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Old 07-15-2018, 01:46 PM   #1
Bobbed72
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Tail light wiring.

Have a weird issue with my license plate lights on my 72 C20. When I bought the truck the rear plate was taped in the rear window by the previous owner. I decided to relocate it to the rear. Truck has no bumper or roll pan so I bolted it to a rear frame member and added some lights. The weird thing is the 12v supply I located at the back goes out when the brake is pushed. Does the circuit for the rear lights go out for when the brake is pressed or is there something else going on?
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Old 07-15-2018, 10:49 PM   #2
ray_mcavoy
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Re: Tail light wiring.

No, there normally isn't a circuit going to the back of the truck that turns off when the brake is pushed so it sounds like you have some other problem going on there.

If the rear wiring is stock, it should have the following wire colors:

Brown = tail lights, live when the headlight switch is pulled out to either the 1st or 2nd notch. This is the circuit that originally powered the rear license plate light.

Yellow = left rear turn signal & brake light.

Dark Green = right rear turn signal & brake light.

Light Green = reverse lights.


The factory ground for the tail, brake, and turn signals made use of the metal light housings screwed to the bed which is bolted to the frame. So it's not uncommon to have ground problems due to rust, corrosion, paint, and such. And depending on which wire you tapped into, you can get some strange behavior with power backfeeding through the dual-filament bulbs when they have a poor ground.
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Old 07-16-2018, 02:30 PM   #3
Bobbed72
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Re: Tail light wiring.

Quote:
Originally Posted by ray_mcavoy View Post
No, there normally isn't a circuit going to the back of the truck that turns off when the brake is pushed so it sounds like you have some other problem going on there.

If the rear wiring is stock, it should have the following wire colors:

Brown = tail lights, live when the headlight switch is pulled out to either the 1st or 2nd notch. This is the circuit that originally powered the rear license plate light.

Yellow = left rear turn signal & brake light.

Dark Green = right rear turn signal & brake light.

Light Green = reverse lights.


The factory ground for the tail, brake, and turn signals made use of the metal light housings screwed to the bed which is bolted to the frame. So it's not uncommon to have ground problems due to rust, corrosion, paint, and such. And depending on which wire you tapped into, you can get some strange behavior with power backfeeding through the dual-filament bulbs when they have a poor ground.
Thanks, that helped. I connected the ground wire to the brake lights. Apparently the wire goes to ground when inactive. Hitting the brakes broke the circuit.
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