Why are my trailer brakes not activating?

Your trailer brakes not activating usually points to an electrical or mechanical fault in the braking system. The quickest path to a fix is to verify power, ground, and brake‑controller settings, then inspect the wiring and brake components.

Common causes at a glance

Understanding the most frequent culprits helps you prioritize what to check first. The issues fall into electrical, wiring/connectors, brake controller, breakaway system, and mechanical wear categories.

• Electrical supply problems: blown fuses or a faulty brake controller, damaged wiring, or a poor ground.


• Wiring and connector issues: corroded, loose, or damaged pins in the trailer connector (7‑pin or 6‑pin) leading to no signal at the brakes.


• Brake controller problems: incorrect gain, incompatible equipment, or a controller that isn’t powered or calibrated correctly.


• Breakaway system faults: a discharged or faulty breakaway battery or a miswired breakaway switch, which can prevent brakes from activating.


• Mechanical issues in the trailer brakes: worn or seized brake magnets, damaged drums, or failing wheel bearings that keep the brake system from engaging even when signaled.

In most cases, electrical faults (power, ground, and controller settings) come first, followed by wiring/connectors and then mechanical wear in the brake assemblies.

Electrical and brake-controller troubleshooting

These steps focus on power delivery, grounding, and proper brake-controller operation. Do them in a safe, controlled environment and with the trailer disconnected from any moving vehicle.

Check power, ground, and controller output

• Verify the tow vehicle’s brake controller is powered and outputting voltage when you apply the brakes. Use a multimeter to measure the brake output voltage at the trailer connector during a brake test.


• Test the trailer connector ground. A poor ground can mimic a failure to activate brakes. Check for a solid ground connection to the trailer frame and clean any corrosion from the ground pin and contact surfaces.


• Inspect fuses and relays in the tow vehicle that feed the brake controller and trailer circuit. Replace any blown fuses and reseat relays as needed.

Electrical faults are the leading cause of non‑activation. If there is no brake output or the ground is flaky, focus on the vehicle side wiring and controller configuration first.

Connector and breakaway considerations

• Inspect the trailer’s 7‑pin or 6‑pin connector for bent pins, corrosion, or damaged insulation. Replace or reseat the harness if pins are misaligned or corroded.


• Check the breakaway system: ensure the breakaway battery is charged, the switch is intact, and the wire from the breakaway harness is properly connected to the trailer’s braking circuit.

Connectivity and periphery components like the connector and breakaway system can prevent any brake activation even when the controller is signaling correctly.

Mechanical and hub-related considerations

When electrical signals reach the brakes but the components fail to engage, the issue often lies in the brake hardware itself. Visual inspection and basic testing can reveal worn or seized parts.

Brake magnets, drums, and bearings

• Worn or damaged brake magnets or coils can fail to attract and apply the brakes. Measure resistance with a multimeter and compare against the manufacturer’s specification; replace if open or shorted.


• Seized or partially seized components: stuck shoes, jammed drums, or rusted contact surfaces can prevent engagement. Inspect for excessive heat discoloration or scoring on the drum and shoe surface.


• Worn or damaged wheel bearings: excessive play or rough rotation can affect braking efficiency. Repack or replace bearings, and check seals for leaks.


• Improperly installed or damaged drums: scoring, cracks, or incorrect drum/shoe clearance can hinder braking. Replace drums and shoes as a matched set when needed.

Mechanical faults can mimic electrical issues. If magnets, shoes, drums, or bearings are compromised, the brakes may not engage even with proper electrical input.

Understanding the system: hydraulic vs electric brakes

Most trailers use electric brakes controlled by the tow vehicle’s brake controller. Some trailers use hydraulic surge brakes, which activate differently. Knowing which system you have helps you troubleshoot correctly:

• Electric brakes: rely on 12V power from the tow vehicle through the connector to energize brake magnets. Check controller output, wiring, and grounding.


• Hydraulic surge brakes: depend on the trailer’s fluid system and actuator. If these don’t activate, inspect hydraulic lines, actuator, and fluid level for leaks or air in the lines.

