The scoop on wiring a trailer to tow
Will that be a four- or seven-pin connector? And make sure you’re not creating electrical problems
Setting up a trailer for your daily driver used to be simple. It was even an easy DIY task for the most tool-challenged. You’d visit your favourite parts store, grab a hitch and a wiring kit, and after a few hours of driveway labour, you and your trailer were ready to hit the road. Today, trailer and automakers seemed to have conspired to make this an almost impossible job.
First, for almost any compact or subcompact passenger cars sold today, there are no manufacturer-approved trailer hitches or wiring because they aren’t rated to tow anything. This has never stopped owners from going to the aftermarket to equip their rides to tow a small utility trailer, but this practice does risk denial of any powertrain warranty claims.
Secondly, many vehicles — even entry-level units — are equipped with some very sensitive wiring systems. Simply patching in a trailer wiring harness can lead to a host of problems, such as anti-lock brake system faults to exterior lighting problems.
Finally, many trailer manufacturers are equipping their units with trailer brakes, even smaller models. This means having to deal with a round, seven-pin plug rather than the older, four-pin flat ones. The latter setups supply power to trailer lights for running, turn signal, and brake lamp functions, while the former configurations do all this, plus provide power for electric trailer brakes and optional reverse lamps. Adding trailer brakes to a light utility trailer might seem like overkill, but in panic stops they can help keep your vehicle and the trailer under control.
As many vehicles use the same bulb for both turn signals and braking,
most proper trailer wiring harnesses come with solid-state adapters that translate these signals into types that will light up the appropriate trailer lamps.
The best types of wiring kits have adapters that install without having to cut or splice wires. These are not always available for specific vehicle applications, so you may be left
to cut or splice on your own. Avoid using the blue plastic connectors known as ‘scotch locks,’ as these clamp around the wire and copper edges inside, piercing the insulation to make contact with the copper wire. They’re almost impossible to weatherproof, leaving a perfect entry point for corrosion.
While many four-pin wiring installs
can be done yourself, seven-pin setups are more involved. These require running a dedicated power supply line from the rear of the vehicle to the engine compartment, as well as a signal lead from the car’s brake system and a trailer brake controller.
These controllers allow drivers to adjust the amount of braking done by the trailer’s units.
The big downside to trailer brake controllers is their mounting point — usually under the dashboard on the driver’s side, where they are constant targets for painful knee hits. A handful of trailer companies now market controllers with remotely mounted switches, allowing the it to be neatly tucked out of the way and still allow the driver an easy reach.