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Old 09-07-2022, 10:24 PM   #1
norcal retiree
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Question 5th-wheel trailer brake wiring?

Just purchased a used Keystone Cougar 5th-wheel RV and hooked it up to my 2018 GMC 3500 for the ride home. That's when I discovered that my truck does not recognize the trailer hook-up and thus, no trailer brakes! Does GMC use a different wiring pattern? Never had a problem with my older RV hooked up to the same truck. Any thoughts/ideas are greatly appreciated.
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Old 09-08-2022, 01:02 PM   #2
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7 pin connector is the same for every truck and trailer. Considering your previous trailer connected with no issues, then it's quite obvious there is a problem with the trailer.

So, a bit more information is needed.

First, do the running lights come on in the camper when you turn the truck head lights on?

Next, does your break controller recognize the "gain" when the trailer is plugged in. If there is no connection, the "gain" when applying brakes will just sit on zero.

Next, any turn signals, 4 way flasher lights, or brake lights working?

If all of this is dead, then my first guess would be you have a bad ground. Are you using one of those silicon fifth wheel hitch pads between the king pin and the fifth wheel hitch? If so, remove it (temporarily) and see if the trailer running lights come on. If they do, then the ground wire on the camper 7 pin is broken. The silicon pad works like an insulator and will not let the truck be grounded.

If removing the pad still does not work, then something bigger is going on.
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Old 09-08-2022, 01:27 PM   #3
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That's awesome! Thanks for the excellent info! I'll report back and let you know what I find. Thanks again!
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Old 09-11-2022, 04:44 PM   #4
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So far, I've found the trailer brake fuse on the truck blown. Replaced fuse, still not recognizing trailer brakes. Thought I had found the problem.
All running, t/s and brake lights work fine. There was some rust/corrosion in the 7-pin connecter, which I cleaned. I also visually inspected the cable and wire connections. Next step is to hook up trailer without the nylon pad.
If no change, then I guess my next move is to start pulling wheels and inspecting the magnets and wiring.
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Old 09-11-2022, 06:21 PM   #5
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Double-checked fuses, then checked for power at the number 2 pin at the 7-pin bed outlet connecter - no power with key in run position. Now it's starting to look like it's a truck issue and not a trailer issue.
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Old 09-12-2022, 04:52 PM   #6
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Run a wire to trailer end of plug top centre, to the next pin to your right. This should turn on your trailer lights. You may have issues truck side and trailer side so this should help nail it down.
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Old 09-14-2022, 07:37 PM   #7
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I crawled around under the trailer to see if I can find anything. I discovered that at one time, in it's past, it had a tire blowout because the brake wiring for that wheel was noticeably longer and obviously had been replaced. Upon closer inspection, I found that the longer wires had been rubbing against the wheel and was almost severed in half. The edge of the wheel was polished like chrome from the wires rubbing. I'm guessing that's what blew the fuse on my truck?
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Old 09-15-2022, 11:24 AM   #8
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Quote:
Originally Posted by norcal retiree View Post
I crawled around under the trailer to see if I can find anything. I discovered that at one time, in it's past, it had a tire blowout because the brake wiring for that wheel was noticeably longer and obviously had been replaced. Upon closer inspection, I found that the longer wires had been rubbing against the wheel and was almost severed in half. The edge of the wheel was polished like chrome from the wires rubbing. I'm guessing that's what blew the fuse on my truck?
Sounds like you found the problem!
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