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Old 02-29-2012, 09:38 PM   #1
JointTech
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Rear Light harness confuzzled

OK so my first task on the new truck (72 C20 cheyenne super) is get the lights working so I can actually drive it to the parts store without getting grease and dirt in my daily driver.

I tested from the plugin at the back of the truck and have power there. I traced that to the front plugin where it goes through the firewall and I have power there (obviously).
After the rear harness plug where it goes out to the lights was a rats nest of cracked spliced wires.

I cut the rats nest out. (thinking back I should have paid more attention to colors then...)
So now on the left side coming from the lights I have Brown/Brown/Light green/Yellow

on the right side coming from the lights I have brown/brown/dark green/light green.

What im confused on is the plug that I cutoff.
It has lt green/lt green/brown/yellow/dark green.

If I hook dk green -> dk green and yellow -> yellow my turn signals work.

I would have thought simply matching the colors back up would get all of them working. not so...

I've read and traced the colorful wiring diagram I found on this site. (thanks to that guy!!!) In fact my eyes are actually bleeding from reading it so many times.

My problem is my colors dont seem to match the diagram..

through different trial and error I have gotten the side/brake/running lights to work. But not at the same time.

heres the connector that plugs in at the back of the truck.


and heres the left side where it goes to the lights.


Can somebody please tell me what goes to what?
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Old 02-29-2012, 10:05 PM   #2
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Re: Rear Light harness confuzzled

here's the diagram I have and the wire colors and what they are.


Yellow ----left side brake/turn signal

Dark Green--right side brake/turn signal

brown -----Tail lights

light green ---back up lights
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Old 02-29-2012, 10:15 PM   #3
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Re: Rear Light harness confuzzled

Hi thanks for the reply
yea ive been looking at that for a couple hours now lol. I dont understand why I have 2 browns from the lights on both sides.
individually i can get the different lights to work on brake/running/turn. but when I just wire them up with the colors you mention above (i just go brown -> both browns) it nothing works.

Is it possible theres some kind of short?
Maybe i just need to go look at it after a break. I'm rusty on car stuff and need to ease back in.
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Old 02-29-2012, 10:25 PM   #4
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Re: Rear Light harness confuzzled

make sure u have good grounds
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Old 02-29-2012, 10:30 PM   #5
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Re: Rear Light harness confuzzled

I didnt have a socket with me so I didnt pull the housings out. The housing is the ground right? I read some other threads where people where soldering a wire off the back to a "real" ground.

The other thing about the ground is you would think if it was bad the light wouldnt work at all. it does one at a time. just not when all are connected.

Im going to go beat on it after dinner and see what I find.

It's freezing out there though. Got to be low 60's.
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Old 03-01-2012, 07:14 PM   #6
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Re: Rear Light harness confuzzled

ok so i went back out today and alligator clipped everything on the left side to the harness. Lights/brake/turn worked !!!
So I wired it all up with connectors. Nothing works. Cut it out and wired it back with alligator clips. Doesnt work...
At the rear harness im only getting Millivolt output. At the harness connector under the hood I get the same millivolt readings.
If I test directly at the fusebox I get 11.5VDC. So it seems like my problem must lie between the fusebox and the firewall connector.

I havent quite figured out how to remove that but I think I may be onto something here. Im thinking I may just get a new front and rear harness but it looks like that wont help if its between the fusebox and firewall. Looks like there are 2 harnesses. one from firewall connector to back connector. and one for back connector to lamps.

Is that a plug in the firewall or just a grommet the wires go through?
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Old 03-01-2012, 08:07 PM   #7
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Re: Rear Light harness confuzzled

ok now im really confused.
Under the hood on the firewall is a harness connector. One side goes all the way back along the framrail to the rear harnes connection. THe other side (which i thought would go back into the cab/fusebox) goes down and follows the framerail back to the drivers side wheelwell. It goes into the wheel well never to be seen or heard from again.
On the drivers side is an extra gas tank and it looks like theres a big wire for power in the same connector.
Did somebody rig this thing up is is this wiring factory?
So what is the connector beside the wheel and
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Old 03-01-2012, 10:19 PM   #8
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Re: Rear Light harness confuzzled

well i figured it out. There was a fifth-wheel in this thing at some point. There was a splice going back to a trailer plug in the fender. I couldnt see it because there was a bedliner in the bed. yanked that out. cut the wires and re-spliced them directly. Didnt have a chance to test but I think im on the the way. the wires going into the firewall that i couldnt find where just electrical taped and grimy. i found them.

