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Running Lights on Trailer do not Work  


I have a gmc sierra 2500 HD , 2006. I wired up a new 7 pin plug and everything works, except the running lights. I tested it with my trailer and a tester. Looks like there is around 20V on the running lights pin. That seems to be a lot. I also checked all the fuses that have anything to do with the trailer. I also made sure the groundwire is clean. Its all good. What am I missing?


Expert Reply:

I would make sure that your tester is on the right setting because unless you have some kind of amplifier on that wire there should be no way that you are reading 20V from the 12V battery in your 2006 GMC Sierra. What you are probably reading is something like 20 amps. Since you are actually getting power to the connector on your Sierra what you will want to do is make sure that your trailer side is connect.

Double check the connector on your trailer and make sure that there is no debris or corrosion that would be causing the signal to malfunction. If the connector looks good then you will want to test the connector using an alternate 12V power source like a direct connection to the battery in your Sierra or another tow vehicle. If you test that and it still doesn't work then you will want to look at all of the grounds on your trailer, including any grounds that your lights might be connected to. Make sure the grounds are connected to clean, bare metal surfaces; even some paint can cause issues sometimes.

If all of that looks good then you will want to inspect the wiring itself. Looks for any places where the wiring might be bent, damaged, corroded, or exposed and replace it using 16 Gauge Wire # 16-1-1 and Butt Connectors # DW05744-5.

Since you have checked all of the wiring on your Sierra, if you do all of this troubleshooting and still cannot find anything then you should probably take your setup into a professional mechanic who can physically inspect it.

expert reply by:
Jon G

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