Quick answer: If your trailer brakes lock only when you slow down, reduce the brake-controller gain and recalibrate it to the controller maker's instructions. If they lock as soon as the trailer is plugged in, stop towing and check the breakaway switch, controller wiring and trailer brake circuit. A GWM Tank 300 does not automatically include an in-cab brake controller: GWM Australia lists one as a separate genuine accessory.
Match the symptom before changing parts
Locks during normal braking
Gain is too high, the controller is not calibrated, or a non-proportional controller is applying too much output.
Locks as soon as you plug in
Treat this as an electrical fault: check the breakaway switch, brake-output wiring and controller installation.
One wheel locks first
Look for local mechanical faults such as incorrect adjustment, a loose part, bearing wear or an out-of-round drum.
Locks after storage
Inspect for corrosion, contamination, stuck components and incorrect shoe adjustment before the next trip.
AL-KO's Australian electric-brake manual lists a malfunctioning controller, incorrect controller wiring, loose brake parts, worn bearings and an out-of-round drum among the causes of locking brakes. That is why the quickest fix depends on when the lock-up happens, not just the fact that it happens.
Safe diagnosis in the right order
- Do not tow with a wheel locked. Park on level ground, chock the wheels and let hot brakes cool. Never disconnect or bypass the breakaway system to continue a trip.
- Confirm the controller setup. Identify the fitted controller and trailer brake type. Use the controller's own calibration procedure; on the proportional controller sold by EVparts4x4, the setup check is performed on a dry, flat, clear road at 40 km/h using the manual control.
- Reduce gain in small steps. If the trailer wheels lock during the test, reduce maximum output. If the trailer pushes the tow vehicle, increase it slightly. The correct setting gives smooth, balanced braking without skidding.
- Check the breakaway switch. Make sure the pin is fully seated and the cable is not snagged. If the brakes remain applied with the pin seated, stop and have the switch, battery and circuit tested.
- Inspect the plug, earth and brake-output circuit. Look for corrosion, crushed cable, loose terminals and chafing under the drawbar or chassis. Follow the wiring diagram supplied with your controller and harness rather than relying on a generic pin-number diagram.
- Check each hub mechanically. With the trailer safely supported, compare wheel rotation side to side. Follow the axle manufacturer's adjustment and service procedure; do not guess at shoe clearance.
Locked solid means stop. Dragging a caravan or trailer with locked wheels can destroy tyres, brake shoes and drums and can make the combination unstable. If the brakes will not release, arrange a licensed auto electrician or trailer-brake specialist before moving it.
Brake-controller gain and calibration
Australian caravan owners commonly search for a "brake controller setting", but there is no single correct number for every trailer. The right output changes with trailer mass, brake condition, tyre grip and the controller model.
A proportional controller measures deceleration and scales trailer-brake output to suit. AL-KO recommends an ADR 38/05-compliant variable proportional in-cab controller for smooth, balanced braking. Once adjusted correctly, the trailer should neither push the Tank 300 nor pull it excessively during a normal stop.
The EVparts4x4 proportional electric brake controller supports 12V negative-earth tow vehicles and electric or electro-hydraulic trailer brakes on one to four axles. It is a universal-fit electrical product, so confirm the stop-light signal, brake-output wiring and exact Tank 300 year/variant with an installer or through the store's fitment check before ordering.
Tank 300 controller and wiring facts
Do not assume the Tank 300 has a built-in adjustable brake controller. GWM Australia lists an Electric Trailer Brake Controller Assembly as a separately priced genuine accessory, together with a required diode kit. A vehicle can have a tow bar and lighting harness without having the in-cab controller needed to operate electric trailer brakes.
For the lighting side, use our GWM Tank 300 trailer wiring guide. Keep the lighting harness and the electric-brake controller conceptually separate: the harness runs trailer lamps, while the controller manages braking output.
Tank 300 towing ratings vary by build date and powertrain, so check the compliance plate, owner's handbook and current GWM specifications for your exact vehicle. Do not copy a later model's 3,000 kg figure onto an earlier build.
Mechanical causes after storage or rough roads
Australian storage and travel conditions can expose faults that are easy to miss in a driveway. Humidity and coastal salt can corrode electrical connections; corrugated-road vibration can loosen terminals or brake hardware; dust can contaminate the drum assembly.
AL-KO specifies that brake adjustment and de-dusting should be carried out after the first 300-1,000 km and then at the service intervals in its manual. It also calls for replacement of bonded linings at a minimum 1.5 mm thickness, or earlier where wear is irregular or the lining is contaminated. Use the specification for your own axle and brake manufacturer rather than a universal 3 mm rule.
| Check | What you are looking for | Next action |
|---|---|---|
| Controller | Incorrect mode, excessive gain, loose mounting or a fault code | Recalibrate or have the controller tested |
| Breakaway system | Pin not seated, damaged cable, battery or switch fault | Repair before towing |
| Plug and loom | Corrosion, crushed insulation, loose earth or crossed wiring | Test against the supplied wiring diagram |
| Drums and shoes | Binding, contamination, scoring or uneven wear | Service to the axle maker's specification |
| Bearings and hardware | Play, broken springs, loose rivets or out-of-round drum | Stop DIY and use a brake specialist |
Australian legal baseline
The Australian Government's Vehicle Standards Bulletin 1 states that trailers over 750 kg GTM need an efficient service braking system. Above 2,000 kg GTM, brakes must operate on all wheels and an emergency breakaway system must apply the brakes if the trailer separates. Registration and in-service rules are administered by states and territories, so check the rules where the trailer is registered.
A trailer with an ATM above 4.5 tonnes is a heavy vehicle under the Heavy Vehicle National Law. If that applies, start with the National Heavy Vehicle Regulator definition and use a qualified heavy-vehicle brake specialist.
When to stop DIY
- The brakes stay applied with the breakaway pin seated.
- The controller or wiring becomes hot, smells burnt or repeatedly blows a fuse.
- One hub locks while the others rotate normally.
- You find bearing play, broken brake hardware, fluid contamination or a scored drum.
- You cannot positively identify the controller, brake type or wiring diagram.
FAQ
Why do my trailer brakes lock as soon as I plug into the Tank 300?
That points to an electrical fault rather than normal gain adjustment. Check that the breakaway pin is seated, then have the controller output, plug wiring, earth and trailer brake circuit tested. Do not tow until the brakes release normally.
How do I reduce brake-controller gain without making the brakes too weak?
Follow the controller maker's calibration procedure on a dry, flat, clear road. Reduce maximum output in small steps until the wheels stop locking, then confirm the trailer does not push the tow vehicle during a normal stop. Recalibrate whenever trailer load or brake condition changes materially.
Does the GWM Tank 300 have a built-in trailer-brake controller?
Do not assume it does. GWM Australia lists an Electric Trailer Brake Controller Assembly as a separate genuine accessory. Check the installed equipment on your exact vehicle; a tow bar and lighting harness alone do not control electric trailer brakes.
Can a trailer lighting harness operate electric brakes?
No. A lighting harness powers functions such as indicators, tail lights and stop lights. Electric trailer brakes need a compatible brake controller and a correctly wired brake-output circuit.
Why does only one caravan wheel lock?
A single locking wheel is more likely to have a local mechanical or wiring fault than a global gain problem. Compare adjustment, wiring, bearing condition, brake hardware and drum condition side to side, and use a trailer-brake specialist if the cause is not obvious.
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