Mismatch between the system type and troubleshooting approach can lead to misdiagnosis. Confirm your trailer’s braking technology before deep troubleshooting.

Step-by-step diagnostic checklist

Follow a structured sequence to isolate the fault quickly. Before starting, secure the trailer, chock wheels, and ensure no movement could occur during testing.

1. Confirm the brake controller is powered and producing a signal when the brake pedal is pressed or the trailer is commanded to brake. Use a multimeter to check the brake output at the trailer connector.


2. Test the trailer connector ground by measuring resistance between the ground pin and the trailer frame. If resistance is high, repair or replace the ground connection.


3. Inspect all fuses and relays on the tow vehicle that feed the brake circuit; replace any that are blown.


4. Inspect the trailer’s connector pins for corrosion or damage; reseat or replace the connector/harness as needed.


5. Visually inspect the trailer brake components at each hub: magnets, wiring, drums, and shoes for wear, corrosion, or damage.


6. Perform a controlled bench test by applying a 12V supply to the brake magnets (via proper wiring) with the trailer unattached to a vehicle. Observe whether the magnets engage and hold.


7. Test the breakaway system if present: confirm the breakaway battery is charged and the switch wiring is intact; trigger the breakaway to verify brakes engage as a final safety check.

Following this checklist helps distinguish electrical faults from mechanical faults, enabling targeted repairs and safer operation.

Safety considerations and next steps

If you detect burning smells, hear grinding, or see fluid leaks, stop immediately and seek professional help. Brake systems are critical to stopping power and trailer control, especially at highway speeds. If you’re unsure about any step, contact a licensed trailer shop or a brake specialist.

Summary

Trailer brakes not activating is usually caused by electrical issues (power, ground, brake controller, or connector problems), breakaway system faults, or mechanical wear in the brake assemblies (magnets, drums, or bearings). Start with verifying the brake controller output, test the trailer connector ground, and check fuses, then inspect the brake hardware at each hub. Understand whether you have electric or hydraulic brakes to follow the correct diagnostic path. If in doubt, consult a professional to prevent unsafe driving conditions.

How to tell if trailer brake magnets are worn?

You can tell if trailer brake magnets are worn by visually inspecting them for wear indicators, such as the four small holes on the magnet's face or exposed windings, and by using a multimeter to check their electrical resistance. Signs of wear include the holes filling in or disappearing, visible copper windings, and a smooth, glazed surface. 
This video demonstrates how to visually inspect trailer brake magnets and identify wear: 59sIntruderTrailersYouTube · Dec 15, 2010
Visual inspection

• Check the wear indicator holes: Most magnets have four small holes on their face, one at 12, 3, 6, and 9 o'clock. If these holes are gone, filled in, or worn down, the magnet is worn out and needs replacement. 
• Look for exposed windings: The magnet's surface should be rough and matte. If you can see the copper wire windings, it is a clear sign that the magnet is worn down. 
• Inspect for glazing: If the magnet has a smooth, polished, or "glazed" appearance instead of a rough one, it has likely experienced excessive wear. 
• Check for damage: Look for any cracks, grooves, or damage to the magnet's surface. 

Electrical testing

• Test resistance with a multimeter: 
  • Remove the hub and drum to access the magnet. 
  • Using an ohmmeter, measure the resistance between the two wires connected to the magnet. 
  • Compare the reading to the manufacturer's specifications for your brake drum size. For example, a 10-inch brake drum should have a resistance of around 3.8−4.03.8 minus 4.03.8−4.0 ohms. If the reading is outside this range, the magnet is likely faulty. 
• Test amperage:
  • While this method is more involved, you can use an ammeter to check the magnet's current draw. Connect the ammeter between a 12-volt battery and the magnet wires. 
  • If the amperage is greater than the specified amount (around 3.23.23.2 amps each), the magnet is bad. If the reading is less, the wires may be bad. 
  • You can also check if the magnet is shorted by connecting one lead to one of the magnet wires and the other lead to the magnet's base. If any amperage is detected, the magnet is shorted. 

Why do my trailer running lights work but not brake or blinkers?

