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DIY Trailer Wiring Harness Diagnosis and Repair

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While towing a trailer, safety should be first and foremost on your mind. Tire pressure, trailer brakes and trailer lights are all critical to ensuring you and your cargo arrive in one piece. Trailer lighting, turn signals and brake lights inform other drivers of your intentions.
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If one or more of your lights aren’t working, you may just have a blown bulb, or you could have a problem with the trailer wiring harness on the tow vehicle or on the trailer.

If you have a trailer lighting or braking problem, check the tow vehicle’s trailer wiring harness first. You should have a digital volt/ohm meter (DVOM) in your toolbox, to make basic circuit checks. Once you confirm the tow vehicle’s plug has the proper ground and voltage, you can go on to check the trailer wiring harness and bulbs for problems. To check the trailer lighting system, plug it into the tow vehicle, and use the DVOM to check for voltage across whatever bulb’s socket you are having a problem with.

Pro tip: While checking exterior wiring on a vehicle or trailer, do not pierce the insulation with your DVOM probes. You can indeed check for voltage and ground this way, but you will also compromise the waterproof insulation. Corrosion will then be in your future, and you will be fixing this issue again.

Trailer Wiring Harness Diagnosis

The typical flat, four-pin connector, for example, should have ground on the white wire, then 12 V on the brown, yellow and green wires, depending on if the taillights, left turn signal or right turn signal are illuminated. Also, brake lights and turn signals are combined, so both yellow and green should have 12 V with the brake lights illuminated. Five-, six-, or seven-pin trailer connectors will have additional connections for the electric brake, reverse lights, brake lights or auxiliary power supply.

Although one can use a DVOM for these checks, a dedicated trailer tester can be an even easier way. Testers for four-pin connectors typically plug right in. LEDs should illuminate for each circuit activated. If nothing illuminates, then you may have a ground problem. If the tow vehicle is equipped with a trailer converter box, you will need to check inputs and outputs of the box itself. Once you determine the problem, make any necessary repairs, but never by-pass a faulty converter box — always replace it.

Trailer Wiring Harness Repair

Because trailer wiring is typically exposed to the elements, wire taps and crimp connectors are asking for corrosion. So to make a long-lasting repair on exposed wiring, follow these three steps:

  1. Crimp the exposed wires using an uninsulated butt connector.
  2. Solder the connection to ensure strength and conductivity.
  3. Use waterproof adhesive-lined shrink tubing, at least one-half inch past each end of the connection, to prevent water intrusion.

Diagnosing and repairing trailer wiring isn’t particularly complicated, but will require patience. Following this guide, you will be able to easily correct any problems with your trailer lighting or brakes and make lasting repairs.

Check out all the towing products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on trailer wiring harness diagnosis and repair, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Wikimedia Commons


Benjamin Jerew View All

Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.

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