A trailer brake controller has a fairly simple job: when you slow down the tow vehicle, it tells the trailer’s electric brakes how much braking to provide.
But using one correctly involves more than installing it and forgetting about it.
Set the braking output too low and the tow vehicle may end up doing too much of the work. Set it too high and the trailer brakes may grab or lock the wheels.
The correct adjustment can also change depending on trailer weight, road conditions and how the trailer is loaded.
And that little manual lever or button on the controller?
It isn’t just there for testing the brakes. In certain situations, such as trailer sway, controlled application of the trailer brakes can be useful for helping stabilize the trailer.
Here’s how trailer brake controllers work, how to set one up, and what the different adjustments actually do.
What Does a Trailer Brake Controller Do?
Many trailers use electrically operated drum brakes.
When braking is required, the brake controller sends electrical power through the trailer wiring to the brake magnets.
More output from the controller generally means stronger trailer braking.
The goal is to have the trailer and tow vehicle work together.
You don’t want the trailer pushing the tow vehicle during braking.
You also don’t want the trailer brakes grabbing so aggressively that the trailer tires lock or the combination feels like it’s being pulled backward every time you touch the brake pedal.
A properly adjusted brake controller should make the combination feel controlled and predictable.
Do All Trailers Need a Brake Controller?
No.
Brake requirements depend on factors such as trailer weight, brake type and the laws where the trailer is being operated.
Some smaller trailers may not have brakes at all.
Other trailers may use braking systems that don’t rely on a conventional in-cab electric brake controller.
Before towing, make sure you understand the braking system installed on your particular trailer and the legal requirements that apply where you’re travelling.
The Two Common Types of Brake Controllers
Most aftermarket electric trailer brake controllers fall into two broad categories.
Proportional Brake Controllers
A proportional controller attempts to determine how aggressively the tow vehicle is slowing down.
Light braking produces light trailer braking.
Harder braking produces stronger trailer braking.
The idea is to make the trailer braking proportional to what the tow vehicle is doing.
For normal road use, this generally provides smoother and more natural braking than simply applying a predetermined amount of braking every time the brake pedal is pressed.
Many newer tow vehicles also have factory-integrated trailer brake controllers that work with the vehicle’s braking and stability systems.
Time-Delayed Brake Controllers
A time-delayed controller works differently.
When the brake pedal is applied, the controller begins applying trailer braking according to its settings, usually increasing the braking output over a predetermined period.
These controllers can work well when properly adjusted, but the trailer braking isn’t necessarily proportional to how quickly the tow vehicle itself is slowing.
Because of that, correct adjustment becomes particularly important.
What Is Brake Controller Gain?
You’ll often hear brake-controller output referred to as gain.
Gain controls the maximum amount of braking power the controller is allowed to send to the trailer brakes.
Think of it as the upper limit.
Gain too low
The trailer may:
- Feel like it’s pushing the tow vehicle
- Take longer to stop
- Provide very little noticeable braking
- Put more braking load on the tow vehicle
Gain too high
The trailer brakes may:
- Grab aggressively
- Lock the wheels
- Jerk the combination during braking
- Feel like they’re pulling backward on the tow vehicle
The goal is enough trailer braking to help stop the combination without excessive grabbing or wheel lockup.
What Are Boost and Sensitivity Settings?
Some brake controllers have additional adjustments such as boost, sensitivity, or similar settings.
The exact terminology varies by manufacturer.
These settings usually affect how quickly or aggressively braking is applied, rather than simply changing the maximum available output.
For example, a heavier trailer may benefit from a different response than a lightly loaded utility trailer.
Don’t assume that another person’s settings are correct for your trailer.
Two trailers behind the same truck can require very different brake-controller adjustments.
How to Adjust a Trailer Brake Controller
Always follow the setup procedure provided by the manufacturer of your particular controller or tow vehicle.
The general process, however, usually looks something like this.
1. Connect the Trailer
Make sure the trailer plug is fully connected and the brake controller recognizes the trailer if it has a connection indicator.
Before travelling, also check the rest of the towing connections, including the coupler, safety chains and breakaway cable.
2. Verify That the Trailer Brakes Work
Before worrying about perfect adjustment, make sure the trailer brakes are actually operating.
