I recently added a T Harness to my car for towing. I traced the problem back from the trailer to the car's factory wiring. A voltmeter with the leads plugged into a tail light socket produces a negative reading when the running lights are turned on. Because of this, my trailer lights don't work properly and the trailer's wiring shows negative voltage on the yellow, green and brown wires simultaneously. The car's lights all work fine, and my trailer has turn signals, hazards, and brake lights, but nothing else.
Thanks Syntax Error for letting us know you figured out the wiring harness was wired incorrectly. It is always a good idea to look up the color wiring standard for the socket and to check the pin positions and what functions .
Sometimes trailers can be wired incorrectly also ?
I wired the licence plate bulb to my brown wire by mistake and this is the brake wire . Took a while for me to figure that one out !
SOURCE: I have a 1997 Ford F150 4x4 Supercab Stepside with
It is a possible option that the relays for the Stop / Turn are having an issue or the fuse in the Engine Mini panel is blown ( page 142 of the 1997 owners manual )
On the 1997/98 MY this is in the aux relay box in between the Engine Compartment fuse panel and the engine mini fuse panel.
The output of the MFS ( Multifunction switch ) triggers the relays, to use the trailer tow fuse to operate the trailer lamps. The fuse to supply power to the Normally Open contacts on the trailer relays should be hot at all times. You can verify this with the diagram below by metering from pin #3 to pin # 2 [ground for coil] or to another known ground.
These are 1/2 size Ford ISO relays, and can be jumpered to make testing easier ( and with this diagram you can bench test them with a meter to check resistance over the Normally Open contacts, when applying power and ground to the coil ).
I am not 100% sure on the 97/98 MY F-150 about the location of this connector, but the 99-03 have all the wires to the rear of the truck via a connector along the driver's side frame rail, that you can check for ( easiest with the spare lowered )
Start with the Fuse in the Engine Mini panel, to see if the 20A fuse is good and work back towards the trailer tow adapter via the relays. I am going with the obvious blown fuse for the Stop/Turn lamps for the trailer.
SOURCE: I have a 2008 Ford F 150 with factory installed
The trailer tow adapter parking lamps is fuse #10 in the cab fuse panel. There is also a trailer tow parking lamp relay that powers them, which could have a problem as well.
Start with the fuse, and maybe check both the 7 pin and the 4 pin for the parking lamps, could be a corroded pin on one of them causing the tester to not see the power on the pin.
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I've checked the headlight switch in the dash for any burned wires or contacts. No problems found there.
I tried a Hopkins Vehicle Converter #48845 yesterday and it didn't change anything.
Just had my rear running lights die this morning when I plugged in my trailer. Checked fuse panel under the dash and found #9 was blown. It's a 15 amp for (quoting the owner's manual here): Autolamps (park), instrument panel illumination, keyless entry module, license plate lamp, parking lamps, and tail lamps. I replaced the fuse after I was done using my trailer and my rear running lights work again, but still have the original problem.
So the question is why did this fuse blow? I'm thinking there's a wire grounding out somewhere under the carpet.
Problem finally solved. After extensive tracing and checking of car's wiring and finding it all fine, I went back to the T harness adapter and did the same with it. I also tested and wrote down the wire color and function of each of the car's wires at the tail lights, and did the same with the T harness. This is how I finally found the problem. Turns out 2 wires on the T harness were switched. Switching them around to match functions with the car's wiring fixed the problem. Lesson learned, always checked the most simple stuff first.
Someone please close this issue.
And never assume a new part is properly built. That's what caused all my frustration to begin with.
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