Hitches 101: Electrical Stuff

Okay, part two of my learning-by-writing series on trailer hitches and towing. Last time I covered the Mechanical Doo-Daws; now for the Electrical Stuff. Dad and I did some of the electrical work ourselves and some we had done professionally, so my expertise here varies accordingly!


Lights and Brakes Requirements

For the sake of road safety, all trailers must have lights and heavier trailers must also have brakes. Specifically, by Washington State law, any trailer that weighs 3,000 lbs or more, or whose gross weight is more than 40% of the gross weight of the vehicle towing it, must have operational brakes. Your vehicle manufacturer may have a different recommendation for when brakes are necessary; our Toyota Sienna's manual says that trailer brakes are required for any trailer that weighs more than 1,000 lbs.

Common sense--or at least, my risk-averse version of it--says you'd want brakes on your trailer even if your tiny house were under 3,000 lbs and your tow vehicle were a humungous truck. Without trailer brakes, your tow vehicle has to work harder to stop itself and the additional weight of the house; with them, the trailer can help out and your stopping power isn't diminished. No reason to risk crashing your house because you couldn't stop in time!


The Wiring Harness

In order for the trailer lights to light up, and the trailer brakes to actuate when needed, there needs to be communication between the vehicle and the trailer. This is achieved with a trailer wiring harness and a brake controller.

The trailer has a plug that plugs into a receiver on the wiring harness. This plug has a certain number of pins--either four or seven, usually, but could be five or six--that each correspond to a wire on the harness, each wire corresponding to a function. Small trailers that don't need brakes tend to have a flat four-pin harness, just to operate the lights--tail lights, left turn, and right turn (the fourth pin is the grounding wire). A seven-pin harness is round and the three additional pins go towards operating the brakes, backup lights, and supplying charging power to a battery on the trailer.

Because it's a big trailer that will have a big load, my tiny house trailer has a seven-pin harness. Each pin is blade-shaped. Seven-pin harnesses with round pins exist, but are rare.

We went about installing the wiring harness in two stages. First we installed a flat four-pin harness, which, in the case of our towing prep package-equipped Toyota Sienna, was a cinch. We bought a wiring harness kit, unplugged and removed the taillights, loosened some interior trim panels from the back end of the car, plugged the appropriate ends of the harness into the taillight plugs, threaded the wires under the panels, and then put the taillights and panels back in place. (I.e., we just followed this etrailer.com video.) If we were to stop there, then we'd wad up the business end of the harness and tuck it into the cavity where the jack lives.

However, we needed a seven-pin plug. So, then, we--that is to say, Dad--installed a four-pin-to-seven-pin adapter. This plugged into the four-pin harness, and then he connected the three additional wires had to be connected by hand. One of these wires connects to the brake controller. The seven-pin plug attaches to the underside of the back bumper, next to the trailer hitch's square receiver. Next to it is a four-pin plug, so we can still tow a little trailer with a four-pin plug.

Caps off to reveal the two different styles of plug.

The Brake Controller

The brake controller is the interface between your brake pedal and the brake wire on the wiring harness--which, in turn, is the interface with the trailer's brakes, themselves. The controller makes it so that when you press the vehicle's brake pedal, the trailer's brakes actuate, as well. Installing it was the step that we farmed out to our mechanic, so I don't know much about how it's wired in. (Some trucks, apparently, are pre-wired so the brake controller plugs right in, but our van was not.) The visible end result is a little box attached under the driver's side dashboard with a few simple knobs and controls that let you vary the power and timing of the braking as well as manually actuate the trailer's brakes, if needed.

And we got our trailer home in one piece, so we must have done everything we were supposed to do!