Lcc9117

I've had my 1999 Cherokee XJ for about 2 months now and finally got around to installing the wiring harness for my trailer. Used a basic T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector, plug and play deal. Running lights work but the brake and turn signals do not work. I checked and all 4 wires going into the converter have power but only 2 coming out have power. I was told by etrailer that the converter was probably bad so I ordered a 2nd harness from Amazon. Installed yesterday and same issue. Any ideas what I could be missing? I checked the fuse for the trailer harness, replaced with a 15A as well as a 20A and issue remains. Brake lights and turn signals on the car work fine and still work when trailer is attached.

fb97xj1

Are you using the correct harness? Everything connected that needs to be? I know there isnt much to it, but just to be sure:

Lcc9117

Yeah, I double checked the harness compatibility. Checked all other connection options. Sanded down where the ground wire goes to make sure it has a good connection. Not sure what I could be missing.

Lcc9117

any other thoughts? I hate to return this one and buy a third only to run into the same issue.

BlueRidgeMark

Okay, I think we can rule out the unit being bad. Either there is something wrong with your wiring or there is something wrong with what you are doing.

Let's dig in a bit here....


Don't do that. Don't ever put in a bigger fuse than should be there in an attempt to troubleshoot. That's just asking for trouble. Find the fault, and fix it. The correct size fuse is not a fault, so it doesn't need to be fixed.


Have you got a meter or test light with a sharp probe? Or a heavy duty safety pin?

You have 4 wires going into your converter, right?

Brake
Left Turn
Right Turn
Running

So you have (somewhere) a connection into the Jeep's wiring, and you have the 4 wires that go into your converter.

Probe the wires going into your converter somewhere between the connection and the converter. Stick that sharp probe into the wire itself, and make sure you have the correct voltages going into your converter. A test light would be great for this.

Let's see if you are getting the right stuff all the way into your converter.

If you are, probe the wires just as they come OUT of the converter, and see if you get the correct results there.

fb97xj1

Make sure the ground is clean and tight (also check the connector to make sure the terminals have not backed out).


I found a post on etrailers website concerning issues with that same harness, and grounds were the first thing mentioned. And to check the wires for power going into, and out of, the convertor. If nothing works, contact etrailer.


https://www.etrailer.com/question-29978.html

BlueRidgeMark

Thanks to DFlinstone for a correction.

Going in to your converter you should have:

1. Ground
2. Power from battery
3. Left turn
4. Right turn
5. Running lights
6. Brake lights

Coming out you should have:

1. Left turn
2. Right turn
3. Running lights
4. Ground

The convertor will light both the left and right turn elements when it sees the brake light on the vehicle lit, so there's no separate brake lead coming out of the convertor.

So, as mentioned by fb97xj1, check those grounds, and do the tests I suggested.

Idratherbewheeling

Sorry for bumping this thread 5 years later but it was one of the first things that came up when I googled this issue and nothing mentioned so far really solved this issue. I kind of solved my issue and thought it might help some other people.

I too bought one of those plug and play trailer harnesses and had the same issue. Brake lights and turn signals didn't work but the running lights did. I did some testing and this is what I found.
If I plug in the trailer while the brakes are pressed in, the brake lights work. If I let off the brakes and press them again, they stop working. Same goes for the turn signals, if they're on when I plug the trailer in, they work, turn them off and on again, they don't.
When I measured the voltages to the plug with the trailer unplugged, I got about 10.5-11 volts the the brake lights, and about 10 volts for turn signals. Next I measured with the trailer plugged in and the brake lights on. This dropped the voltage to about 8.5-9 which is too low for the converter to work correctly. I suspect it works fine above 10 volts.
To fix this I just replaced the trailer lights with led lights. I know this is sort of a bandage fix and the real issue is probably a not so great ground somewhere in the jeep. Haven't found it yet.

So yeah if you have this issue, make sure you have enough voltage for the converter box to work. Your trailer lights might push it below its threshold. Just because the lights on your jeep are working doesn't mean you don't have a bad ground on the jeep.