Question about 1991 GMC C1500
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?
Posted on May 23, 2009
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
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
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
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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