How to Adjust Electric Brakes on a Trailer

Electric trailer brakes don't self-adjust the way some drum brake systems do — or when they do, the auto-adjustment can fall out of calibration over time. Knowing how to adjust them correctly keeps stopping distances predictable, reduces brake wear, and prevents the kind of trailer sway that turns a routine haul into a dangerous situation.

How Electric Trailer Brakes Work

Electric trailer brakes use electromagnets mounted inside drum brake assemblies. When you apply the tow vehicle's brakes, the brake controller sends a voltage signal to those magnets. The magnets energize, grab the rotating drum, and use that motion to force the brake shoes outward against the drum — slowing the trailer.

Two adjustments govern how well this system works:

1. The mechanical brake adjustment — how closely the shoes sit to the drum
2. The brake controller gain setting — how much electrical current the controller delivers to the magnets

Both have to be right. A perfectly calibrated controller does nothing if the shoes are worn down or sitting too far from the drum.

Adjusting the Brake Shoes (Mechanical Adjustment)

This is the physical side of the job — setting the correct clearance between the brake shoes and the drum interior.

What you'll need:

• Floor jack and jack stands
• Brake spoon (adjuster tool) or a flathead screwdriver
• Flashlight
• Wheel chocks

General process:

1. Chock the tires on any axle you're not working on. Safety first — you're lifting a loaded or semi-loaded trailer.
2. Raise the axle so the wheel spins freely. Support it on jack stands.
3. Remove the rubber plug from the adjuster slot on the back of the brake backing plate. This exposes the star wheel adjuster.
4. Insert your brake spoon or screwdriver through the slot and engage the star wheel.
5. Expand the shoes by rotating the star wheel in the correct direction (typically downward on the tool to expand — but verify for your specific axle, as some are reversed).
6. Spin the wheel by hand while adjusting. Stop expanding when you feel noticeable drag — the shoes are making contact.
7. Back off the adjuster slightly — usually 8 to 12 clicks — until the wheel spins freely with only a light, even drag.
8. Replace the rubber plug, lower the wheel, and repeat on each brake assembly.

The goal is minimal clearance without the shoes riding against the drum while driving. Too tight causes overheating and premature wear. Too loose means delayed braking response and excessive electromagnet effort.

Adjusting the Brake Controller Gain ⚙️

The gain setting on your in-cab brake controller determines how aggressively the system applies trailer brakes relative to your tow vehicle's braking input.

This isn't a one-number answer — it depends on:

• Trailer weight and load (heavier loads generally need higher gain)
• Tow vehicle weight (a heavier truck handles the same trailer differently than a lighter one)
• Number of braking axles on the trailer
• Road conditions and typical terrain

How to set gain correctly:

1. Find a flat, open road or parking lot with no traffic.
2. Drive to approximately 25 mph.
3. Apply the manual override on your brake controller — this activates only the trailer brakes, not the tow vehicle's.
4. Observe what happens:
   • If the trailer wheels lock up or you hear/feel harsh grabbing, gain is too high — reduce it.
   • If you feel no resistance or very little pull, gain is too low — increase it.
   • The correct setting produces a firm, steady pull from the trailer without wheel lockup.
5. Repeat until the response feels balanced and controlled.

Most controllers use a dial or digital setting on a scale (often 0–10). Start in the middle and work from there.

Variables That Change the Right Adjustment 🔧

Trailer brake adjustment isn't universal. Several factors affect what "correctly adjusted" looks like in practice:

Brake shoes also wear unevenly depending on how often the trailer is loaded, how hard braking tends to be, and whether previous adjustments were accurate. An adjustment that was right six months ago may not be right today.

When Mechanical Adjustment Isn't Enough

If you've adjusted the shoes and set the gain correctly but the trailer still brakes poorly — or brakes unevenly from side to side — the problem may be:

• Worn or glazed brake shoes that need replacement
• Weak or failed electromagnets that aren't generating enough pull
• A failing brake controller sending inconsistent voltage
• Corroded wiring or connectors in the trailer's brake circuit

A multimeter can help you check magnet resistance (typically 3.0–3.5 ohms per magnet is normal, though specs vary by manufacturer). Values significantly outside that range usually mean a magnet is failing.

What Your Specific Setup Changes

The right adjustment process and the right settings depend on your trailer's axle manufacturer, the number of braking axles, your tow vehicle's weight, your brake controller model, and how you typically load and use the trailer. A lightly loaded single-axle utility trailer and a fully loaded three-axle fifth-wheel live in completely different adjustment ranges — even if both have "electric brakes."

The mechanical steps above reflect standard drum brake practice, but your axle manufacturer's service documentation is the authoritative source for torque specs, adjuster direction, and shoe clearance tolerances specific to your equipment.