How to adjust electric trailer brakes
Two adjustments to worry about: the brake controller gain in the truck, and the manual brake shoes on the trailer. Step-by-step for both.
There are two separate adjustments that get called “adjusting the brakes” and people mix them up. The brake controller gain in the truck sets how hard the trailer brakes hit when you press the pedal. The star wheel inside each drum sets how close the shoes sit to the drum. Both matter. Both are easy.
If the trailer feels like it’s pushing the truck during stops, you need more gain. If a brake test stop locks up the trailer wheels, you need less. If the gain is right and the trailer still won’t stop properly, that’s when you adjust the shoes themselves.
When you need to adjust
- Every 3,000 miles or roughly every 3 months of use
- After the first 200 miles on a new trailer (initial wear-in)
- Any time the trailer sits unused for more than a few months
- When the brake controller stops feeling responsive even at higher gain settings
Setting the controller gain in the truck
This is the dashboard adjustment, not the shoe adjustment. Do it first.
- Find an empty road or parking lot with no traffic.
- Accelerate to 20 to 25 mph on flat, dry pavement.
- Without using the truck brake, press the controller’s manual override (the slider or paddle on the brake controller) all the way.
- Note what the trailer does.
The right gain feels like firm, smooth braking with no skid or lockup. Most setups land between 4.0 and 7.0 on a 0-to-10 scale, but heavier trailers need higher numbers.
If the wheels lock or skid, the gain is too high. Drop it by 0.5 and retest.
If the trailer barely slows the rig, the gain is too low. Raise it by 0.5 and retest.
A loaded trailer needs more gain than an empty one, so you’ll re-tune when the load changes meaningfully. The 12,000 lb fifth wheel loaded for a month of camping won’t have the same setting as the same trailer empty on the way home.
Setting the gain on the road
Once you’re rolling on the highway, the trailer should feel like a passive follower during stops. Not pushing, not dragging.
| Symptom | Adjustment |
|---|---|
| Trailer pushes truck during stops | Gain up 0.5 |
| Trailer drags truck during stops | Gain down 0.5 |
| Wheels lock on a 20 mph test stop | Gain down 0.5 |
| Wheels barely engage on a 20 mph test stop | Gain up 0.5 |
Make changes in small steps. The controller is a multiplier, and one click can flip the feel.
Adjusting the brake shoes themselves
If the gain is maxed out and the trailer still won’t stop well, the shoes are probably too far from the drum. This is a mechanical adjustment under the wheel, done with the trailer jacked up.
You’ll need: a jack rated for the trailer’s axle weight, jack stands, wheel chocks on the wheels that stay on the ground, a brake spoon or 1/4 inch flathead screwdriver, and rubber gloves.
Step 1. Chock the wheels you’re not lifting. Jack up the side you’re working on under the frame, not the axle. Put a jack stand under for safety.
Step 2. Spin the wheel by hand. It should turn freely with just a faint scrape if the shoes are close. If there’s no resistance at all, the shoes are too far from the drum.
Step 3. Find the brake adjustment slot. It’s a small oval slot in the backing plate (the part behind the brake drum) or in the bottom of the drum itself. Some trailers cover it with a rubber plug. Pop the plug out.
Step 4. Reach the star wheel through the slot with a brake spoon. Rotate it one click at a time, spinning the wheel between clicks. The shoes are getting closer to the drum.
Step 5. Stop when the wheel becomes hard to turn by hand. Back the star wheel off by 8 to 10 clicks so the wheel spins freely with a slight drag.
Step 6. Replace the plug. Lower the trailer. Repeat for every braked wheel on the trailer (most tandem-axle trailers have brakes on all four).
If the star wheel won’t move, the threads are likely seized. A shot of penetrating oil and 20 minutes of patience usually frees it. If it still won’t move, the brakes are due for a full teardown and rebuild.
Quick check after adjusting
Hitch up. Roll at 5 mph in a flat parking lot. Tap the brake controller’s manual override. The trailer should feel like it’s gently grabbing. If you don’t feel it, gain is too low or shoes need more adjustment. If it lurches to a stop, back off.
Why this matters
Electric trailer brakes are a magnet pulled toward a rotating drum. When the magnet attracts, a lever pushes the shoes outward against the drum. If the shoes are too far away, the magnet pulls but the shoes barely contact. If the shoes are too close, the wheel can drag and overheat the drum.
A trailer with proper brakes stops the rig together with the truck. A trailer with bad brakes pushes the rear of the truck forward at every stop, which is how jackknifes happen on slick roads.
Federal regulation requires brakes on any trailer over 3,000 lb in most states. California cuts in at 1,500 lb. Some states require brakes on every axle, not just one. Check the state law where you live and where you tow.