How to Adapt 4-Pole Trailer Wiring for Connection to Truck with a 7-Way Trailer Connector  

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Expert Reply:

No worries, we can help you!

If your trailer still has a 4-pole plug like the one shown in the linked photo then you may be able to use a simple adapter like # PK12716 or # HM47065 to connect your trailer to your truck's 7-way.

If you have just the wire leads then you can make a more reliable permanent modification to the trailer so it matches your truck's connector without the potential reliability issues that can occur when using adapters and the extra connections they create. Usually a 4-wire trailer harness has white, green, yellow and brown wires that are USUALLY for ground, right turn/stop, left turn/stop and running lights. If the trailer has been re-wired (it sounds like it) then different color wiring may have been installed. So you will test!

First you'll identify the function of each trailer wire with a 12V battery, either the truck's or a spare if you have one. Simply connect the white ground wire to the negative battery terminal and then one at a time connect each of the others to the positive terminal to see what light function is activated.

Once you know each wire's function you can then wire in a 7-way trailer plug such as # H20046 by installing a junction box on the trailer, such as # 38656. You will match the FUNCTIONS of each wire, not their colors, based on your testing of the trailer. Since your trailer has only 4 wires for its lighting, you'll make 4 wiring connections inside the junction box rather than 7. The other wires on the 7-way plug can simply be taped off and remain inside the junction box.

On the # H20046 harness the functions for each wire color are as follows:

Ground = WHITE
Left/Stop = RED
Right/Stop = BROWN
Running Lights = GREEN

Just match the FUNCTIONS and make your connections within the junction box using ring terminals # 44-5310A. You'll need eight of these.

expert reply by: Adam R

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