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Randy's Electrical Corner - September 2013

Wiring Diagram
Randy | Photographer
Posted August 13, 2013

Trailer Rigging

Ah, trailers—the little red wagon of the Jeep world. I used to love going riding in the little red wagon with my dad pulling it along. Of course, Earl would try to knock me out of it, but that's another story. A trailer is like a good dog, it is always there when you need it, follows you wherever you go, and doesn't ask for anything back. If I had a driver's license, I'd get a job doing something that had me towing trailers. How cool would that be? Getting paid to play with a grown-up version of a little red wagon? I think that would be a great job.

So I'm going on about trailers because I just wired up a '91 Comanche for trailer towing, and it got me thinking about other trucks and cars t hat I've helped people hook to trailers. Sure, the '91 and '92 Comanche wiring is kind of unique. They don't have yellow turn signals, so they are different than Cherokees, and they sure aren't a Wrangler, but maybe some of the tips here can help you in your future wiring.

Now this isn't as easy as just wiring up a flat four plug. No, no. Of course not. This truck needed to get a trailer brake controller and a 7-prong plug. The trailer brake controller is kind of cool. It varies how many amps it sends to electronic trailer brakes based on how you turn the dial and how hard you stop. That way, the trailer stays behind you instead of passing you. It also helps save the brakes on the tow rig. Of course car hauler trailers have brakes, but so do many off-road camping trailers, ATV trailers, and so on.

Most newer trucks and SUVs come from the factory pre-wired for a brake controller, but most of your Jeeps aren't pre-wired so you Jeep guys have more work to do. Although only one new wire needs to be run to the back of the Jeep, you can pick up all the other wires at the back of the Jeep.

A trailer brake controller and trailer plug should have been a really easy installation, and it pretty much was. But I hit a few curves along the way and remembered stuff that just might help you guys if you ever want to wire in a trailer plug or a trailer brake controller. Of course, I had to use a relay. What Randy install is complete without a relay?

Step By Step

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  • A trailer brake controller has four wires. The ground and constant 12 volts are pretty easy. The other two wires are a brake light input and the brake controller output. Older Jeeps didn’t come pre-wired for a brake controller from the factory, so I had to run a new wire all the way to the back of the truck. The brake trigger in a Jeep with red turn signals is the tricky part. If I just hooked to the brake light wire, then when the hazards or turn signals come on the trailer brakes would as well. That’s probably not good.

  • The ’91 and ’92 MJs have a whopping six wires coming off the brake light switch. Some feed info to the ECM for stuff like cruise control and torque converter lockup. The white wire with a pink trace had a ground at all times except when the brake was pressed. So, I wired up a relay like this to get to a 12-volt positive feed for the brake controller. I used a switched 12-volt input because otherwise the relay would be on even when the Jeep was off. This would eventually kill the battery.

  • Rather than use a Scotchlock connector that cuts the wire, I used a pair of strippers to cut the insulation and then a razor knife to cut a piece of it off. Be careful not to cut any of the actual copper of the wire. I cleared 1⁄2- to 3⁄4-inch and wrapped the feed wire going to the relay around it before soldering. If I ever want to remove it, I can de-solder it, or just cut the new wire right next to the solder joint.

  • It seems every old trailer wiring setup is different from every other. But every new truck has the same wiring on their flat seven plugs. When you wire in a new flat seven plug, do it this way. This is looking at the back of the plug on the tow rig. If you are wiring a trailer up, you will have to flip this diagram over or hold the plug so you can see the part that actually plugs in.