Properly wiring trailer brakes is crucial for safety when towing. The braking system relies on electrical current to activate the brake shoes, so it’s important that the wiring provides power correctly. Most trailers use electric brakes that require a 12-volt power source from the tow vehicle. How this 12-volt power reaches the brakes depends on the specifics of the brake system, but in most cases, one wire will provide positive power and the other will be ground. So does it matter which wire is positive? Let’s take a closer look at trailer brake wiring to find out.
Trailer Brake Systems
There are two main types of electric brake systems used on trailers:
Surge Brakes
Surge brakes activate when the trailer pushes against the hitch ball during braking. This hydraulic pressure actuates the brake shoes. An electrical connection is only used to disable the surge brakes when backing up. During normal braking, it doesn’t matter which wire is positive.
Electric Drum Brakes
Electric drum brakes use magnets activated by electrical current to spread the brake shoes. The wiring provides power to control the magnets for smooth, proportional braking. With electric drum brakes, proper polarity in the wiring is needed.
Electrical Circuit
The trailer brake circuit relies on basic electrical principles to work correctly:
- A complete circuit must be made from power source (tow vehicle battery) to brakes and back.
- Like any electrical circuit, it requires both a positive and negative to function.
- Power flows from positive to negative to operate the magnets in electric brakes.
So in the brake wiring, one wire must provide positive voltage and the other ground/negative to create a closed circuit. If the wires are reversed, the circuit won’t work properly.
Wire Color Standards
To avoid confusion, trailer manufacturers follow a basic color code for brake wiring:
- White wire is ground/negative
- Blue, red or black wire is positive
The blue (or sometimes red or black) wire always connects to the cold side of the tow vehicle brake controller. The white wire connects to ground.
Trailer Wiring Diagram
This generic trailer wiring diagram shows the standard color code and function:
- Blue wire provides power from the brake controller to the electric brakes.
- White wire completes the circuit back to ground.
Following this wiring color standard provides the proper polarity for the trailer brakes to function.
Testing Polarity
The easiest way to test that your trailer is wired correctly is by using a circuit tester. This handy tool will indicate if you have the proper positive and ground.
To use a circuit tester on a trailer:
- Connect trailer and tow vehicle.
- Attach tester to blue and white wires at brake coupler.
- Make sure tester shows blue as positive and white as ground.
- Reverse tester connections if needed to get proper polarity.
Following this procedure ensures your trailer brakes will function as intended.
What Happens with Wrong Polarity
Reversing the positive and ground wiring won’t immediately damage the brakes, but will cause a few issues:
- Braking performance will be compromised.
- Shorter brake life due to added heat and wear.
- Brake controller may show fault code or not function.
- Brakes may drag or not release fully.
While reversed polarity won’t make the brakes completely fail, it will decrease their efficiency and durability over time. Proper polarity enhances braking capacity and extends brake system life.
Causes of Miswired Brakes
If you find your trailer brakes have the incorrect polarity, a few things could cause it:
- Wires crossed at controller or coupler connection.
- White ground wire color faded to blue/red over time.
- Previous repair work switched wire positions.
- Manufacturing defect with reversed colors.
Carefully examining all wire connections and colors can help determine the cause of miswired brakes.
Fixing Reversed Polarity
The fix for reversed trailer brake wiring is simple – just swap the positive and ground wires:
- Disconnect trailer wiring connector.
- Identify blue and white wires.
- Switch wire positions in connector.
- Reconnect wiring to test for proper polarity.
- Repeat as needed until polarity is correct.
Doing this at either the tow vehicle or trailer connectors will remedy the reversed wiring. Just be sure to verify correct polarity with a circuit tester afterwards.
Proper Brake Controller Settings
Along with properly wired brakes, your brake controller must have the right polarity settings:
- Electric over hydraulic – Set for positive ground polarity.
- Time delayed – Set for positive ground polarity.
- Proportional – Automatic or connected according to manufacturer instructions.
Mismatched polarity between the brakes and controller can also cause problems, so be sure to check both.
Preventing Future Miswiring
A few simple steps can prevent trailer brake polarity issues down the road:
- Use dielectric grease on connections to avoid corrosion.
- Occasionally recheck polarity with circuit tester.
- When rewiring, maintain color code standards.
- Avoid using old faded wires when repairing brakes.
Proactive maintenance and adhering to wiring best practices will keep your trailer brake polarity in the proper positive/ground configuration.
The Bottom Line
While reversed polarity won’t completely disable electric trailer brakes, it can reduce braking power and cause faster wear. Following the standard color code convention makes it easy to identify proper positive and ground wires. Periodically testing polarity with a circuit tester provides inexpensive insurance your brakes are wired correctly. Investing a few minutes upfront to get the polarity right ensures optimal braking performance and safety.
| Trailer Brake Type | Does Polarity Matter? |
|---|---|
| Surge Brakes | No, wiring only disables when reversing |
| Electric Drum Brakes | Yes, required for proper brake function |
| Wire Color | Function |
|---|---|
| Blue | Positive from brake controller |
| White | Ground/Negative |
Conclusion
Properly wired positive and ground connections are required for electric drum trailer brakes to work safely and effectively. Following standard color codes, periodically testing polarity, and properly setting the brake controller will ensure optimal brake performance. Taking a few simple steps to maintain proper polarity in your trailer wiring provides peace of mind while towing.