2004 GMC Sierra Not Sending Current to Trailer Tail Lights

Question

Question:

I have a 2004 GMC Sierra 1500 Z71. The tail lights on my trailer do not work but the other functions do. The adapter plug has red LEDs that illuminate when each function is activated. An LED lights up for the working brake and turn signals but does not light up for the tail lights that dont work on the trailer. Brake, turn, and tail lights work on the truck. I bought a new adapter which also has test LEDs and got the same result. I replaced the factory plug on the trucks wiring harness and still did not have tail lights. I checked the trailer fuse section in the dash and did not see any blown fuses. I replaced broken tail lights on the trailer with LED lights and used the existing wiring which did work until I broke the old tail lights. If it is a trailer ground issue, shouldnt I still get power to the test LED on the tail light section of the adapter plug? It appears to me that the tail light wire is not getting power from the truck somewhere before the trailer connection. The truck harness plug appeared to be free of corrosion and as mentioned above I plugged it into a new connector and 4 wire adapter plug.

asked by: Robert

0

Expert Reply:

I recommend testing the ground connections on the vehicle and the trailer. When you find that some of the lighting functions work, but not all of them, it is always best to check the ground connections.

Check the ground on the connector on your vehicle to make sure it is attached to a good, clean ground on the vehicle frame. Also, check the ground on the trailer connector which should be attached to a good, clean ground on the frame of the trailer.

Next we will check the fuse for the trailer running lights. On the 2004 Silverado/Sierra, the fuse for the trailer running lights is in the fuse box under the hood, not in the passenger compartment like the fuses for the right turn/brake and left turn/brake. Replace the fuse, but before plugging the trailer back in, you need to search for a short circuit on the trailer that could cause the fuse to blow. Trace the running light circuit from the trailer connector back to the lamps, looking for pinched or broken wires, or cracked or worn wire insulation.

Now we can turn our attention to the trailer. Check the ground connections on the trailer. The taillights are suspect for ground problems especially if they are grounded through their mounting bolts. If the mounting brackets for any of the lights are bolted to the trailer frame, the ground connection can be lost there also.

If all the ground connections are solid, look for a broken pinched or frayed wire that is making contact with the trailer frame or another wire in the harness.

I am including a link to a video on troubleshooting a trailer wiring harness.

You should also check the owner's manual for fuses and relays related to the tow package wiring. The only way to really test a relay is to use a known good relay in its place.

I have included a FAQ article on testing 4 and 5 ways that will show you how to test the wiring with a circuit tester such as # PTW2993.

If the problem is occurring when the trailer is connected, and if the taillight circuit on the truck side trailer connector work when the trailer isn't connected, then you could have too many lights on the trailer drawing power and overloading the circuit. This may explain why the LED lights work since they draw less power.

expert reply by: Chris O

0

2004 GMC Sierra Not Sending Current to Trailer Tail Lights

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