I have a large dovetail trailer and a small tractor that I occasionally tow. Over the last couple of weeks my truck shows "trailer disconnected". I just replaced the trailer plug thinking it could be the issue but it isn't. After about 6 miles down the road the truck will recognize the trailer and the trailer brake will start working. My theory is that the issue is with the trailer brake battery. I have never replaced it and it is about 5 years old. Am I on the right track with my theory?

 

I would better guess that you have a grounding issue. I would run a simple ground from trailer to truck frames, even if it was simple jumper cables to make the ground, then take for a test drive.

 

I'll check the battery and the trailer ground this weekend.

 

What year/make/model of truck? IIRC, there was/is a recall or TSB related to trailer disconnect for newer F150s in the not so recent past.

 

The trailer battery is not connected to the brakes until it departs from the tow vehicle and the breakaway gets pulled. The tow vehicle is perfectly capable of providing full braking power to the trailer brakes on it's own.

What the truck is telling you is that it's not seeing the brakes when it checks them every few seconds. The ITBC puts a minute voltage on the brake wire, not enough to engage the brakes but enough to see some current flow. If current doesn't flow, there is an open somewhere.

Most often this is due to corrosion in the connections. There's the 7-pin connector in the bumper, a plug that goes into the back of it behind the bumper, a connection in the harness near that, and the ground connection at the frame. Unplug the two connections behind the bumper, spray some electronics cleaner into them, use a soft toothbrush or electronics brush, spray again, let dry, fill with dielectric grease and re-connect. Check that the ground connection is tight and not corroding, and the frame is not rusting. Sand if required bolt back up and covering with something protective like paint.

Scrub out the 7-pin in a similar manner, then lightly grease all 7 pins with dielectric grease. Lightly, as it's hard to keep dirt and grime out of the connector and you don't want it sticking to the pins.

If this does not solve your issues, check the wiring on your trailer. Trailers are more often than not wired very poorly, with non-water tight connectors.

 

Truck is 2012 F150 supercab with 3.5L Eco Boost.
I played around with the trailer ground a little yesterday and did not correct the issue. I am fairly sure the trailer battery is dead but I have not confirmed that yet. The male 7 pin plug on the trailer is new. I'll dig into the truck side of the plug when I get some time and clean it. What os recommended to clean the contacts/plugs?

 

You can use brake cleaner in a pinch. Electronics cleaner can be used as often as you want as it won't degrade the silicon seals used in connectors.

Isopropyl alcohol.

 

The trailer breakaway battery has nothing to do with it. I tow trailers with surge brakes or no brakes and the truck knows the trailer is connected even though there is obviously no break away battery.

 

I have spray contact cleaner, plastic safe. You can find it at Walmart, Autozone etc. Good to have on hand. CRC is one brand, a few out there.
https://www.google.com/search?q=elec...&bih=924&dpr=1

 

I picked up a can of CRC electronics cleaner and got to work on it yesterday. Had to remove the spare tire to get to the wiring. Removed the ground wire and cleaned the attach point (sanded some paint off) then added some dielectric grease to it. Cleaned the separated plug connectors and added a small amount of dielectric grease there. Put that all back together. Cleaned the bumper mounted 7 pin plug. Hooked up my trailer and still have the same problem. I can turn off the truck, unplug the trailer and plug it back in, turn the truck back on and still nothing. I then left the truck on and unplugged the trailer then plug it back in and it works. There is no rhyme or reason on when/if it works.

I realize that the trailer battery has nothing to do with this but I did remove it from the trailer and it is dead. Have a new one on order. Thinking about adding a solar charger to that battery since I let my trailer sit for long periods of time.