Electric Brake Operation, Synchronization, and Wear
- Explain how magnet current becomes shoe application force.
- Apply safe burnishing, synchronization, and adjustment procedures.
- Identify lining and magnet conditions requiring replacement.
- Trailer jack and adequately rated jack stands — Raise and support the trailer according to its manufacturer's instructions; never support it on the axle or suspension.
- Brake adjusting spoon — Turn the star wheel on brakes without self-adjustment to establish slight lining drag.
- Straightedge — Check magnet faces for uneven wear.
- Lining thickness gauge — Check lining thickness against the replacement threshold.
- Damp cleaning brush — Clean brake components without dry brushing or generating airborne brake dust.
Electric trailer brakes apply through electromagnets rather than hydraulic wheel cylinders. The energized magnet grips the rotating drum armature surface and moves the actuating lever. The lever applies the primary shoe, which transfers force through the adjuster to the secondary shoe. Increasing magnet current increases the braking force.
The controller must be set according to its manufacturer's instructions. Dexter describes approximately 2 volts when the brake pedal is first pressed, increasing gradually toward 12 volts as braking demand rises. A sudden high-voltage application can produce harsh braking or wheel lockup. Forward self-adjusting brakes compensate for lining wear during forward and reverse stops and do not require manual adjustment.
Before synchronization, burnish the brakes with 20-30 applications, each reducing speed by approximately 20 mph, such as 40 mph to 20 mph. Allow cooling between applications. Synchronization then requires road testing on dry pavement without sand, gravel, vehicles, or pedestrians. Adjust gain near impending lockup rather than sustained sliding; not every loaded brake system can lock its wheels.
Proper synchronization eliminates the sensation of the trailer pushing or jerking the tow vehicle. Dexter recommends trailer brake application just slightly ahead of the tow vehicle brakes. Do not use either vehicle's brakes alone to stop the combined load. For electric brakes without self-adjustment, adjust after the first 200 miles, every 3,000 miles thereafter, and as performance requires.
Inspect and service immediately when braking performance declines; annual service is usually adequate for normal use. Replace linings at 1/16 inch or less, or when contaminated with grease or oil, abnormally scored, or gouged. Hairline heat cracks in bonded linings are normally acceptable. Replace both shoes in each brake and both brakes on the axle. Check magnets with a straightedge, replace unevenly worn magnets or magnets with exposed coils, and replace magnets in axle pairs using genuine Dexter parts.
Clean without generating airborne brake dust because older linings may contain asbestos. Use a damp brush rather than compressed air or dry brushing. Lubricate the specified anchor, arm, and backing-plate contact points lightly, keeping lubricant off linings, drums, and magnets. Replacement shoes must be burnished again using the stated application and cooling procedure.
How It Works
Electrical input becomes mechanical braking
The energized electromagnet grips the rotating drum armature surface, moving the actuating lever to apply the primary shoe; force then passes through the adjuster to the secondary shoe. Increasing magnet current increases braking force, so a sudden high-voltage application can create harsh braking or wheel lockup. Follow the controller manufacturer's instructions; Dexter describes approximately 2 volts when the pedal is first pressed, increasing gradually toward 12 volts as demand rises.
Burnishing and synchronization serve different purposes
Burnishing beds the lining against the drum: use 20-30 applications, each reducing speed by approximately 20 mph, such as 40 mph to 20 mph, and allow cooling between applications to limit heat buildup. Synchronize afterward on dry pavement clear of sand, gravel, vehicles, and pedestrians, adjusting gain near impending lockup rather than sustained sliding; not every loaded brake system can lock its wheels. Proper synchronization removes pushing or jerking, with trailer application just slightly ahead of the tow vehicle brakes; never use either vehicle's brakes alone to stop the combined load.
Clearance, wear, and friction condition
Forward self-adjusting brakes compensate for lining wear during forward and reverse stops and do not require manual adjustment; brakes without self-adjustment require adjustment after the first 200 miles, every 3,000 miles thereafter, and as performance requires. Excessive clearance delays shoe contact, while insufficient clearance can cause drag and heat; on manually adjusted brakes, expand the shoes until the drum becomes very difficult to rotate, then back off until it rotates freely with slight lining drag. Inspect and service immediately when performance declines rather than waiting for annual service, which is usually adequate for normal use; lightly lubricate specified anchor, arm, and backing-plate contact points while keeping lubricant off linings, drums, and magnets.
