Electric Brake Service and Electrical Diagnosis

Electric brake current energizes the magnet, which is attracted to the rotating drum and moves the actuating lever. The actuating cam pushes the primary shoe outward, and primary-shoe force brings the secondary shoe into contact. More magnet current increases braking force. During normal towing, the trailer and tow-vehicle brakes must work together; synchronization is established by road testing.

After shoe and lining replacement, burnish the brakes with 20–30 applications that reduce speed by 20 mph, such as 40 mph to 20 mph, with cooling time between applications. Replace linings worn to 1/16 inch or less and replace shoes contaminated by oil or grease or damaged by scoring, pitting or gouging. Replace both shoes in each brake and the shoes on both brakes of the axle together. Replace magnets immediately if their coils are exposed; magnet faces should be flat, and uneven wear calls for replacement. Refacing the drum armature accompanies magnet replacement, and replacing the opposite-side magnet is recommended for even braking.

Inspect and service immediately when brake operation is abnormal; annual service is normal for ordinary use. Return cleaned components to their original brake assembly. Before drum reassembly, apply a light film of white grease or anti-seize to the anchor pin, actuating-arm bushing and pin, and backing-plate contact areas for the shoes and magnet lever; lightly grease the actuating block. If manual adjustment is needed, safely support the trailer, remove the slot cover, expand the shoes until the wheel is difficult to turn, then back off to free rotation with slight drag while holding the automatic lever clear. Replace the cover and set all brakes to the same clearance.

For the disc-brake option, inspect pads and rotor surfaces every 36,000 miles or 12 months, whichever comes first. Deep rotor grooves can indicate a caliper-piston, slider-bolt or residual-pressure problem. Turn the rotors when replacing pads. Caliper mounting bolts require 40–50 ft-lb and blue thread-locking compound on reassembly; lubricate slider-bolt bushings only with silicone-based grease because petroleum grease is incompatible with their rubber.

Begin electrical troubleshooting at the controller and use its manufacturer’s adjustment and test instructions. Measure brake voltage across the magnet’s two leads with the tow-vehicle engine running and the trailer coupler connected. Voltage should rise gradually from zero to approximately 12 volts as the brake pedal is applied. A rapid rise can cause grabbing, while failure to reach at least 12 volts can point to wiring, battery or alternator issues.

For a maximum-current check, set any system resistor to zero or bypass it. Measure an individual magnet by disconnecting one lead connection and placing the ammeter in series, then reconnect and seal the wires after testing. The chart lists 3.0 amps per magnet, 6.0 amps for two brakes, 12.0 amps for four and 18.0 amps for six. High current can indicate a short: if it falls to zero when the trailer is unplugged, the short is in the trailer; if it stays high with all magnets disconnected, the short is in the trailer wiring. Loose or corroded connections increase resistance and reduce voltage available to the magnets.