Adapting Truck with 5-Way Connector to Trailer with 7-Way Connector

Expert Reply:

A 5-way to 7-way adapter is not offered. The best way to proceed would be to install the 7-way connector on your Truck, and if you should need to tow a boat trailer in the future, use a 7-way to 5-way adapter, such as part # A75TB.

In order to utilize the electric brakes on your trailer with the 7-way connector, you will need to install a brake controller to actuate them. I would recommend a proportional controller, such as the Tekonsha Prodigy, part # 90185. As the 1998 GMC trucks were not prewired for installing a brake controller, we offer an Installation Kit part # ETBC7 which would include everything needed to complete the installation, including the 7-way connector for the vehicle.

Some of the wiring needed to install the controller might already be in place on your vehicle. Take a look under the rear of the truck, along the drivers side frame rail and the area above the spare tire. If you can locate an orange and a blue wire, you can save yourself some time. The blue wire will be used to carry the brake output signal from the brake controller to the 7-way connector. The orange wire is used to carry the 12V accessory power from the battery to the trailer connector. The other end of these wires can be found under the master cylinder in the engine compartment. If you will follow the brake lines down from the brake booster, you can find the wires wrapped in electrical tape down by where the brake lines turn to pass under the truck cab. The orange wire would be extended and run to the positive battery terminal via a 40 amp circuit breaker included in the install kit. The blue wire, which carries the voltage for the trailer brakes, would run through the firewall to connect to the brake controller. (When passing wiring through the firewall, make sure that the opening uses a grommet, to prevent damage to the wires which could create a short-circuit).

The brake controller would then be installed in the cab. Connect the blue wire on the controller to the blue wire that you have run through the firewall, the white wire would be extended and run through the firewall to the negative battery terminal, the black wire would be extended and run to the positive battery terminal via a 20 or 30 amp circuit breaker included in the install kit. The red wire would attach to the cold side of the brake stoplight switch. The brake stoplight switch is located at the top of the brake pedal. It will have several wires coming out of it, use a circuit tester, such as part # 40376 and probe each wire until you locate the one that only goes hot when the brake pedal is pressed. Tap the red wire into this circuit.

expert reply by: Mike L