A brake controller can’t compensate for:
- Worn brakes
- Poor brake adjustment
- Wiring problems
- Bad grounds
- Damaged connectors
- Faulty brake magnets
- Mechanical brake problems
If the controller shows output but the trailer barely brakes, don’t immediately assume you need more gain.
There may be a problem at the trailer.
3. Find a Safe Place to Test
Use a flat, open area away from traffic where you can safely test the trailer brakes at low speed.
Follow the brake-controller manufacturer’s recommended testing procedure.
The objective is to determine how strongly the trailer brakes respond without testing adjustments for the first time in traffic.
4. Adjust the Gain
Start with the manufacturer’s recommended initial setting.
Test the braking response.
If the trailer provides too little braking, increase the gain gradually.
If the trailer brakes grab or lock, reduce it.
Make small adjustments rather than jumping from one extreme to another.
5. Test Normal Braking
Once the initial setting feels appropriate, pay attention to how the trailer behaves during normal stops.
Ideally, the truck and trailer should feel like they’re slowing together.
You shouldn’t feel the trailer aggressively pulling backward.
You also shouldn’t feel as though the trailer is pushing the tow vehicle through the stop.
Your Brake Controller Setting Isn’t Necessarily Permanent
One setting may not be correct forever.
Suppose you adjust the controller while towing an empty cargo trailer.
Then you load it with several thousand pounds of cargo.
The braking requirements have changed.
The same thing can happen with an RV.
Water, supplies, equipment and cargo can significantly change trailer weight.
Road conditions matter too.
A setting that works well on dry pavement may produce wheel lockup more easily on a slippery surface.
If the trailer’s weight or road conditions change substantially, pay attention to the braking response and adjust according to the controller manufacturer’s instructions.
What Does the Manual Brake Control Do?
Most trailer brake controllers have some form of manual control.
Depending on the controller, this may be a:
- Lever
- Slider
- Button
- Paddle
The manual control allows you to activate the trailer brakes without applying the tow vehicle brakes in the normal way.
This has several useful purposes.
Testing Trailer Brake Operation
Manual control can help verify that the trailer brakes are responding.
At low speed in a safe testing area, applying the manual control should produce noticeable braking from the trailer.
The exact testing procedure depends on the controller, so follow its instructions.
Checking Your Gain Setting
Many manufacturers use the manual control as part of the initial gain-adjustment procedure.
Again, follow the procedure specified for your controller rather than relying on one universal setting.
Manual Trailer Braking and Trailer Sway
The manual brake control can also have an important use if trailer sway develops.
If the trailer begins moving from side to side, carefully applying the trailer brakes alone can create drag behind the tow vehicle and help bring the trailer back into line.
This is different from abruptly stepping hard on the tow vehicle’s brake pedal.
The objective isn’t to lock the trailer wheels.
You’re trying to apply controlled braking at the trailer to help stabilize the combination while reducing speed.
SAFETY STOP — KNOW YOUR CONTROLLER BEFORE YOU NEED IT
Don’t wait until a trailer is swaying behind you at highway speed to figure out where the manual control is.
Before towing:
- Know where the manual control is located
- Know how it operates
- Make sure you can reach it without looking away from the road
- Understand how your particular controller responds
- Make sure the trailer brakes are working properly
If sway occurs, keep steering inputs smooth, avoid accelerating, reduce speed safely and pull over once control has been restored.
Then find out why the trailer swayed.
A brake controller or sway-control system shouldn’t be used to hide an improperly loaded or unstable trailer.
Related: Trailer Sway — What Causes It, How to Prevent It, and What to Do If It Happens
How Do You Know If the Brake Controller Is Set Too High?
Common signs include:
- Trailer tires locking during ordinary braking
- Trailer brakes grabbing
- Jerking when the brake pedal is applied
- The trailer feeling like it’s pulling hard against the tow vehicle
If this happens, don’t simply assume the controller is defective.
Check the gain and other applicable settings first.
However, brake adjustment or mechanical problems can also cause uneven or aggressive braking.
How Do You Know If the Brake Controller Is Set Too Low?
You may notice:
- The trailer pushing the tow vehicle
- Longer stopping distances
- Little noticeable trailer braking
- Excessive reliance on the tow vehicle brakes
Increasing the gain may help if the system is simply under-adjusted.
But there’s an important distinction.
Maximum gain doesn’t fix trailer brakes that aren’t working properly.
If you need unusually high output to get very little braking, inspect the trailer brake system.
