Troubleshooting Voltage Drop from 12V to 7V When Trailer Brakes are Installed

Question:

I checked my brake wire coming from my truck all the way to where it connects to my magnets for my electric brakes. I have almost 12v until I hook the wires up for my brake magnets and it drops to 7v. What could be my problem with my brake controller turned all the way up to 10.

asked by: Alexander W

Expert Reply:

It sounds like you may have a poor ground connection which is causing low voltage when you are testing. Sometimes it may look like we have a really good ground point but some paint or debris is getting in the way and causing issues.

What I recommend doing is taking your trailer brake ground wires and running them up to your main trailer ground location. You will likely need more 10 Gauge Wire # 10-1-1 and I recommend using Heat Shrink Butt Connectors # DW05745-5 because they hold up much better than standard butt connectors in the elements.

If you are still having issues then you may need to route the ground wires into your trailer 7-Way (5 o'clock pin) and then from there up to your tow vehicle battery to ensure that you have a constant ground on that wire.

Let me know if you're still having issues and I can help assist you further.

expert reply by: Jon G

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Wes

3/13/2025

I have the same issue, all new wires clean surface for grounds. Have 12 volts on the brake wire once I engage the trailer brake system it drops to 7.5

Jon G.

3/14/2025

@Wes Are your 7-Way plug and socket clean and free of corrosion?

Wes

3/14/2025

@JonG everything is brand new for this trailer besides the trailer itself. 3.1 ohms at the magnets. 12 volts out of the truck but once I engage the electric brake volts drop to 7.5. Only thing I haven’t done is rewire the trailer again. The lights are all self grounding. I had a ground from the axel to the trailer bare meta, and a ground from the plug to the tongue of the trailer bare metal