How To Install A Trailer Wiring Harness
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How To Install A Trailer Wiring Harness

Installing a new trailer light wiring harness on a car, truck, or SUV is an easy project that any mechanically inclined person can do.

Installing a wiring harness in your car, truck, or SUV is an easy project that any mechanically inclined person can complete successfully. Every installation is slightly different, so you need to consult the service manual for your vehicle for the exact procedures. Barring any unforeseen complications, you should be able to complete the basic installation in a couple of hours on a Saturday morning. Saturday Morning? Back in the time when I got my start in auto mechanics there was a magazine that ran a monthly section called "The Saturday Morning Mechanic." Before that magazine went where all legitimate magazines go when they go out of print, they had changed the name of the department to "The Shade Tree Mechanic."

What You Will Need

  • Basic mechanics hand tools
  • 12 volt circuit tester or a digital Multimeter (DMM)
  • Wiring harness
  • 804C Scotch Lock Connector - Blue Gel 3M connectors
  • Service manual

Step 1: Remove The Tail Light Assemblies.

Remove the tail light assemblies on your vehicle. You want to access the wiring harness, and that will require the removal of the Tai lights on most pick up trucks and SUVs. Consult your service manual to find out how to go about removing them. All the fasteners that you will have to remove are not always in full sight.

Step 2: Sort Out The Vehicle Wiring Using A Schematic

Determine the function of each wire in the wiring harness supplying the tail light assembly. You need to know which wire serves the brake lights, the right and left directional signal lights, and the tail/running lights. If you have a service manual this is easy because there will be a wiring schematic in the manual that gives their colors. If you do not have a schematic, you will have to sort them out using a 12-volt test light or a DMM. It will help if you have a helper during this step.

Step 3: Sorting Out The Wiring Using A Test Light Or DMM

  • Connect the ground wire from the voltage tester to a good ground.
  • Have you helper press the brake pedal while you touch the tip of the test light to each of the terminals in the harness plug. When the test light come on, you have found the wires for the brake lights. Mark it down on a slip of paper, its color, and its function.
  • Have your helper turn the right directional on while you probe for that wire. Repeat for the left directional.
  • The remaining wire will be the tail light/running light wire

Step 4: Cut The Wires On The Vehicle Wiring Harness

  • Cut the wires for the brake lights, directional signals, and the taillights. Insert the two cut ends of each wire into the opposite sides of a Scotch Lock connector. If you take a close look at these two pictures of a Scotch Lock connector, you will see that one side has two openings while the opposite side has one opening. The circuit will work no matter how you position the wires but it is customary to put the original wires on opposite sides so they face one another.

  • Push the wires all the way in the connector until they butt against the opposite side of the connector.
  • Using a pair of pliers, push the metal plate down on the three wires, piercing their insulation.
  • Close the connectors cover sealing it against moisture.

Typical Trailer Harness Color Code

  • The brown wire goes to the parking lights or taillights.
  • The green wire goes to the right turn signal/stoplight.
  • The yellow wire goes to the left turn signal/stoplight.
  • The white wire goes to the common wire or to the chassis ground

The "Female" half of the connector connects to the vehicle while the "Male" half connects to the trailer wiring.

Step 5: Verify Wiring

Using your test light, make sure that each wire has voltage to it at the proper time.

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Comments (2)

You are always the expert, Jerry, well done.

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