Troubleshooting Trailer Brakes Locking Up and Brake Controller Reading No Trailer Connection


Ok, have a single brake axle on a recently purchased 16 ft trailer. The brakes lock when the trailer is connected to truck and the brake controller indicates no trailer connection. The trucks electrical and any brake controller malfuctions has been eliminated by testing as well as hooking up to multiple other trailers with no problems....and I can find no obvious problems in the trailers electrical....any suggestions as to the cause?

Expert Reply:

This could be a ground problem, a problem with the trailer connectors, or a problem with the brakes. The first thing I would check is the trailer connectors on the truck and trailer. Make sure they are clean and free of corrosion. Corrosion could cause a short between the 12 volt power wire and the brake output wire. You should also check for dirt and corrosion on the back side of the connectors.

If this does not solve the problem, you can also make sure that the trailer is pinned out the same as the connector on the truck. I have included a link to our wiring FAQ article that shows the functions of both the vehicle side and trailer side connectors.

Next, make sure the ground coming from the trailer side plug is properly attached to a clean and corrosion free bare metal surface. Do the same for the brake magnet ground wires. Last, you can check the brakes themselves to make sure they are functioning properly or if they need to be replaced.

I would check the magnets for wear before going on to test them with a meter. If the magnets show abnormal wear or if you can see the windings beginning to show through the friction surface of the magnet, they will need to be replaced.

To further test your magnets, first make sure the trailer is not connected when performing this test. If the trailer is connected, you run the risk of shorting out a brake controller or various other electrical components.

To test the brake magnets with an ammeter, connect the ammeter between the positive battery terminal of a separate battery and one of the magnet wires. Then, connect the other magnet wire to the negative battery terminal. Check the amperage, if the amperage is greater than the specified amount in the chart below, replace the magnet. If the reading is less than the specified amount, then the leads are bad and the magnet should be replaced.

Electromagnet Specifications:

7 inch Diameter Brake Drum:
Max Amps at 9-10.5 Volts
2 Brakes 4.7-5.6
4 Brakes 9.5-11
6 Brakes 14.3-16

Max Amps at 12-13 Volts
2 Brakes 6.3-6.8
4 Brakes 12.6-13.7
6 Brakes 19.0-20.6

10 and 12 Inch Brake Drums
Max Amps at 9-10.5 Volts
2 Brakes 5.6-6.6
4 Brakes 11.3-13.3
6 Brakes 17.0-20.0

Max Amps at 12-13 Volts
2 Brakes 7.5-8.2
4 Brakes 15.0-16.3
6 Brakes 22.6-24.5

Next, disconnect the negative wire from the brake magnet wire and connect it to the base, housing, of the brake magnet. If there is any amperage, the magnet is shorted and will need to be replaced.

Finally, using an ohmmeter, check the resistance between the brake magnet wires. If the resistance is not within the specified range below, the magnet will need to be replaced.

7 inch brake magnet resistance should be 3.8-4.0 Ohms
10 and 12 inch brake magnets resistance should be 3.2-3.5 Ohms

expert reply by:
Michael H

Ask The Experts a Question >>

Product Page this Question was Asked From

Similar Expert Q&A Pages

See More Q&A Expert Answers >>

About Us
photos and videos

Original Photos & Videos

Produced to make sure you know what you are getting and you get exactly what you need.


Installations Completed

To make sure products work and fit the way they are supposed to.

etrailer call center

Phone Calls & Emails Answered

1,125,924 phone calls and 1,350,587 emails to help find the right solution.

etrailer training

Average Hours of Product Training

We get to know our products firsthand so experts can better help you.

etrailer service

Years of Quality Customer Service

Assisting our neighbors and customers, face to face at the counter.

etrailer experts

Pages of Expert Information

Created to make sure you have all the answers to your questions, from real experts.