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Dave Nova
Dave Nova, GMC Technician
Category: GMC
Satisfied Customers: 19450
Experience:  ASE Master Certification. GM World Class Certification
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GMC Canyon: Issues with trailer T harness. When Parking/Headlights

Customer Question

Issues with trailer T harness. When Parking/Headlights are on... I have no Brake Lights or Signals. When turning OFF headlights and Parking lights, Trailer lights work perfectly. I have '04 GMC Canyon. T harness and trailer wiring are all new and correct as it works fine with other vehicles. NO trailer/running lights fuse in fuse block... or at least not labeled as such. Please Help
Submitted: 7 months ago.
Category: GMC
Expert:  Dave Nova replied 7 months ago.
The problem you are experiencing is due to a bad ground to the trailer. It should ground through the hitch, but may not. Find the white wire in the harness and make sure it attaches to a good chassis ground.
Customer: replied 7 months ago.
I have a good ground on the trailer as it works well on other trucks. The Hopkins wire kit has the ground integrated into the circuit however I did test it with a jumper wire from the harness directly to the truck frame via cleaned surface, ring and screw. It made no difference. Everything works perfectly until I turn on the headlights... then I have no signal/brake lights.
Expert:  Dave Nova replied 7 months ago.
The T harness has a built in adapter to go from a 5 wire system on the truck (separate brake and turn) to a 4 wire trailer system (integrated stop and turn). If you have good grounds on the harness and to the trailer lights, the problem has to be in the adapter that is built into the T harness.
Customer: replied 7 months ago.
Thank you for the replies. My problem is still unsolved as I have exchanged the T harness for a new one. That makes 3 now that didn't work, two new ones and my old original that suddenly stopped. I attempted various grounding points from different points on the harness still to no avail. I guess I'll look into tracing the wires through the truck until I find the answer. If that doesn't work, I'll pull power the old way directly from the taillights.Thanks again for the help, but in my case it hasn't helped.
Expert:  Dave Nova replied 7 months ago.
When turning on the headlights, do the truck brake and turn lights quit working, or is it just the trailer lights that quit?
Customer: replied 7 months ago.
Just the Trailer lights.... The truck lights work work fine all of the time, with or without the trailer and with or without the headlights. The trailer lights (and plug tester) works fine until the running lights (headlights) come on or get turned on. One thing I noticed is that if I disconnect the trailer taillights (Brown wire) at the plug, then the brake and signals work again but no taillights. I'm thinking the Brown wire may have rubbed bare and is grounding out somewhere. Now that I have a day off work, I'm going to trace the wires as far back as necessary to find a solution. I'll post what I find if I get it fixed.
Expert:  Dave Nova replied 7 months ago.
I know you said the trailer works fine on other vehicles, so the trailer lights are likely okay, but I do still feel this is a ground problem. Double check trailer light grounds and temporarily run a ground from trailer light bulb socket to trailer frame to see if this makes a difference.
Customer: replied 7 months ago.
OK, first thing in the morning I will check the trailer ground and make a jumper to be sure. Currently the white wire (ground) runs the length of the trailer with Brown, Green and Yellow where it joins 2 others and is screwed to the frame. The 2 ground wires are then paired 1each with the Brown/Yellow and Brown/Green and run directly to the taillights. The three connections are then made at the (LED style) lights. Other than creating a "T" with the White and Brown, they have not been broken or connected to anything. See Diagram attached.
Customer: replied 7 months ago.
Here is complete image... sorry
Expert:  Dave Nova replied 7 months ago.
Sounds like the ground should be good, but best to double check.
Customer: replied 7 months ago.
AArrghhh... I'm so frustrated! Wires all look good from taillights to the trailer harness connector. As far as I can see they look good from connector onward as well. Truck lights work perfectly. I separate the factory connector, insert the Hopkins harness 5-4 adapter. Truck lights still work perfectly but no Brake/directional on the trailer. Disconnecting just the BROWN allows everything to work perfectly except no taillights (for obvious reasons). I know it's not your fault and not every problem can be solved through talking. Some just need to be dealt with hands on. I feel like I've wasted my money here. I want to rate you 5 stars for your help, but my problem is still not solved. Any further testing I can do to rule some things out barring a complete new harness to the rear of my truck?
Expert:  Dave Nova replied 7 months ago.
Connect all wires as normal, but disconnect the brown wire just before each taillight and test operation again. Do turn/stop lights work?