Electric brake controllers: which type you need, and the rules

Which type?

Type	How it brakes the trailer	Suits
Proportional (inertia)	Senses the vehicle's deceleration and applies matching force	Caravans and heavier trailers; smoothest
Time-based (delay)	Ramps up to a set level over a set time when you brake	Lighter trailers, occasional towing
Wireless or in-trailer	A module in the trailer, adjusted from a phone or remote	Swapping between vehicles; tidy installs

All of them have a manual override, so you can apply the trailer brakes alone to settle sway.

What the rules require

Under Vehicle Standards Bulletin 1, trailers over 750 kg gross trailer mass must have brakes, and except for over-run (override) brakes, the braking must be "activated by a control signal generated by the tow vehicle". Over 2,000 kg, the brakes must work on all wheels and the trailer needs a breakaway system. See electric trailer brakes for the full rules.

Installation

A controller needs a heavy-gauge feed from the battery with a circuit breaker, an earth, a stop-light signal, and an output to pin 5 (blue) of the trailer socket. On modern vehicles the wiring often runs through a vehicle-specific harness. An auto electrician is the safe choice; set the gain with the loaded trailer on a quiet road.

Planned range

When we open, we plan to stock proportional under-dash and wireless controllers, wiring kits and breakaway kits. No prices or stock yet.

Proportional controllers in more detail

A proportional controller contains a sensor, usually an accelerometer, that measures how quickly the tow vehicle is slowing down. Brake gently and it sends a small signal to the trailer brakes; stop hard and it sends a strong one. Because the trailer brakes in step with the vehicle, the combination stops in a straight line without the trailer pushing or snatching. That smoothness is why most caravan owners, and most auto electricians, recommend a proportional unit for anything heavier than a small box trailer.

Proportional controllers must be mounted at the angle the maker specifies, usually within a set range of level, and pointed the right way along the vehicle. A unit mounted crooked or loose reads the wrong deceleration and brakes too much or too little. Most have a calibration or levelling step when first fitted.

Time-based controllers

A time-based controller does not sense deceleration. When you press the brake pedal, it ramps the trailer brakes up to a level you set, over a time you set. It works, but a light touch of the pedal can still bring the trailer brakes on hard, and a hard stop can feel like the trailer lags. They suit lighter trailers and occasional towing, and cost less.

Wireless and in-trailer controllers

Some controllers mount in the trailer rather than the vehicle, sensing deceleration there and taking power through the trailer plug. You adjust them from a phone app or a small remote in the cabin. They are handy if you tow with more than one vehicle, because the controller stays with the trailer, and they avoid drilling under the dash. Check that the vehicle's trailer plug supplies enough power through the right pins, and that the app connects reliably in the vehicle you drive.

Setting the gain

Gain is how strongly the controller applies the trailer brakes. Set it with the trailer loaded as you will tow it, on a quiet, dry, flat road. Drive at a low speed, brake firmly and feel the combination: the trailer should slow itself without locking its wheels or pushing the vehicle. Increase the gain until the trailer brakes are working hard just short of locking, then back it off a little. Re-check after changing the load, when roads are wet, and on long descents. Many controllers also have a boost setting for heavy loads.

The manual override

Every controller has a manual lever or button that applies the trailer brakes on their own. If a caravan starts to sway, gently applying the trailer brakes alone, without the vehicle brakes, pulls the combination straight. Practise reaching it without looking, so you can use it calmly in an emergency.

Breakaway systems

Trailers over 2,000 kg must have a breakaway system that applies the trailer brakes automatically if the trailer comes off the tow vehicle. It has its own battery in the trailer and a cable attached to the vehicle; if the trailer separates, the cable pulls a pin and the brakes apply. Check the breakaway battery is charged before each trip, and that the cable is attached to the vehicle, not the coupling.

Choosing a controller

• What you tow: a caravan or heavy trailer suits proportional; a light trailer, either type.
• How many vehicles: an in-trailer unit if you swap tow vehicles often.
• Your vehicle: some late-model vehicles need a specific harness or have integrated controllers.
• Display and controls: clear gain and output display, and an override you can reach easily.
• Support: a local supplier or auto electrician who knows the unit.

Testing before every trip

Before each trip, plug in the trailer, check the controller shows the trailer is connected, and test the brakes at walking pace in the driveway with the manual override. A controller that shows no connection points to a plug, socket or wiring fault, often a dirty or bent pin, which is far better found at home than on the highway.

Common faults and fixes

• No trailer connection shown: check the plug pins are clean and straight, the socket is not corroded, and the trailer's brake wire reaches pin 5.
• Brakes grab at low speed: reduce the gain, or check the controller's mounting angle.
• Brakes feel weak: increase the gain, check the brake magnets and drums are not worn, and check the wiring gauge is heavy enough.
• Controller loses power: check the circuit breaker near the battery and the earth connection.
• Brakes work on one side only: check the trailer's brake wiring and magnets.

Many brake problems come from the trailer, not the controller, so service the trailer's brakes as its maker recommends.

Installing in modern utes

Many current utes have a plug-and-play brake controller harness from the vehicle's accessory range or from the controller maker, which connects behind the dash without cutting factory wiring. Others need a brake controller feed run from the battery to the rear of the vehicle. Some vehicles have an integrated brake controller set through the dashboard screen. Ask the auto electrician which method suits your vehicle, and keep the controller where you can reach the override easily.

Controllers and electric-over-hydraulic brakes

Some heavy trailers and boat trailers use electric-over-hydraulic brakes, where an electric pump drives hydraulic disc brakes on the trailer. These need a controller that is compatible with the system; some controllers have a setting for it. Check with the trailer maker and the controller maker before buying, because a mismatched controller can make these brakes behave poorly. Boat trailers that are dunked in salt water often use this type because disc brakes cope better with immersion.

Cost of a controller

A brake controller itself is a modest cost compared with a caravan, but the installation matters as much as the unit. Budget for the controller, a vehicle-specific harness or a battery feed with circuit protection, the trailer plug if it needs upgrading, and an auto electrician's time. Proportional controllers usually cost more than time-based units. Wireless in-trailer units cost more again but save labour in the vehicle. Ask for a fitted price that includes testing with your trailer.

Our summary

For most caravans, choose a proportional controller, have it fitted by an auto electrician, set the gain with the van loaded, practise the manual override, and check the breakaway system before every trip. That combination meets the rules and makes the trailer stop straight and smoothly.

Keeping the controller working

Check the controller's display each trip, keep the trailer plug and socket clean and dry, and have the trailer's brakes adjusted and inspected as its maker recommends. Worn brake magnets and drums reduce braking power even with a perfectly set controller.

Where to go next

See electric trailer brakes for the full rules on which trailers need brakes, and towbar installation cost if you are fitting a towbar and controller together. Check your vehicle's towing limits with our towing capacity calculator before choosing a caravan.

Common questions