Brake Controller Troubleshooting
Controller Says “No Trailer” or “Disconnected”
Start with the trailer connector.
Check for:
- Plug not fully inserted
- Dirty or corroded terminals
- Damaged wiring
- Poor ground connections
- Loose connections
Intermittent trailer connections can sometimes appear only when the combination is moving, so inspect the plug and wiring carefully.
Controller Shows Output but Trailer Doesn’t Brake
If the controller appears to be sending power but the trailer brakes aren’t responding properly, the problem may be farther downstream.
Possible causes include:
- Wiring problems
- Poor ground
- Brake magnet problems
- Incorrect brake adjustment
- Worn brake components
- Connector problems
Don’t automatically replace the controller because the brakes aren’t working.
Trailer Brakes Are Too Aggressive
Check:
- Gain setting
- Boost or sensitivity setting
- Trailer brake adjustment
- Brake condition
Reduce excessive controller output if appropriate, but investigate mechanical problems if one wheel or axle behaves differently from the others.
Brakes Work Intermittently
Intermittent problems often point toward electrical connections.
Inspect:
- Trailer plug
- Tow-vehicle socket
- Ground connections
- Wiring near the axles
- Brake wiring
- Connectors and splices
Movement and vibration while towing can expose connections that appear fine while the trailer is sitting still.
Only Some Trailer Brakes Work
This is less likely to be a controller-setting problem.
The controller normally supplies the trailer braking circuit as a system.
If one wheel isn’t braking while the others are, inspect the wiring and brake components at that wheel or axle.
Don’t Forget the Trailer Brakes Themselves
A perfectly adjusted brake controller can’t make poorly maintained trailer brakes work properly.
Depending on the brake design, trailer brakes may require periodic:
- Inspection
- Adjustment
- Cleaning
- Bearing/service-related inspection
- Replacement of worn components
Some electric drum brakes are self-adjusting, while others require manual adjustment.
Know which system your trailer uses.
If braking performance gradually becomes worse even though your controller settings haven’t changed, inspect the brakes rather than continually increasing the gain.
What About Factory-Integrated Brake Controllers?
Many modern trucks and SUVs have integrated trailer brake controllers.
These can communicate with other vehicle systems and may provide features that aftermarket controllers don’t.
The controls may look different, but the basic objective remains the same:
The trailer and tow vehicle should work together during braking.
Follow the tow vehicle manufacturer’s instructions for trailer setup, gain adjustment and manual brake operation.
Don’t assume that the factory controller automatically determines the perfect setting for every trailer.
Brake Controller Quick Reference
Trailer feels like it’s pushing the truck
The gain may be too low, or the trailer brakes may not be providing enough braking.
Trailer brakes grab
Gain, boost or sensitivity may be too high, or the trailer brakes may need inspection or adjustment.
Trailer tires lock
Reduce excessive output as appropriate and verify brake condition and adjustment.
Controller says trailer disconnected
Check the trailer plug, socket, wiring and grounds.
Controller shows output but braking is weak
Check the trailer brakes and wiring before assuming the controller needs replacement.
Trailer weight changed significantly
Recheck braking performance and adjust the controller according to the manufacturer’s procedure.
Trailer starts swaying
Controlled manual trailer-brake application may help stabilize the trailer while you safely reduce speed. Once stopped, investigate the cause of the sway.
The Bottom Line
A trailer brake controller isn’t simply an on/off switch for the trailer brakes.
It determines how the trailer participates in stopping the entire combination.
Set correctly, the trailer and tow vehicle should feel like they’re working together.
Set incorrectly—or connected to trailer brakes that aren’t working properly—you may experience weak braking, grabbing, wheel lockup or a trailer that pushes the tow vehicle during stops.
Learn what the gain and other adjustments on your controller actually do.
Know where the manual control is.
Test the trailer brakes before you need them.
And if changing the controller setting doesn’t solve the problem, stop adjusting the controller and start looking for the actual fault.
Sometimes the brake controller isn’t the problem at all.
Related Articles:
- Tow Ratings Explained: What Can My Vehicle Safely Tow?
- Trailer Sway: What Causes It, How to Prevent It, and What to Do If It Happens
- Weight Distribution Hitches Explained: How They Work and How to Adjust One
- Choosing the Right Trailer Hitch for Your RV
Written by Dan Harvickson (Founder of Last Mountain Tech) RV Specialist