Quick answer: Tank 300 trailer-brake lock-up needs diagnosis by symptom. For grabbing during braking, check controller gain and calibration. For brakes applying immediately on connection, stop and investigate breakaway activation or unintended brake-circuit power. One locking wheel needs a local brake inspection. Confirm which controller is fitted before changing settings.
Technical references reviewed: · Australian towing setup
When do your trailer brakes lock?
During normal braking
Start with gain, boost, brake mode and calibration. If the problem persists, inspect the brakes rather than repeatedly lowering output.
As soon as you plug in
Check for an activated breakaway switch or unintended brake-circuit power. This needs an electrical investigation before a road test.
At one wheel only
Look for local adjustment, wiring, contamination, bearing or drum faults. Whole-trailer gain changes can hide an uneven brake problem.
After storage
Inspect for corrosion, stuck components and contamination. A binding wheel needs service before controller calibration.
The AL-KO electric-brake manual lists controller faults, incorrect wiring, loose brake parts, worn bearings and an out-of-round drum among lock-up causes. Record when the problem starts and which wheels are affected before replacing parts.
What is the safest order of checks?
- Stop and secure the combination. Park on level ground, apply the appropriate parking brakes and chock the trailer. Let hot brakes cool. Do not drag a locked wheel or bypass the breakaway system to continue towing.
- Identify the fitted equipment. Note the Tank 300 year and variant, controller model, trailer brake type and any displayed fault code. Electric drum, electro-hydraulic and mechanical over-run systems require different checks.
- Inspect the breakaway pin and cable. Check that the pin is correctly seated and the cable has not snagged or pulled it out. Brakes that stay applied with the pin seated need the switch, battery and circuit tested.
- Inspect visible wiring. Look for corroded plug pins, loose terminals, damaged adapters and crushed or chafed cable. Have an auto electrician test the earth return and brake circuit against the actual installation diagram.
- Investigate a binding or uneven wheel. A specialist should compare brake adjustment, hardware, bearings and drum condition. Any lifting requires rated stands, wheel chocks and the axle maker's procedure; never work under a jack-only trailer.
- Calibrate only after faults are resolved. If the brakes release normally and no electrical or mechanical fault remains, follow the fitted controller's setup procedure in a safe, clear test area.
Brakes applied without a command? Stop the road test. An electrical fault needs circuit testing, and a seized wheel needs mechanical service. A new controller will not repair a damaged trailer brake assembly.
How should you set controller gain and boost?
There is no universal gain number for a Tank 300 and caravan. Load, brake condition, tyre grip and controller design all affect the setting. A proportional controller scales trailer output with deceleration, but still needs correct installation and calibration.
Follow the controller manual for mounting, levelling and brake-mode selection. Where gain and boost are separate, set maximum output first, then check initial response and balanced pedal braking. Reduce excessive output in small steps; the trailer should neither skid nor push the tow vehicle.
For the EVparts4x4 proportional controller, the published setup procedure uses the manual control on a dry, flat, clear road at 40 km/h. That is specific to this product, not a universal speed for other controllers. Use the instructions supplied with your fitted unit.
Does your Tank 300 have the right controller and wiring?
A tow bar and working trailer lights do not confirm electric-brake control. GWM Australia lists an Electric Trailer Brake Controller Assembly as a separate accessory, with a required diode kit. Confirm the equipment fitted to your exact vehicle and its installation requirements.
For lighting and harness selection, see our Tank 300 trailer wiring harness guide. The controller's stop-light input senses a braking command; its brake output supplies the trailer brake circuit. Those functions must be correctly connected.
| Circuit | VSB 1 reference | What to confirm |
|---|---|---|
| Earth return | Pin 3 · white | A sound wired return, including connections under load |
| Service brakes | Pin 5 · blue | Correct controller output; no auxiliary-power cross-connection |
| Stop lights | Pin 6 · red | Correct lighting circuit; distinct from regulated brake output |
Reference: VSB 1, section 7.2, standard 7-pin and corresponding first seven 12-pin circuits. Check plug versus socket viewing direction, adapters and the actual loom. Wire colour alone is not proof of function.
Is an earth fault the same as brake lock-up?
No. Poor earthing can cause weak or intermittent braking and controller faults. Brakes applying without a command also require checks for unintended brake-circuit power and breakaway activation. Record a fault code exactly and use that controller's manual; warning colours are not universal.
Ask the installer to check the fused supply, earth return under load, stop-light trigger and brake output. Include any plug adapter and trailer-side wiring in the test. Do not feed battery power into an unidentified terminal or rely on a tow-ball connection as the earth return.
Check towing limits against your exact vehicle's handbook, compliance information and applicable GWM specifications. Model year, powertrain and build date matter; a figure from another Tank 300 variant is not a safe substitute.
Need a compatible proportional controller?
Our controller is listed for 12V negative-earth tow vehicles and compatible electric or electro-hydraulic trailer brakes on one to four axles. Confirm the Tank 300 installation, brake type and mounting requirements before ordering.