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Old 03-03-2012, 07:06 AM   #9
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Re: Rear Light harness confuzzled

Quote:
Originally Posted by JointTech View Post
Hi thanks for the reply
yea ive been looking at that for a couple hours now lol. I dont understand why I have 2 browns from the lights on both sides.
individually i can get the different lights to work on brake/running/turn. but when I just wire them up with the colors you mention above (i just go brown -> both browns) it nothing works.

Is it possible theres some kind of short?
Maybe i just need to go look at it after a break. I'm rusty on car stuff and need to ease back in.
The 2 browns on each side: one should go to the taillight and the other to the side marker. Also, another brown should feed your license plate light. These will all light up with your running lights on. The "Ground" usually goes bad because the housing must make good contact to solid metal. Some guys will add a jumper wire to insure a solid ground.

Just take your time and sort things out. I have similar issues, P.O. had taps and splices and wires hanging loose. I've cleaned it up and everything works for now. But, my taillight housings are rusting through. When I change those, then I'll replace the brittle wiring all the way back to the plug.
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Old 03-04-2012, 08:02 AM   #10
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Re: Rear Light harness confuzzled

well i got it!!
i went ahead and took all the lights off to wash the lenses and replace the bulbs since Im having som much fun with lights these days
No to the muffler shop next weekend and it will be road worthy. ugly as sin but driveable.
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Old 03-28-2012, 11:55 PM   #11
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Re: Rear Light harness confuzzled

I started to try and get my lights working today lots of spagetti back there I have had a tail light a directional no brake lights yet I played with it for 2/3 hours bad bulbs and now after reading I realize no ground cause I had the housings out for blasting and priming....the bed is from a 67 and the truck is a 72 so the rear taillight harness has no wires for the markers because there are none is your diagram year specific ?....is the harness from the front to the back cross member the for same 67-72 ? so if I'm using a 67 far rear taillight harness is that gonna cause a problem? If you know ......I'll be back at it tomorrow night thanks for any info from anyone this thread has helped already wish I read it before I wasted a few hours


Quote:
Originally Posted by VetteVet View Post
here's the diagram I have and the wire colors and what they are.


Yellow ----left side brake/turn signal

Dark Green--right side brake/turn signal

brown -----Tail lights

light green ---back up lights
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Old 03-29-2012, 12:08 AM   #12
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Re: Rear Light harness confuzzled

the full size images are here:

he says its the diagram for 67-72 so im not sure on yor questions really. I just battled my way through with the help of that diagram and the forum
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Old 03-29-2012, 10:08 PM   #13
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Re: Rear Light harness confuzzled

Ok thanks

Quote:
Originally Posted by JointTech View Post
the full size images are here:
[url="http://www.flickr.com/photos/boundstaffpress/3396610207/sizes/o/in/photostream/"]All sizes | 1967-72 Chevy truck Cab and chassis wiring | Flickr - Photo Sharing![/url]

he says its the diagram for 67-72 so im not sure on yor questions really. I just battled my way through with the help of that diagram and the forum
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Old 10-07-2012, 01:05 PM   #14
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Re: Rear Light harness confuzzled

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Old 12-18-2014, 01:56 AM   #15
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Re: Rear Light harness confuzzled

Don't mean to highjack but I was wondering if anyone had the correct routing for the rear light harness. I got a replica to replace the hacked harness that one of the PO's made for it and need to figure out exactly where its all supposed to run. Thanks.
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Old 12-18-2014, 12:48 PM   #16
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Thumbs up Re: Rear Light harness confuzzled

I want to add a simple way to figure out the rear plug wiring. I hope this helps people. This is how I did mine and a few other vehicles.
This is for a 71 Chevy truck. It should work for any model though if you follow it.

The factory harness should run the drivers side frame rail.

You will need a helper to do this. To sit in the truck and go through the task of turning the lights on and off when you give them directions.
Now under the truck with a test light and paper & pencile. Have the rear plug undone so that you can check each wire.

Tell them to put on the brakes. Then check to see which wire in the plug will light up your test light. Mark it brakes.
Then do tail lights, mark it.
Now do right turn signal. Mark it.
Then left. Mark it.
Once this is done then do it with the brakes on. You should have a solid test light on the brake side wire and a blinking test light on the signal side. Do this for both sides.
This is very simple and it's a lot harder to type than it is to do. Just take your time.
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