Confusing electric brake issue

After helping my brother replace all the bearings, brakes and drums on his hi-lo, he had the same problem he'd been having for some time: the brakes engage but not sufficiently to help in stopping. TV is 2016 F-250 and we're using the built-in brake controller set to 10.

I pulled the emergency brake pin, the trailer brakes lock, and we get a full 12.8V on the blue brake terminal (trailer batteries measure 12.8V).

However, with the trailer connected we cannot get the trailer brakes to lock although we can hear them actuate. Never more than a slight drag on the TV.

The voltage on the TV disconnected measured 13.8V. After connecting the trailer the voltage on the blue terminal drops to 10.3V. The receptacle was badly corroded so we bought a new cable for the camper and installed it expecting it to take care of the problem. It didn't. We now get 11V with trailer connected to the TV but still only a slight drag, no brake lock ever.

I don't understand why we don't more voltage when the brakes are actuated from the Ford brake controller and I'm not sure what to do next.

Any suggestions for troubleshooting are welcome.

Hi smoldt, welcome to the forum. You've done a good job troubleshooting the system, I think. From your description, I'd say your trailer wiring is sound, since you have full battery voltage when the safety brakes are engaged and they lock.

So, that leaves your tow vehicle and its wiring. I appreciate that you have 13.8V with everything disconnected, but that is under a no load condition too. Also, you are measuring voltages with everything connected when you get the low values. With jumper wires, try connecting ONLY the trailer brakes to the tow vehicle. (Connect the tow vehicle's brake wire and its ground to the trailer). If you get full lock and voltage then, there's a problem in the connection, most likely in the tow vehicle's plug or wiring. It would be allowing "leakage" through one of the other lines into ground.

OR, for some reason, the built-in brake controller is not able to supply full voltage with the trailer connected. You'd see the same low voltage and wimpy brakes with just the brake circuit connected. This is possible - you can have full voltage with no load, but as soon as current is allowed to flow, the voltage (pressure) drops. I saw this in a new battery once. No load, it read full voltage, but with even a tiny load the output dropped quickly.

Let us know what you find. You're having an interesting problem!

- Jack

Thanks, Jack. I’ve thought about excess current causing the voltage drop and that makes sense. It would draw the same current via the safety brake circuit but that’s coming directly off the battery so I might see a higher voltage than via the TV. I’ll have my brother try another TV again - he’s off on a trip right now.

Another TV would work too. Good idea!

BTW, you live just a bit north of my Sister. She's in Black Forest.

- Jack

Another long shot possibility is a grounding issue. The harness from the TV connector may have a higher resistance connection to the 'frame' ground than the trailer's breakaway brake. My 2200 trailer now has a 'ground' terminal block for all grounds and a dedicated ground return wire from the electric brake actuators to that terminal block. I was seeing 'poor' ground issues on my tank monitors and other circuits.