View controller specifications and current price
Send your vehicle year, variant and trailer brake details to our fitment team, or browse GWM parts and towing accessories.
Why does one wheel lock, especially after storage?
When one wheel grabs first, inspect that assembly and its wiring before changing settings for every wheel. Compare adjustment and component condition side to side. Storage corrosion, contamination or loose hardware after rough-road travel can all warrant a brake service.
| Area | Possible finding | Next action |
|---|---|---|
| Shoes and adjustment | Binding, unequal clearance or uneven wear | Adjust or replace to the brake manufacturer's specification |
| Drum and linings | Scoring, contamination, corrosion or an out-of-round drum | Assess and service the affected assembly |
| Springs and hardware | Broken springs, loose parts or displaced components | Repair before a road test |
| Bearings | Play or damage | Service the hub and check related brake damage |
| Local wiring | Damaged cable or poor connections at one brake | Repair and compare brake operation across wheels |
For the assemblies covered by its manual, AL-KO specifies initial adjustment and de-dusting after 300–1,000 km. Its bonded-lining replacement limit is 1.5 mm, with earlier replacement for irregular wear or contamination. These are AL-KO specifications, not universal limits for every trailer brake.
Source: AL-KO Electric Braked Axles Operating and Service Manual. Follow your axle maker's later service intervals and adjustment procedure.
What is the Australian braking requirement?
The following summary comes from VSB 1 Revision 6, August 2026, section 8. Its scope is trailers up to 4.5 tonnes ATM, but the braking thresholds below use GTM. Check state or territory registration and in-service requirements as well.
| GTM | Braking baseline |
|---|---|
| 750 kg or less | Brakes are not required by this VSB 1 threshold. |
| Over 750 kg to 2,000 kg | An efficient service braking system is required. Over-run brakes are permitted; brakes may be fitted to one axle. |
| Over 2,000 kg | Brakes must operate on all wheels. An efficient breakaway system must apply the brakes immediately on separation and hold them for at least 15 minutes. |
Electric trailer brakes need a compatible controller; an over-run system works differently. Meeting a mass threshold does not prove that an installation is correctly wired or that the Tank 300 can legally tow that load. VSB 1 is construction guidance, and applicable ADRs take precedence.
When should you stop DIY and call a specialist?
- The brakes apply without a command or stay applied with the breakaway pin seated.
- Wiring overheats, smells burnt or repeatedly blows a fuse.
- One wheel locks, a hub is abnormally hot or the trailer pulls during braking.
- You find bearing play, broken hardware, contaminated linings or drum damage.
- You cannot identify the controller, trailer brake type or correct installation diagram.
Give the specialist your vehicle year and variant, controller model, trailer brake type, fault-code photo and the exact symptom. Ask for both electrical and mechanical checks where needed, followed by calibration and a controlled brake test.
Frequently asked questions
Why do my trailer brakes lock as soon as I plug into the Tank 300?
If the brakes apply without a pedal or manual-control command, check for breakaway activation or unintended power on the brake circuit. A technician should test the controller output, socket, adapters and trailer wiring. Do not tow until the fault is repaired and the brakes release normally.
How do I reduce brake-controller gain without making the brakes too weak?
Follow the fitted controller's calibration procedure on a dry, flat, clear test area. Reduce maximum output in small steps if the trailer wheels lock, then check balanced braking with the pedal. Recheck after a significant load change or brake service; the trailer should neither skid nor push the tow vehicle.
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 with a required diode kit. Confirm the installed equipment on your exact year and variant. A tow bar and lighting harness alone do not provide adjustable electric-brake control.
Can a trailer lighting harness operate electric brakes?
A lighting harness alone cannot regulate electric trailer brakes. They need a compatible controller and a correctly wired brake-output circuit. Working indicators and stop lights do not prove that the brake circuit or its earth return is correct.
Can a bad earth cause trailer-brake problems?
Yes. Poor earth connections can cause weak or intermittent braking and controller faults. They do not, by themselves, prove the cause of a lock-up. Have the earth return tested under load and investigate unintended brake-circuit power if the brakes apply without a command.
Why does only one caravan wheel lock?
A single locking wheel calls for a local brake and wiring inspection. Compare shoe adjustment, contamination, springs, bearings, drum condition and connections side to side. Do not reduce the whole trailer's gain to hide a fault at one wheel; have the affected assembly serviced.
Which Australian trailer-plug pins are earth and electric brakes?
For the standard Australian 7-pin and corresponding first seven 12-pin circuits shown in VSB 1, pin 3 is the white earth return and pin 5 is the blue service-brake circuit. Confirm the connector type, viewing direction, adapters and actual wiring before testing; do not identify terminals by colour alone.
Can I disconnect the breakaway system to stop the brakes locking?
Do not bypass it to continue towing. Park safely, chock the wheels and inspect whether the pin or cable has activated the switch. If the brakes remain applied with the pin correctly seated, have the switch, battery and brake circuit tested before moving the trailer.
Sources and related reading
