Troubleshooting Electric Drum Brakes That Do Not Grip Despite Functioning Magnets

Question:

I have a persistent problem with the left hand Dexter Brake unit not working. I have adjusted the unit. The magnet is energizing when I test it getting correct voltage and not shorting to earth. I havent done a current check as I dont have an ammeter but voltage and resistance are fine. It magnetises and appears to work fine It will attract steel tools etc and can feel resistance to turn the drum when fitted. However when driven the left hand brake doesnt work effectively while the right hand side works well it has to as its only one trying. I have readjusted and retested number of times and still cant get it to work properly. Would appreciate some help as it has got me puzzled.

asked by: Josh

Expert Reply:

Based on your troubleshooting efforts so far I suggest that you test the trailer brakes without using a vehicle and brake controller; you can test the trailer brakes directly by applying +12V power and ground to the appropriate contacts on the trailer plug. Please refer to the linked photo. A set of jumper cables and 10-gauge wire like # 10-1-1 or # 10-2-1 make it simple enough to "hot wire" the brake circuit with the full power from your vehicle battery or a well-charged spare.

If you cannot adjust the brake's star wheel adjuster to get enough brake shoe tension with this full power signal then either there is a mechanical issue within the brake (a broken spring, sticking actuator arm or worn brake shoes) or perhaps your one drum has an issue with grease contamination. If grease did get on the brake shoes this CANNOT be cleaned off since it will get into the pores of the brake shoe material. New shoes are needed if grease was the original problem.

The drum could simply be worn to the point that its material thickness no longer can make enough contact with the shoe. Hubs are cast with a minimum armature surface thickness spec molded right into the hub casting. If your hub has more wear than this it will not grip the shoes.

I linked our main pages for all replacement electric drum brake parts and also for complete brake assemblies.

expert reply by: Adam R