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Troubleshooting Intermittent Trailer Brake Signal for 2008 GMC Sierra

Question:

I have reviewed a number of posts referencing trailer brake problems but still have questions as to why my new fifth wheel trailer brakes stop working. I have determined the fault is not in the trailer and have also determined it is in fact in the truck. I have been using the circuit tester as described in related posts. I ordered, but havent been able to locate on the truck, the trailer brake relay GM Part# 20904439. However, there is a device in the rear of the truck where four modular plugs are terminated. I unplugged each one and sprayed circuit cleaner as a process of elimination even before doing this the control circuit appeared to be working. To date, I have replaced the controller, male and female trailer connectors, added new ground connection to truck frame and unplugged and cleaned the four modular plugs. Im hoping the brake signal will continue working properly. BTW, the truck is a 2008 GMC Sierra one ton dually 6.6 duramax crew cab 4WD. Im interested in knowing if anyone else has experienced this gremlin.

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Expert Reply:

You have checked quite a few things so far but you have you checked the fuses and relays under the hood of your 2008 GMC Sierra? That's where the circuit protection is located. If you check out the help article I attached you can see at least where the fuse box is and what wiring needs to be connected there for the circuits to be active. On the fuse box lid there will be a diagram for the fuses and relays as well.

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Jameson C

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