Troubleshooting Trailer Brake Magnets That Receive Only 9.5 Volts from Trailer Wiring

Updated 05/30/2018 | Published 05/26/2018

Question:

Trailer brakes won’t lock up ! Got 12 at junction box but 9 and half at new magnets how many volts should I hav at magnets?

asked by: Michael

Expert Reply:

You can expect some minor voltage drop between your trailer's junction box and the brake magnet wiring but losing 2.5V suggests you have a short in your brake circuit.

I suggest tracing all brake circuit wiring from the j-box to each of the brake assemblies. Look for worn insulation, cracked insulation, points where the wiring may touch the trailer frame or moving suspension parts. Also check that any splices are solid. Also make sure that the magnet wire you're using as ground is solidly attached to clean bare metal on the trailer frame (unless you are grounding the brakes at the j-box, in which case you'll want to check the main ground).

I linked an article that explains how to test your brake magnets using a multi-meter like # PT89ZR.

expert reply by: Adam R