Diagnosing Problem with Tow Package Wiring of 1999 Chevy Silverado

Updated 08/15/2013 | Published 08/13/2013

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Question:

I have the above mentioned pickup and the wiring is stock with the 7 pin factory tow package connector. This morning I plugged in my trailer that has a matching plug and even all lights on the truck work perfectly, none of the trailer lights worked except the left turn signal. I took out my 12 volt tester and then found that none of the wires at the female receptacle on the truck have power. The trailer is almost new and I am sure that there is no problem with the lights as my son had just used in on his vehicle with the same plug and everything worked fine. As I said, I used my tester and did not have any power coming to the receptable on the truck. I checked the fuses in the fuse box on the drivers side of the dash by the door and all fuses were good. I looked at an on line wiring diagram and see that there is something called a body control module that may or a black box uner the truck that appears to be related to the towing plug in. Do you have any suggestions to get this thing working? Thanks for you insight in advance.

asked by: Greg H

Expert Reply:

Since you have checked the 7-way on the truck, and are not getting signals on any but the left turn pin, try unplugging the wiring harness from the rear of the 7-way. Use your meter on the pins of the connector that plugs into the rear of the 7-way, as each lighting function is activated.

If you have signal at the plug, the 7-way is faulty and should be replaced. Part # 74682 would be a good replacement. If you are not reading a signal at the plug, you likely have blown fuses in the power distribution box under the hood on the drivers side fender. The lid of the distribution box will have a legend on it showing which fuses protect which circuits. Check the towing fuses, and replace if necessary. Before plugging in the trailer again, you should take a few minutes to examine the wiring on the trailer, looking for a short that caused the fuses to blow. Even through the trailer is fairly new, it is entirely possible that a rock or other road debris could have been kicked up and damaged the wiring. Starting at the connector on the trailer, trace each circuit back to its lamp. Look for pinched wires or nicked insulation that would allow the wiring to short out on the trailer frame.

If the fuses are not blown, your least complicated and expensive option bet would be to bypass the lighting portion of the tow package wiring using a taillight converter, part # C59496. You could cut off the 4-pole and wire the outputs into your 7-way as shown in the wiring FAQ I will link you to.

expert reply by: Mike L