No Tail Light Function on 16 ft Car Hauler Towed by a 2004 GMC Sierra 2500

Question

Question:

I have a 2004 GMC Sierra 3/4 ton pickup. I hook it to my 16foot car hauler and only the signal lights and brake lights work. When I turn on the tail lights I have nothing. I have checked power from the pickup and its good. I have also checked all the wiring on the trailer and at both junction boxes on the trailer. I am not sure of what the position for the yellow, and blue wire. On the pickup the center pin is not hot . I used a test light and went the old school way and still am not sure what is going on. All the wiring looks brand new and all connections are sealed as well as the junction boxes. Any suggestions on this would greatly be appreciated. I am at a loss.

asked by: Wayne

1

Helpful Expert Reply:

Based on your description, it sounds like your truck has a 7-way trailer connector. Anytime your trailer lights are not functioning properly, it is best to isolate the issue to either the truck or trailer. It sounds like you have already started some of the troubleshooting, but I will list the steps from the beginning just in case.

The first and easiest step to take is to check the vehicle side and trailer side connectors for any corrosion. You said your wiring looks new, so this may not be a problem, but it is still best to check just to be sure. Corrosion could show up as green or white discoloration on the pins. Clean or replace as necessary.

The next step is to use your test light and test the function of each pin of the 7-way trailer connector on your truck. I have attached a diagram that shows what the function of each pin should be. Have a helper active each of the functions while you test the pins.

If the pins are not functioning as they should on your 7-way connector, then we know there is an issue on your truck. If you have a factory installed 7-way, common causes for these problems are blown fuses or damaged wiring. You can check for blown fuses in your power distribution box and you can check for damaged wiring behind your trailer connector. If you have an aftermarket 7-way, check for loose wire connections or damage sections of wire that could cause a short and repair or replace as necessary.

If the 7-way connector on your truck is functioning properly, then we know the issue is somewhere on your trailer. One of the most common causes for trailer light failure is poor ground connections. Check the ground connections for your trailer connector as well as your trailer lights. Make sure they are clean and free of corrosion. If any grounds are not ran through the main trailer ground, make sure they are attached to a clean, bare metal surface on the trailer frame. Also check for any pinched, frayed or damaged sections of wire that could cause a short.

If the grounds and wiring look clean, use a circuit tester to probe the tail light wires on the trailer. Start at the 7-way connector and work your way back toward the tail lights. This will tell you where the signal terminates. If there is no signal for the tail lights just behind the 7-way connector on the trailer, then there may be an internal wiring issue in the trailer connector. I have attached a diagram for the trailer side 7-way connector for you as well.

I have also attached an FAQ article on trailer wiring that should help when troubleshooting the wiring on your trailer. If you are not able to find a solution or the problem after following these steps, let me know what you come up with and we can take further steps.

expert reply by: John H

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