Question about 1991 GMC C1500

4 Answers

Both rear turn signal lights flash

Got a weird one for ya. I'm a tech with 30 years experience and I can't figure this one out! No matter how good ya are, sometimes you get stumped! Truck is my personal ''winter beater'' and tow vehicle for my drag car. When the R or L turn signals are activated, BOTH the rear lights flash. Front is fine. Headlighs on or off makes no difference. Using an unpowered 12V test light with the firewall harness plug disconnected I checked power feed from the column switch. Both sides work as they should, so the problem is not in the turn signal switch. Problem SHOULD be a short in the wiring harness between the firewall connector and rear lights, but it is NOT! Unplugged the connector at the rear of truck and used a test light. This cut out of the circuit ALL rear lights and tralier light connector. Power was fed to both sides, the Yellow and Dk Green wires. i.e. both sides flash at the same time. Used a DVOM set on ''continuity'' to check wiring harness between firewall and rear connector. There is NO SHORT in the wires. Checked and cleaned the rear frame ground while back there. Additionally, when I hit the brakes the signals cease to flash. Hazard flashers work and I checked both relays. Both click and I can feel them cycle when I touch 'em. Everything else, brake lights, park lights, headlights, all bulbs, etc are fine. I haven't checked the brake switch and circuit yet, but looking at wiring diagrams I can see no way where power could feed back into the rear wiring harness and cause this problem, unless coming thru the column switch, which checked out OK. The problem was intermittent for awhile, usually when wet or raining, but now is permanent. Anyone got any ideas here? What am I missing?

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  • Gar
    Gar May 01, 2009

    Carhob...re-read my post. I cut the trailer connection completely out of the loop when I disconnected the rear connector. Continuity test on wiring harness from firewall to rear lights showed no shorts. Front lights don't flash because they are on a different circuit and use separate output connections from the column switch and completely different wires from the firewall connector to the front turn signals.

    Raffaiel...I'll try running a temporary ground cable from the rear of the truck to my negative battery post. And replace the flasher units. Also, gonna check the motor ground straps. I'll let ya know if it works.

    Motor1258...I missed something here. Since there is nothing wrong with my wiring harness from the firewall connector to the rear lights, where do you suggest I splice the wires in? Something is causing power feedback into the rear light harness. I have to locate where and why voltage is being injected into the Dark Green and Yellow turn signal wires. It's NOT from a short in the rear wiring harness. I know it's complicated, but re-read my post. Voltage has GOT to be coming from an auxiliary circuit like the brake switch or??? Could a problem in the front turn signal harness be feeding back thru the column switch to the rear turn signals?


  • Gar
    Gar May 02, 2009

    Motor1258....Yeah you did misunderstand. It's hard to explain a problem as complex as this by text. The wires between the firewall and taillights is where the problem SHOULD be, but it's NOT!! Used my DVOM set on "continuity" (it buzzes if the circuit is complete).and checked each wire. The R and L turn signal wires ARE NOT fused together, nor are any of the other wires in the harness. No broken wires either. That's what's got me stumped. Everything points to the Yellow and Dk Green turn signal feed wires making contact. Voltage has to be coming from somewhere and backfeeding into the turn signal harness.

  • Gar
    Gar May 09, 2009

    REPLYING TO COMMENTS:

    Already checked all fuses, fuse box wiring. Nothing wrong there.

    No daytime running lights. Truck is a 1991.

    Power feed from column switch to turn signals is Yellow on the left, Dk Green on the right.. Already have wiring diagram from Mitchell Manual and Auto Zone. Both the same.


  • Anonymous May 11, 2010

    did u look at fuse box wiring!

  • Anonymous May 11, 2010

    what i find weird is frt lights dont flash both sides too,seems u did not check trailer plug connections, wiring damage wich often is cause.

  • Peter
    Peter Mar 20, 2014

    Ok may or may not help. I had a $200K Cryogenic trailer I noticed the tractor pulling it left turn signal would blink several times then the right and left would flash back and forth then pop everything wwas out . later on the same thing would happen. The driver told me that it had been doing it for a month since they had picked it up at the manufacturer in MN.. Long story short. I started by diconeccting the trailer from the tractor ( a freightliner) and no problem hten I started on the trailer. I initially looked in the relay box on the left front fender of the truck and OMG someone had put jumper wires all over the place in there and the relays were shorting out. I ripped all the jumpers out and started rom scratch testing the wiring for a short . Turns out on the circular junction box on the rear of the trailer someone had forgotten to tighten one of the ground wire nuts so ther was excessive resistence and a major drop in amperage. Tightened it up poped the clear lens back on called the manufacturer and back billed them labor under warranty. Their department couldn't figure out the loose ground so they tried to make it work by installin a bunch of jumper wires in the relay box. Geeezzzzz. Loose grounds or corroded connections cause a heck of a lot. Corroded show up more in damp weather.

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Bad Ground!

Posted on May 23, 2009

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Why don't you just cut 2 wires where you know they're good, add 2 new wires, and run them to the back, and by-pass whatever the problem area is, and your done?

Posted on May 01, 2009

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  • Mike Butler
    Mike Butler May 01, 2009

    So maybe I am missing something? I read that when right or left signal is on, both rear lights flash, but front work as they should tells me that the right & left signal wires to rear may be fused together somewhere between firewall where all is good, and rear . If they are fused together, one will illuminate the other, thus was the reason for running separate wires. Am I still reading it wrong?

  • Mike Butler
    Mike Butler May 02, 2009

    According to wiring schematic on autozone, yellow goes back to direction switch. Each socket has a yellow correct. If you sign up at autozone.com for free you'll get access to the diagram for your own info. Good Luck

  • Mike Butler
    Mike Butler May 02, 2009

    Has that truck also got a daytime running light module? Another consideration too.

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What about diode assy used to light the middle brake light? It it was passing voltage backwards it would light both or the power side of activation source.

Posted on May 26, 2013

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  • Expert
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We have a problem like that also and some of it has not enough ground and some have low relay that is the most of it...

Posted on May 01, 2009

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Posted on Jan 02, 2017

Anonymous

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SOURCE: Both rear turn signal lights flash

I did 15a and electrical for over 30 for chevrolet and seen some pretty weird stuff.To me you still have a ground problem.your idea of checking the brake light switch is a good start.both rear lamps flashing to me means the hazards are shorting across the turn and brake light circuits,because hazards stop flashing when brakes are applied.The brakes light switches in the 90's were really weird because they also incorporated the abs system.Good luck,let me know what happens.Thanks

Posted on May 26, 2009

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