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73-87 Chevy _ GMC Trucks => Electrical => Topic started by: brody287 on June 27, 2020, 12:55:26 AM
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79 Chevy C10, I was doing some work to wire up a 4-wire harness for pulling a trailer, and discovered that I have signal light issues (I haven’t wired up the harness yet so this is before that)
Basically, it began with no left turn signal at all in the front or the back. I replaced the flasher relay and now it works in the front. I still don’t have any brake light or signal light in the back. The light simply won’t come on, even when the front left flashes or the brake light is on on the other side.
The crazy thing is, if I pull out the lights then the turn signal/brake light lamps on both sides come on. What gives?
Can anybody help me figure out the wiring here or figure out why I’m having these issues? Thanks
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Perhaps the Ground cable is broken, or not good fixed at the ground point.
If you move the light (and also the cables then)
and it works, then i would start with your search near by.
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Did you try replacing the left rear tail/turn/brake lamp (1157) bulb?
If the bulb is good, either there is an open in the yellow wire that runs between the turn signal switch and the left rear lamp or the turn signal switch is faulty. Use an incandescent test light to probe the yellow wire where it enters the flat turn signal switch connector that attaches to the lower right side of the steering column under the dash. If you step on the brake or switch on the ignition and the left turn signal the test light should illuminate.
If the test light illuminates:
there is an open in the yellow wire between the point you just probed and the back of the truck. Locate the rear connector located in the vicinity of the spare tire and probe the yellow wire there. If the test light illuminates, remove the left rear lamp from the bedside and probe for power there. Be sure to inspect for corrosion in all of the various connectors you disassemble. If the test light fails to illuminate at the rear connector, work your way forward, probing the yellow wire periodically until you find power. The break in the wire is between where you find power and the closest probe point rearward that was dead. Carefully inspect the rear lamp wiring harness where it crosses over the rear axle and comes into close proximity with the exhaust. Failures are common there due to excessive exhaust heat. Also, inspect for chafing of the wires that might allow water and salt intrusion through the insulation to corrode the conductors.
If the test light does not illuminate:
replace the turn switch.
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Thanks BD, that was the key piece of information I needed. I didn't know where to begin testing underneath the dash. I tested at the connector on the steering column underneath the dash and found that it was giving power, so it wasn't my switch. Phew. Didn't really want to pull the steering wheel. I found some wire underneath the bed that looked chaffed so I tried testing there by sticking a pin through the shielding. But it was fine there.
The problem ended up being right before the harness at the tail of the truck. Somebody tried to splice into it using one of those 3M plastic splices. Maybe I'm not the first one to try and put a trailer harness on it? Anyway, whatever they were doing they never finished and the yellow left turn lamp wire was actually completely mangled and broken in several places inside a wrap of electrical tape. Not sure why I didn't notice the wrap of electrical tape before ::), but it's done now. Soldered in a new length of wire and sure enough she works now! And I got the trailer harness wired up too!