Brake controllers and Trailer brakes.

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Well-known member
Oct 4, 2007
I am a little confused (what else is new??). I have never towed a trailer with brakes and had a brake controller. I have towed a lot with smaller trailers and no trailer brakes but.... The trailer in the avatar here is mine but No, I did not tow it!!

Does the brake controller actually supply the 12V necessary to operate the brake magnets or is a separate 12V either fed to the trailer or supplied by the trailer battery required? If the controller does all the supplying, is the current (amp) level controlled by the controller unit for softer or harder braking?

(I understand that the trailer battery connection, switch and brake-away control wire is needed if the trailer requires a break-away system.)

Curious minds want to know!!!
The brake controller is powered by the truck's 12 volt system and sends voltage directly to the brake magnets through the brake wire on the 7 way connector.  The trailer batteries are not involved in any way.  The adjustments on the controller determine how much voltage is sent to the brakes according to how hard the vehicle is decelerating.

Where the trailer battery is involved is in the breakaway switch.  When the lanyard is pulled out of the switch socket full trailer battery voltage is sent to the magnets.  This way the trailer brakes will apply even if the electrical cable has been yanked out of the truck.

Both the truck and trailer share a common ground because their frames are tied together through the 7pin connector, so voltage from either system can activate the brake magnets.

There is a separate 12 volt pin in the connector, but it just charges the trailer batteries from the truck alternator.
Thank you Lou for the precise explanation. From this explanation, it is the voltage that is varied slightly rather than the amperage being controlled. Hope I got that right!!!

Thanks again.
My pleasure, Stu.  Voltage and current are related - if the resistance in the circuit stays constant both voltage and current will vary in proportion to one another.

The brake controller regulates the voltage that's applied to the brake line - the amount of current (amps) flowing through the line is determined by applied voltage and the circuit resistance.

You can use the voltage/current relationship as a troubleshooting tool - if you're sending voltage (pressure) down the line but the current (quantity) changes, you have a resistance change in the circuit.  Most likely causes would be a bad connection that causes more resistance, or an open brake magnet that's not drawing the proper amount of current.
Thanks Lou, yes, I understand the voltage /current relationship a little bit!! I was an  aircraft instrument tech in the Air Force and retired from the telephone switching industry. Heh heh!!

I enjoy your detailed explanations of a lot of this stuff that is not readily apparent, please keep them up!!!!
I have a 2009 cougar T T.  When I hook up my truck to the travel trailer to check my trailer lights all turn signals and running lights work correctly. But when i depress my brake pedal I blow the 20 amp fuse in my truck?  My LED on the brake controller does not lite and my left turn signal stops.  I have replaced the 30 and 50 amp breakers that are mounted to the side of the tongue of the trailer. Last season i had this happened and had the RV center troubleshoot this only to have them do the same repair? In an effort to save money I did the same repair. it worked until on our way home from our trip. does anybody have any ideas?  would appreciate any help!!!!!
Possible magnet shorting out?? Had T.T I worked on, after pulling tires and drum, the 12volt to magnet was grounding out to backing plate, as brakes were applied the actuation moved wires enough and would ground out. Had been going on long enough insulation worn from wires, wire clip support had either not been installed, or had fallen off and became dust???

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