Electric trailer brakes must be controlled by a brake controller mounted in the towing vehicle within easy reach of the driver. This allows quick and easy adjustments to the controller to provide for different levels of brake application for varying trailer loads. This adjustment regulates the voltage available for the electric brakes; higher voltages apply the brakes more firmly than lower levels do. A sudden loss of the ability to modulate the level of brake application may indicate controller failure. Testing for this problem is well within the ability of most weekend mechanics, and requires a minimum amount of equipment.
Things You'll Need
Press down on the brake pedal while watching the brake controller. The display on the controller should light up and hold a relatively steady reading, which should not vary by more than 1/10 of a volt up and down. It may be necessary to turn the ignition key to the "On" position for this step.
Check the fuse to the controller if the display does not light up. Replace the fuse if it is burned and repeat the test. Another burned fuse indicates a short circuit that must be found and repaired before further testing of the controller can be done.
Go to the trailer connector on the back of the tow vehicle with your multimeter. Have your helper hold down the brake pedal while you probe the terminal that is connected to the blue wire in the connector; in a properly wired connector this wire is the brake control wire and will have voltage present when the tow vehicle brake is applied. The amount of voltage present will vary as your helper moves the adjustment control on the brake controller. No voltage, or a voltage that does not vary, indicates a faulty controller or a problem with the blue wire itself.
Test for a problem with the blue wire by probing for voltage in the blue wire at the back of the controller. A lack of voltage here, or a voltage that cannot be modulated, indicates a failed controller.
Tips & Warnings
- Sometimes finding a suitable ground for the second multimeter probe is a problem. Removing any rust and exposing bare metal where the multimeter ground probe makes contact is a good idea before probing the blue brake wire.
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