Quick answer
An electric brake controller mounts in the tow vehicle and needs four connections: a fused heavy-duty power feed from the battery, a solid earth, a signal from the stop-light circuit, and an output wire to pin 5 (blue, electric brakes) at the trailer socket. Mount it where the maker says, set it up with the loaded trailer, and test at low speed. Because trailer brakes are safety-critical and modern vehicles often need a vehicle-specific connection, many owners have it professionally installed.
Is this a DIY job?
- Difficulty: Pro (a confident DIYer with auto electrical experience may do it)
- Time: 2 to 4 hours
- Tools: controller kit and harness, heavy auto cable, crimp tool for large lugs, circuit breaker or fuse, multimeter, trim tools, drill and grommets
- Skills: high-current wiring, routing cable through the firewall, reading the controller’s instructions
Getting it wrong can mean trailer brakes that don’t work, lock up or stay on. If you’re not confident with high-current wiring and your vehicle’s brake light circuit, have it installed by an auto electrician.
Do you need one?
Under the national guide for small trailers (VSB 1), a single-axle trailer up to 750 kg gross trailer mass (GTM) doesn’t need brakes. Above that, brakes are required. Up to 2000 kg GTM, brakes on the wheels of at least one axle are enough, and over-run (mechanical) brakes are allowed. Any brakes other than over-run brakes must be operable from the driver’s seat. Over 2000 kg GTM the brakes must work on all wheels, be operable from the driver’s seat, and apply automatically if the trailer breaks away. Trailers with electric brakes need a controller in the tow vehicle. Check your state’s rules for your trailer.
Types of controller
- Proportional (inertia-sensing): senses how hard the vehicle is slowing and applies matching trailer braking. Generally smoother.
- Time-delayed: applies a preset braking level that ramps up over time once you touch the brake pedal. Simpler, but the trailer may brake too little or too much for the situation.
- Remote-head or in-dash units: the electronics hide behind the dash with only a small control head visible.
Step by step
- Read the controller’s instructions first. Cable sizes, fuse or circuit breaker rating, and mounting angle all come from the maker. They vary between models.
- Check for a vehicle-specific harness. Many controllers have plug-in harnesses for particular vehicles, and many modern vehicles monitor the brake light circuit. A plug-in harness, or a towbar harness with a brake signal output, avoids cutting into factory wiring. (see Do I need a vehicle-specific towbar wiring harness?)
- Disconnect the battery negative. Do this before running power cable.
- Choose the mounting spot. Within the driver’s reach, where it won’t hit your knees, and clear of airbags and their deployment paths (for example, not on the knee airbag panel). Proportional units may need a specific orientation, or may self-calibrate: follow the maker.
- Run the power cable. From the battery positive, through a circuit breaker or fuse mounted close to the battery, to the controller. Use the cable size the maker specifies. Pass through the firewall with a grommet and protect the cable in loom tube away from heat and moving parts.
- Run the earth. Heavy cable to the battery negative or a solid chassis earth point, as the maker specifies. A poor earth is a common cause of weak trailer brakes.
- Connect the stop-light signal. From the brake light switch circuit, via the vehicle harness or a suitable connection. The controller needs this signal to know you’re braking.
- Run the brake output to the trailer socket. From the controller output to pin 5 (blue) on the 7-pin or 12-pin socket at the towbar, using the cable size the maker specifies. Fuse or protect it as instructed.
- Reconnect the battery and check. With the trailer disconnected, the controller should power up. With the trailer connected, many controllers show that they can see the trailer brake magnets.
- Set up with the loaded trailer. Follow the maker’s setup: some self-calibrate after a few stops, others need you to set gain (output) manually. Start low.
- Test at low speed. In a safe, empty area, use the manual control to apply trailer brakes alone at walking pace. You should feel the trailer pull back firmly without locking the wheels. Then test normal braking. Adjust gain until the vehicle and trailer stop smoothly together.
- Check the breakaway system on the trailer separately, following its maker’s test.
Specs table
The four connections
| Connection | From / to | Notes |
|---|---|---|
| Power | Battery positive, through a circuit breaker or fuse near the battery | Cable size and breaker rating per the controller maker |
| Earth | Battery negative or solid chassis earth | Heavy cable; clean bare-metal contact |
| Stop-light signal | Brake light switch circuit | Use a vehicle-specific harness where available |
| Brake output | Controller to trailer socket pin 5 (blue) | Cable size per the maker; the trailer’s brake earth returns via pin 3 |
Trailer brake requirements (VSB 1, check your state)
| Gross trailer mass | Requirement |
|---|---|
| Up to 750 kg (single axle) | Brakes not required |
| Over 750 kg to 2000 kg | Brakes on at least one axle (over-run brakes allowed) |
| Over 2000 kg | Brakes on all wheels, operable from the driver’s seat, applying automatically if the trailer breaks away |
What you’ll need
- electric brake controller
- brake controller harness (vehicle-specific, if available)
- auto cable twin core / heavy duty single core auto cable (size per the controller maker)
- circuit breaker or inline fuse holder
- cable lugs and adhesive heat shrink
- firewall grommet and split loom tube
- 12 pin flat trailer socket or 7 pin flat trailer socket
When to call an auto electrician
- You’re not confident with high-current wiring, firewall routing or crimping large lugs.
- Your vehicle uses CAN-bus lighting or monitors the brake light circuit and there’s no plug-in harness.
- The trailer brakes are weak, grab, or stay on after setup.
- You also need a 12-pin socket, Anderson plug or DC-DC charger for a caravan. It’s often efficient to do these together.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: Proportional or time-delayed brake controller: which is better? A: Proportional controllers match trailer braking to how hard the vehicle is slowing, which is generally smoother. Time-delayed units are simpler. Choose one that suits your trailer and how you tow, and check it’s compatible with your vehicle.
Q: Does my trailer need a brake controller? A: If it has electric brakes, yes: they won’t work without one. Brake requirements depend on the trailer’s mass, so check VSB 1 and your state’s rules.
Q: Why are my trailer brakes weak? A: Common causes are low gain, a poor earth, thin cable, corroded plug pins, or worn brake magnets. Check the gain setting and the earth first.
Q: Can I move the brake controller between vehicles? A: The controller can often move, but each vehicle needs its own wiring. Plug-in harnesses make this easier.
Sources
- Department of Infrastructure, VSB 1 Revision 5 – trailer brake requirements by GTM (section 15); brakes on at least one axle up to 2 tonnes GTM; brakes other than over-run operable from the driver’s seat; breakaway requirement; pin 5 for service brakes. https://www.infrastructure.gov.au/sites/default/files/documents/vehicle-standards-bulletin-1-revision5-trailers-with-an-aggregate-trailer-mass-of-4-5-tonnes-or-less-june2009-pdf.pdf (checked October 2026)
- RACQ, “A guide to trailer brakes” – electric brakes need a control unit in the tow vehicle, with a manual control within the driver’s reach; brakes on at least one axle up to 2000 kg GTM; breakaway systems required over 2000 kg GTM. https://www.racq.com.au/articles/rvs/2018/6/a-guide-to-trailer-brakes (checked October 2026)
- REDARC, “A beginner caravanner’s review on buying your first electric brake controller” – proportional vs time-delayed; professional installation recommended. https://redarcelectronics.com/blogs/blog/a-beginner-caravanners-review-on-buying-your-first-electric-brake-controller (checked October 2026)