Troubleshooting Weak Brakes on Tandem Axle Trailer

Updated 01/29/2026 | Published 08/22/2024

Question:

Sir: I have a Tekonsha P3 controller with a two axle trailer, four brakes. I have poor braking with the brakes fully adjusted. I ran the diagnostics on the P3 the battery voltage is 14.0, output voltage is 11.1 and output current is 9.5 amps. This is using the manual control level fully engaged. Your articles state that this indicates the magnets need to be replaced. Could you please advise how I can confirm this and make sure the controller is not faulty. Regards, Jim

asked by: Jim T

Expert Reply:

Hi Jim; I don't think it is the brake controller based on what you have already tested. However, depending on where you tested it could be the vehicle or trailer wiring. If you tested at the 7-way on your vehicle then that checks out. If you tested the wiring running out of the brake controller go ahead and test the vehicle 7-way for power on the brake output pin. If this checks out check for power at the brakes. If you have low power here there could be a short somewhere or a bad ground.

A bad ground is a fairly common problem and could definitely cause weak braking. It is always a good idea to check the ground or grounds on the vehicle and trailer. These should be secure on a clean, bare-metal surface. If all of this checks out you can test the brake magnets.

It is unlikely they're all bad, but nothing is impossible. For 10 and 12 inch brake assemblies, each magnet should read 3.2 to 4.0 amps at 12 to 13 volts. If the amperage readings are greater than the specified amounts, replace the magnets. If it is less, the leads are bad and the magnets should be replaced.

expert reply by: Robin H

Robin H.

1/27/2026

Hey Jim, what did you find when you checked the grounds and tested the brake magnets on your trailer? Knowing whether the readings matched the specs could really help others chasing weak braking issues with their Tekonsha P3 setups.

Jim T.

1/29/2026

@RobinH Robin, it turns out I had two problems. The brake magnets were in bad shape, lots of scoring and visual evidence of magnet overheating. They had maybe 25000 miles on them , so I had new brake assemblies installed. I never did measure the amperage or resistance of the magnets. I also replaced the 7 pin plug and pigtail on the trailer side with a Hopkins unit from eTrailer. I had a voltage drop from the 7 pin socket on the tow vehicle to the junction box on the trailer that the pigtail connects into. This suggested to me that the plug was not making good contact with the socket on the tow. vehicle. Cleaning the contacts did not help. After I installed the new pigtail, I noticed that the running lights on the trailer were quite a bit brignter than before, confirming there was a bad connection at the 7 pin socket. So it was two problems at once. I also think the problems were exacerbated by brake service at a sub standard shop, with little experience with trailers. I had the complete brake job done at a local reputable RV service shop. So one must pick quality shops for good service.