How to Make a 9 Pin to 7-Way Adapter for Connecting Semi Truck to Trailer  

Question:

Expert Reply:

I have a solution for you that will allow you to make an adapter, but we do not sell one pre-made.

To adapt your 9-pin trailer plug to work with the 7-way connector on the truck, you would need the 9-pin vehicle side connector that you referenced, part # R9CB. A length of 7-wire cable # DW04914-1 from the 9-pin would be tapped into the relevant circuits behind the 7-way, with the two remaining terminals being connected as needed. For a more temporary install, the 9-way R9CB could be temporarily attached to the truck, with a length of # DW04914-1 wired to the appropriate terminals on the rear of the connector. Wire the other end of the cable to a 7-way trailer end like # A7WCB, which would plug into the 7-way on the truck when needed. The remaining two terminals on the 9 pin would be connected as needed.

I attached a diagram that has a chart with how a 7-way typically gets wired up. To determine the functions of the pins on your 9-way you would need to use a circuit tester like part # 3808 to test each of the pins.

If your trailer has combined turn signals from the brake lights and your truck has separate then you would also need to use a signal converter like part # RM-732.

Most likely you will need to activate the trailer's brakes so an electric brake controller like the P3 # 90195 would be an excellent option. I attached a help article on how to hardwire this controller in.

expert reply by: Jameson C

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