Step-by-Step Procedure
0/7 doneElectric Brake Service Values
| Initial brake-pedal controller output | Approximately 2 volts, increasing gradually toward 12 volts |
| Burnishing | 20-30 applications with about a 20 mph speed reduction; allow cooling |
| Manual-adjust brake intervals | First 200 miles, then every 3,000 miles, or as required |
| Lining replacement thickness | 1/16 inch or less |
Diagnostic Chart
| Symptom | Check | Spec / Expected | Likely cause & fix |
|---|---|---|---|
| Trailer pushes the tow vehicle or jerks during braking. | Confirm burnishing is complete, review controller instructions, and evaluate synchronization in a clear, suitable road-test area. | Trailer brake application just slightly ahead of the tow vehicle brakes; gain near impending lockup, not sustained sliding. | Controller synchronization may not match trailer braking demand; abrupt application can produce harsh braking. Basis: LDServiceOnline |
| A manually adjusted brake drags or has inconsistent clearance. | Raise and support the trailer as instructed by its manufacturer, never on the axle or suspension. Confirm free wheel and drum rotation, remove the adjustment-slot cover if fitted, and expand the shoes until rotation becomes very difficult. Back off, refit the cover, lower the wheel, and repeat on all brakes. | Drum rotates freely with slight lining drag; matching clearance across all brakes. | The star-wheel setting may leave the shoes too tight or clearances unequal. Basis: LDServiceOnline |
| Braking performance declines or linings show visible damage. | Inspect immediately for lining thickness, grease or oil contamination, abnormal scoring, and gouging. Distinguish hairline heat cracks in bonded linings from replacement conditions. | Replace linings at 1/16 inch or less, or when contaminated, abnormally scored, or gouged. Replace both shoes in each brake and both brakes on the axle; burnish replacement shoes again. | Worn, contaminated, or damaged friction material may no longer provide dependable braking; hairline heat cracks in bonded linings are normally acceptable. Basis: LDServiceOnline |
| Magnet inspection reveals an irregular face or visible coils. | Check the magnet with a straightedge and inspect for exposed coils. | Replace unevenly worn magnets or magnets with exposed coils; replace magnets in axle pairs using genuine Dexter parts. | Magnet wear has reached a condition requiring replacement. Basis: LDServiceOnline |
At the Bay: Diagnose the Case
CASE 1 / 4
Trailer pushes the tow vehicle or jerks during braking.
- Test
- Compare
- Correct
Select the next check
Basis: LDServiceOnline. A likely cause is a hypothesis; confirm it with the applicable service procedure before repair.
Common Bay Mistakes
- ✕ Do not apply the manual-adjustment procedure to forward self-adjusting brakes.
- ✕ Do not chase sustained wheel lockup to prove synchronization.
- ✕ Never use compressed air or dry brushing on brake dust; older linings may contain asbestos.
- ✕ Do not mistake normally acceptable hairline heat cracks for an automatic lining rejection.
Quick Check
Practice only — not scored.
Plan the Service Sequence
Planning practice only. Follow the applicable manufacturer's instructions, employer safety program and qualified supervision on real equipment.
Plan the first action. Compare the service steps with the bay mistakes.
Hands-on: Shop Test Equipment
Dial Indicator — Bearing End Play
Magnetic-base dial indicator (0.001 in) on the hub face, spindle nut adjustment
🛠 Equip your bay for this test →Symptom: Bearing service — set end play after repack.
Spec limits used
- 0.001–0.010 in — Allowable end play, double-nut wheel end (Dexter 9-15K service manual, p.64)
- 0.008 in — Nev-R-Lube 50 mm max axial end play (Dexter 059-831-00)
- illustrative only — Face runout limit (General shop practice)
Simulated readings for practice. Always follow the manufacturer's procedure and values on a real trailer.
Source: LDServiceOnline — 6-18