How do electric caravan brakes work?
Electric brake controllers (EBCs) are more necessary than ever, with most vans now fitted out with electric brakes. There are different types of EBCs but to understand them you will first need to know some trailer brake basics.
Trailer brakes are required by law for any trailer, except for single-axle trailers weighing 750kg or less. For trailers with an ATM of 751kg to 2000kg, trailer brakes are required on at least one axle; at 2001kg ATM or more, brakes must be fitted on all axles.
By law, you also need to have self-activating emergency brakes for trailers weighing more than 2000kg ATM that will stay activated for at least 15 minutes. These are called breakaway brakes and the purpose of this is to automatically apply the caravan’s brakes if the van decouples from the towing vehicle on the move – clearly an important safety item.
The breakaway brakes are activated by a switch on the A-frame. The switch energises the system when a pin in the switch housing is pulled out. The pin is attached to a small, rubber-encased steel cable which, in turn, you secure to the tow vehicle, preferably not on the towbar assembly. These brakes and how they’re activated is independent of the EBC.
The brakes used for almost all caravans – whether drum brakes or disc brakes – are activated electrically (although some are partly hydraulic, but I’ll explain this further on).
There are a few basic elements to an electric trailer brake system. Firstly, the brakes on the caravan themselves, which are usually 12V electro-magnetic drum brakes. ‘Electric’ disc brakes cannot be wholly electrically activated (they require hydraulic fluid to activate their piston-in-caliper design) and require much more sophistication to interact with an electric controller, so are less common. One example is the Sens-a-Brake system, which uses the same principle as electric brakes but relies on an air compressor (triggered by the EBC) mounted on the van’s A-frame to apply force to the hydraulic trailer brakes.
The brake system that used to be fitted to almost all caravans (and still are to European vans and some camper trailers) are override brakes, either cable or hydraulic. These are wholly self-contained on the trailer and do not require any power from vehicle or van to operate.
These brakes operate when the weight of the trailer forces against a pushrod behind the coupling when vehicle brakes are applied. This mechanical force either forces the pushrod against a lever to pull a cable, or push fluid into the brake lines, to activate the brakes.
It’s a simple and durable system, but is not a very flexible braking device as it’s not remotely adjustable – it requires a manual method to lock it, such as a collar, when reversing (otherwise the force of the vehicle pushing back against the trailer may activate the brakes). These brakes are not compatible with load-levelling hitches, either.
I won’t go into too much detail about these basics here but, needless to say, you need to ensure your van has brakes and that the brakes are plugged in (via a seven-pin or 12-pin trailer plug connection). Under the Australian Design Rules, the connector must adhere to Australian Standard 2513-1982 ‘Electrical Connectors for Trailer Vehicles’, and this includes specifically that the electric brakes run through circuit five of the connector.
A brake controller is an aftermarket component that is wired into the vehicle and provides the activation of 12V power to the electric trailer brakes. There are a few different types, but the main part of a brake controller kit is usually a small rectangular box that is commonly fitted on a metal bracket on the lower steering column panel or leading edge of the lower dash area of the tow vehicle. A newer design, with a remote head, can be fitted almost anywhere near the driver, most often in the place of a vehicle’s switch blank (if it has any) with the control unit installed under the dash.
Don’t assume that a brake controller will be installed just because you’ve bought a vehicle with a heavy-duty towbar: it is rarely fitted when a towbar and trailer plug is installed, unless it has been specifically requested.
While there are a couple of different types of caravan brakes commonly used, they operate on similar principles. The brake controller is powered by the vehicle battery and is activated by pressing the foot brake in the towing vehicle. The brake controller is wired into the vehicle brake light switch circuit, so when the switch gets power to turn on the brake lights, it also activates the brake controller.
The brake controller is the brain behind the activation of the trailer brakes, but its interaction with the vehicle’s electrics is very simple. When the brake light switch is activated when you put your foot on the brake, all the controller does is feed a current back to the caravan through the trailer plug to the caravan brakes. The controller does not interfere with the towing vehicle’s brake system at all.
Some old-school hydraulic trailer systems used to tap into the vehicle’s brake hydraulics and vacuum system. This design is not used any longer and, in fact, most manufacturers stipulate that they must not be used with their vehicles.
SLOW IT DOWN
How long an EBC delays activating the caravan electric brakes, and how much braking force it signals the caravan brakes to apply, depends on the design of the controller.
The most basic controller is a time-activated type. It simply sends a predetermined current to the caravan brakes at a predetermined interval after the vehicle brakes have been applied. This type of controller can be mounted anywhere convenient in the vehicle (within reach of the driver, of course) and is compact.
All EBCs have an activation button or sliding control so the driver can activate the caravan’s brakes independently of the vehicle (usually to ‘stretch out’ the rig, to reduce sway).
The most popular caravan electric controller is the proportional brake controller, which has a pendulum within the controller box, which senses deceleration. This type can vary brake force on the caravan brakes, based on the amount of braking force applied. So the more severe the braking force applied to the vehicle brakes, the higher the force applied to the sensor and the more current it sends to the trailer brakes.
The pendulum controller has to be fitted level so the pendulum can accurately sense the amount of braking force being applied, which is why it has to be mounted on a horizontal plane in the vehicle.
A third type of controller is equipped with accelerometers that measure G-force as the towing vehicle’s brakes are applied and apportions the required braking to the caravan brakes accordingly.
The Next Steps
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