If your trailer running lights work but the brake and turn signals don't, the most likely causes are a burned-out bulb, a bad ground connection, a blown fuse in the tow vehicle, or a wiring issue on either the truck or the trailer. Start by checking the bulbs and ground connections on the trailer, and then inspect the tow vehicle's fuses, plug, and wiring. 
On the trailer

• Check the bulbs: Since the running lights still work, it's possible only the brake and turn signal filament in a dual-filament bulb has burned out. Replace any suspect bulbs. 
• Check the ground connections: A bad or corroded ground connection is a very common cause of this problem. Check that each light's ground wire is securely fastened to the trailer's metal frame and is free of rust, paint, or corrosion. 

This video demonstrates how to check the ground connections on a trailer: 58sThomas Fix ItYouTube · Mar 17, 2023
On the tow vehicle

• Check the fuses: The tow vehicle has specific fuses for the trailer's brake and turn signals. Check your vehicle's fuse box (and the owner's manual to locate them) for any blown fuses related to the trailer's towing package. 
• Inspect the wiring and plug: Examine the wiring harness from the truck and the trailer's connector for any signs of damage, such as pinched, cut, or frayed wires. Also, check for corrosion on the pins in the truck's socket. 

You can watch this video to learn how to check for damaged wires on a trailer: 1mHomes for BeginnersYouTube · May 22, 2020
If the problem persists

• Use a test light: A circuit tester or test light can help identify where the signal is being lost. You can use it to check for power at different points, starting with the truck's connector and working your way back to the trailer. 
• Check for shorts: A short in the wiring can blow a fuse. If a new fuse blows immediately, it indicates a short that needs to be found and fixed. 

How to tell if a trailer brake module is bad?

A bad trailer brake module can be identified by symptoms like weak or delayed braking, a "pushing" sensation when stopping, grinding noises, or an unresponsive brake controller display. To diagnose further, perform a "tug test," check for error messages on the controller, and use a multimeter to test for proper voltage and resistance at the wiring and individual brake magnets.
 
This video demonstrates how to perform a tug test and check for warning lights: 57sShaners Mechanic LifeYouTube · Aug 11, 2023
Common symptoms

• Poor braking performance: The trailer doesn't slow down enough, or the brakes feel weak, delayed, or are sluggish. 
• "Pushing" sensation: The trailer seems to shove your tow vehicle forward during stops, indicating the trailer brakes are not engaging properly. 
• Unusual noises: Listen for squealing, grinding, or scraping sounds that can point to worn parts or debris. 
• Uneven braking: The trailer pulls to one side when braking, which can be caused by an electrical imbalance or one brake assembly not working correctly. 
• Brake controller issues: The controller may display error messages, a "trailer disconnected" warning, or have an unresponsive display. 

This video explains how to troubleshoot common problems with trailer brakes: 1metrailerYouTube · Dec 30, 2024
Diagnostic steps

1. Perform a tug test: While driving slowly, apply the trailer brakes (using the manual slide on the controller) without using your foot brake. You should feel the trailer brakes engage. If you don't, it suggests an issue with the system, such as the module or wiring, says this YouTube video. 
2. Check the brake controller display: Look for any error codes or warning lights. Some controllers have a "trailer disconnected" warning that can indicate a problem with the module or the connection to the trailer. 
3. Test the voltage at the controller: Use a multimeter to check the voltage on the wires going into the brake controller. There should be power on the black wire at all times, and power on the red wire only when the brake pedal is pressed, notes this etrailer.com article. 
4. Test the brake magnets: If the controller is getting power, test the individual brake magnets on the trailer's axles. Set your multimeter to ohms and measure the resistance between the two wires on each magnet. Compare the reading to the specifications for your brake magnet size. 
5. Visually inspect the magnets: Look for signs of wear, such as the coil being worn down, pitting, grease, or oil residue on the magnet's surface. If any of these are found, the magnet needs to be replaced, according to Hitchweb. 

Why won't my trailer brakes engage?

This can be caused by various factors, including worn brake shoes, a faulty brake controller, or electrical problems. If your trailer brakes work inconsistently, it could be due to loose wiring connections, a failing brake controller, or corroded brake components. Sometimes, trailer wheels may lock up unexpectedly.