How to Wire a Trailer Breakaway Switch

by K.K. Lowell

All trailers with electric brakes are required to have a breakaway switch to apply the brakes if the trailer becomes disconnected from the tow vehicle. Many older trailers do not come equipped with a breakaway switch. If you own an old trailer, it's easy to install a breakaway switch. Purchase a kit that contains the switch, a battery and a battery box. For trailers that already have an on-board battery, a breakaway switch kit is not necessary.

Step 1

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Select a location to mount the trailer breakaway switch. It should be located on the trailer tongue with the cable facing forward; the cable must be able to be pulled straight. Mount the switch to the trailer with a self-drilling screw through the hole in the mounting tab.

Step 2

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Mount the battery box inside the trailer frame in a protected location. The battery is equipped with four 5/16-inch holes, but it is only necessary to use two of them. Install the mounting bolts diagonal from each other. Position the box and mark the holes, then remove the box and drill the holes. If the trailer has an on-board battery, skip this step.

Step 3

Install a solderless ring terminal on the breakaway battery's negative wire. Scrape the paint from a small area near the battery box. Attach the ring terminal to the bare spot with a self-drilling screw.

Step 4

Connect one lead of the trailer breakaway switch to the breakaway battery's positive wire. Perform the splice with a solderless butt connector. If necessary, extend this wire with 14-gauge automotive primary wire and solderless butt connectors. Tape all connections with electrical tape. If the trailer already has an on-board battery, it is only necessary to extend the wire from the breakaway switch to the battery's positive terminal.

Step 5

Locate the trailer connector's blue wire that supplies power to the electric brakes. You may have to cut into the trailer wiring's sheathing to find the wire. Connect the breakaway switch's other lead to the blue wire with a Scotchlok connector. If necessary, extend the wire from the trailer breakaway switch with 14-gauge automotive primary wire and solderless connectors.

Step 6

Test the breakaway switch by pulling on the cable until the insulating tab is removed from the switch. The trailer brakes should lock up. Make sure to put the insulating tab all the way back in the switch after performing this test.

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