and hwo to test with a multimeter (wiring looks like black, white, light blue and green).
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Saturday, April 21st, 2012 AT 3:10 AM

11 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
I don't feel this is a wiring harness issue. Check the headlight switch.
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Saturday, April 21st, 2012 AT 3:12 AM
Tiny
JASON_ROSS
  • MEMBER
  • 7 POSTS
I did notice that the headlight switch is not switching on the same. Kind of clunking into place. I will check it out. Thanks.
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Saturday, April 21st, 2012 AT 3:18 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
Glad to help. Let me know what you find.
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Saturday, April 21st, 2012 AT 3:33 AM
Tiny
JASON_ROSS
  • MEMBER
  • 7 POSTS
Ok So I took apart the switch.A little bit of scoring but I cleaned it all up. I did notice something while I took the switch out. When I put my foot on the brake and then put the turn signal on it blinks fast. I put the switch back into the circuit and it still does the same thing when I engage the parking and headlights. No tail, no reverse, no license plate. One thing of note is when I put the blinker on it blinks once and dies.
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Saturday, April 21st, 2012 AT 6:45 AM
Tiny
CARADIODOC
  • MECHANIC
  • 33,871 POSTS
Hi guys. Sounds like a bad ground for the rear lights. Turn the running lights on, then watch what happens to the brightness of the left rear one when you remove the right rear bulb. If it changes, there's a bad ground. The rear lights are finding an alternate, poor ground through the front signal lights. You can also verify this by measuring the voltages on one of the rear sockets. One wire will have 12 volts. The other two must have 0 volts. If you find some voltage on all three wires, you have a bad ground.
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Saturday, April 21st, 2012 AT 8:48 AM
Tiny
JASON_ROSS
  • MEMBER
  • 7 POSTS
I think this is it.I found a snapped ground on the factory rear wire harness that attaches to the frame. I will keep you updated once I fix it.
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Saturday, April 21st, 2012 AT 8:58 AM
Tiny
JASON_ROSS
  • MEMBER
  • 7 POSTS
Ok. You know your truck is old when the ground wire is disintegrated inside the shielding. So I had to undo the the factory harness about a foot and use the ground from my trailer harness (I don't use this truck to pull anything)and spliced then crimped them together. I know this is low tech and kind of shoddy but I didn't have anything else. Regardless, I want to say thanks because I learned a ton about the inner workings of the headlamp switch and the wiring. Now my old reliable is just that. Old and reliable :)
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Saturday, April 21st, 2012 AT 10:08 AM
Tiny
CARADIODOC
  • MECHANIC
  • 33,871 POSTS
Old! What you callin' old? :)

My daily driver is a rusty trusty '88 Grand Caravan. My three newer vehicles, (one with 4,200 miles), sit in the garage because they're more likely to develop a computer-related problem.

Will wait to hear your problem is solved. Jacobandnickolas would have gotten you there too.
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Saturday, April 21st, 2012 AT 7:13 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
Thanks, Cardiodoc. I was thinking switch because he mentioned it felt different. However, it could be a ground issue. Actually, now that is what it sounds like.
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Saturday, April 21st, 2012 AT 8:31 PM
Tiny
CARADIODOC
  • MECHANIC
  • 33,871 POSTS
You're welcome. I was fidgeting on the edge of my chair and couldn't help sticking my nose into your conversation.
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Sunday, April 22nd, 2012 AT 5:09 AM
Tiny
JASON_ROSS
  • MEMBER
  • 7 POSTS
Ok. Lights started blinking on and off now.
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Tuesday, April 24th, 2012 AT 12:24 PM

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