Trailer brake voltage

Barnbuilder · Aug 11, 2015

I have a 99 and a 15 F350. I have an enclosed gooseneck that I just put new brakes on. And I have a new 10 ton gooseneck. Neither of these trailers seem to stop well with either truck. I was checking voltage and at the junction box on the trailers I was getting 9 volts and at the axles 8 volts. Is there that much voltage drop? I can't get the brakes on either to lock up. The 99 truck has a P3 and the 15 has the factory controller.

George2615 · Aug 11, 2015

My brake controller varies the voltage to the trailer brakes. The voltage applied to the trailer (0 to 12+) is controlled by the controller as you are stopping.
The manual override lever should apply full battery voltage to the trailer brakes. If you are applying the manual override lever and only getting 8-9 V there may be a controller problem. If the controllers are set up properly you may have a wiring problem.
Here's a link on the P3 controller. Owners manual should describe setting the factory unit.

p3 trailer brake controller adjustment - Bing Videos

tcartwri · Aug 11, 2015

We had to run heavier gauge wire from the controller on our 03. The difference was night and day. I'm assuming you're using the Ford harness.

Diamondpilot · Aug 11, 2015

How are the grounds?

Chris

Barnbuilder · Aug 12, 2015

Yes I was using the manual control lever.

Barnbuilder · Aug 12, 2015

Grounds are good the big trailer is brand new. Both controllers are set at max.

George2615 · Aug 12, 2015

If you are getting 8-9 V at the trailer end, what V do you have right at the controller. If it's full voltage you may have to run a heavier wire back to the trailer plug like tcartwri mentioned. Also, double check your ground as Chris mentioned or try adding another ground with a temporary jumper from truck to trailer and recheck voltage.

Your 2015 should still be covered under warranty. If you can't figure it out take it back to the dealer and ask them to diagnose / fix the low voltage problem.

Diamondpilot · Aug 12, 2015

Check the voltage at the plug on the back of the trucks and compare. This will give you a starting point.

Chris

Barnbuilder · Aug 12, 2015

With the trailer unplugged I checked voltage at the socket and it was 12.5.

Yes I took it back to the dealer with a load on it. They tested the truck and said it was good. Turn the gain down to 3 and pulled it around the gravel parking lot and said it was stopping good. Well that's great but running 60 and hit the brakes it's not. I don't know if I'm expecting too much but I would think a 10 ton running gear would lock up when empty and controller at max gain. I did adjust the brakes and it helped. Maybe the brakes need time to get worn in.
I know on the enclosed trailer I just replaced the brakes and pulled them apart again and maybe half the shoe is in contact with the drum.
Get tired of having to fix things even when new.

tcartwri · Aug 12, 2015

Measuring the voltage will tell you very little. To calculate the actual voltage drop at the brakes, you need to know four things. Voltage, current, distance, and resistance of the wire. We know that there are already considerable losses in the factory harness if you're only reading 12.5 at the connector.

Typical electric brakes pull about 15A at full load for four brake drums.

Plugging those known numbers into a voltage drop calculator will show you very quickly why having the right size wire will make a huge difference in your braking performance on a heavy trailer.

In our case we went from a gain setting of 9 or 10, to 3 or 4 when loaded. The factory Ford harness simply was completely undersized for the job.