Explainer

Trailer brakes are among the most overlooked legal requirements in towing, partly because the trailer may have them while the truck lacks any means to operate them.

The requirement

Most states require a trailer to have functioning brakes above a defined gross weight. The threshold varies meaningfully, and a trailer legal to tow unbraked in one state may not be in the next.

Many states also require a breakaway system — a device applying the trailer brakes automatically if the trailer separates, powered by its own battery.

That battery is a real maintenance item. A breakaway system with a dead battery satisfies nothing.

What the controller does

Electric trailer brakes need a controller in the tow vehicle to tell them how hard to apply.

Time-delayed controllers apply a preset amount over a set ramp. Simple and inexpensive; the trailer braking does not respond to how hard you are actually stopping.

Proportional controllers sense the tow vehicle’s deceleration and match it. Smoother, and considerably better in an emergency stop — which is the situation that matters.

Many trucks now include an integrated controller from the factory. That is generally proportional and correctly wired, and it is also another electronic system that can fail, as Ford’s integrated trailer module recall demonstrated.

Setting the gain

Controllers have a gain adjustment, and leaving it at default is both common and wrong. It needs setting for the trailer and load, and resetting when either changes.

The usual method: find a clear, safe stretch, tow at low speed, and apply the trailer brakes alone with the manual override. Harsh grab or lock-up means gain is too high. Barely slowing means too low.

Why it matters more than it looks

An unbraked or under-braked trailer pushes the tow vehicle during a stop, and that push is what initiates a jackknife.

Trailer brakes are also what let you stabilise a sway event — applying them alone pulls the rig straight, where using the tow vehicle brakes can make it worse.

This article was researched and drafted with the assistance of AI tools and reviewed, fact-checked and edited by Leonidas Clark before publication. General information, not advice for a specific vehicle.