Troubleshoot Electric Trailer Brakes

Most electric trailer brake malfunctions, that cannot be corrected by either brake adjustments or synchronization adjustments, can generally be traced to electrical system failure.  Voltmeters and ammeters are essential tools for proper troubleshooting of electric brakes.  Mechanical causes are ordinarily obvious i.e. bent or broken parts, worn out linings or magnets, seized lever arms or shoes, scored drums, loose parts, etc.  Replace defective or worn parts.  

How to Measure Voltage

System voltage is measured at the magnets by connecting the voltmeter to the two magnet lead wires at any brake.  This may be accomplished by using a pin probe inserted through the insulation of the wires.  The engine of the towing vehicle should be running when checking the voltage so that a low battery will not affect the readings.

Voltage in the system should begin at 0 volts and, as the controller bar is slowly actuated, should gradually increase to about 12 volts.  If the controller does not produce this voltage control, consult your controller manual.

The threshold voltage of a controller is the voltage applied to the brakes when the controller first turns on.  Lower threshold voltage will provide for smoother braking.  If the threshold voltage is too high, the brakes may feel grabby and harsh.

How to Measure Amperage

System amperage is the current flowing in the system when all the magnets are energized.  The amperage will vary in proportion to the voltage.  The engine of the tow vehicle should be running with the trailer connected when checking the trailer braking system.

One place to measure system amperage is at the BLUE wire of the controller which is the output to the brakes.  The BLUE wire must be disconnected and the ammeter put in series into the line.  System amperage draw should be as noted in the following table.  Make sure you ammeter has sufficient capacity and note polarity to prevent damaging you ammeter.

If a resistor is used in the system, it must be set at zero or bypassed completely to obtain the maximum amperage reading.  Individual amperage draw can be measured by inserting the ammeter in the line at the magnet you want to check.  Disconnect one of the magnet lead wire connectors and attach the ammeter between the two wires.  Make sure that the wires are properly reconnected and sealed after testing is completed. 

The most common electrical problem is low or no voltage and amperage at the brakes.  Common causes of this condition are; poor electrical connections, open circuits, insufficient wire size, broken wires, blown fuses (fusing of brakes is not recommended), improperly functioning controllers or resistors.

Another common electrical problem is shorted or partially shorted circuits (indicated by abnormally high system amperage).  Possible causes are; shorted magnet coils, defective controllers or bare wires contacting a grounded object.

Finding the cause of a short circuit in the system is done by isolating one section at a time.  If the high amperage reading drops to zero by unplugging the trailer, then the short is in the trailer.  If the amperage reading remains high with all the brake magnets disconnected, the short is in the trailer wiring.  

All electrical troubleshooting procedures should start at the  controller.  Most complaints regarding brake harshness or malfunction are traceable to improperly adjusted or nonfunctioning controllers.  See you controller manufacture's data for proper adjustment and testing procedures.  For best results, all the connection points in the brake wiring should be sealed to prevent corrosion.  Loose or corroded connectors will cause an increase in resistance which reduces the voltage available for the brake magnets.