Trailer not charging / 7 pin harness - very strange

MO

MooseMan-HILO

Advanced Member
Joined
May 8, 2011
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Taking the advice of this forum and trying to clean up some electrical woes I have been having. Primarily related to arriving after a drive with a dead battery. Got a new battery, new terminals and diagnosing the truck's power out - there is none. But there's a twist.

All fuses are good. Weird thing is that when the truck is running my volt meter says 14 volts. When I put a test lamp on it, it doesn't work. It's so confusing I made a video - here it is. Could this be a relay?

 
With out being there to check for myself it's hard to say, but I would try a new fuse in the plus line. I have seen fuses cause your exact problem. Does your trailer lights an brakes work ok?
A volt meter puts no load on the circuit but your test light does.
 
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I THINK you're connecting to the wrong pins in the plug. On my F150, with factory installed tow package, the +12V pin is at 1:00 O'clock in the TV socket and the Gnd pin is at 7 O'clock. I would expect your Expy to be wired the same. The "keyhole" is at the top of the socket in my setup.

Actually, I just reviewed your video. I see the keyhole in your socket is at 9 O'clock, so your socket is rotated 90 degrees counter clockwise compared to mine. That puts the +12V at 10 O'clock and the Gnd at 4 O'clock. Is that what you were tapping?

But, do you have a 7-pin plug or a 6-pin plug? A 7-pin plug uses a center tap for backup lights. The 6-pin plug puts the +12V in the center tap and there is no backup light supply.

- Jack
 
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His key is on the left, I just checked my TV and it is the same pins that he is checking. . Positive is the first pin clock wise from the key slot.
That would be the same as 1:00 on yours jack. His is a 7 pin.
 
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PopRichie77 said:
With out being there to check for myself it's hard to say, but I would try a new fuse in the plus line. I have seen fuses cause your exact problem. Does your trailer lights an brakes work ok?
A volt meter puts no load on the circuit but your test light does.
Click to expand...

Replaced all fuses related to trailering. I have an LED test lamp that did light up when I tested it on the distribution block in the trailer (trailer battery disconnected), vehicle running and connected. My normal test lamp did not work. Voltmeter still registering 12-14 volts.

So it has to do with load. Maybe a mild short in the line? Wonder if there's a relay that went bad (I have no idea how those work).
 
PopRichie77 said:
His key is on the left, I just checked my TV and it is the same pins that he is checking. . Positive is the first pin clock wise from the key slot.
That would be the same as 1:00 on yours jack. His is a 7 pin.
Click to expand...

Yep it's turned 90 degrees to the left
 
I finally found a link to a wiring diagram that corresponds to my truck: Trailer Wiring Diagram light plug brakes hitch 7 pin way wire brake lights connector utility boat

The top diagram is the way my socket is wired if you're looking at the pins with the cap lifted. The bottom diagram is my trailer plug looking into the pin openings of the plug. My socket also has the pinout molded on the cap cover, so I have no trouble telling what is what there.

There are a lot of confusing diagrams to be found and they don't always agree with this one.

Edit: at the risk of beating a dead horse, since you saw 14+V across two of the pins, you were either seeing battery charging voltage or, running light voltage. Nothing else would be hot, unless you had the turn signals on or the brakes applied. So, with the trailer hooked up and your engine running, are the trailer running lights on? And do you see close to 14V on across the trailer battery posts? If you don't see alternator voltage on the battery, it is not being charged.

- Jack
 
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One other thought, the connection in the TV plug could be loose. Tow packages are some times added by the new car dealer, if so the plus wire may be a separate wire running to the engine compartment and may have its own fuse.
 
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I was thinking that Ford may have addressed the problem of the trailer draining the TV battery and put a relay in to solve the problem, after reading your info, it looks like that is what they did. My explorer doesn't have the factory tow package. I installed a relay in the trailer to prevent blowing the fuse when I raise the trailer top. If you wire yours direct put a 30 amp fuse in the line, and use #10 wire, also you will have to unplug the trailer before raising the top or you will blow the fuse, it may not at first because you have a new battery in the trailer but after some use it defiantly will blow the fuse. Trying to raise the top with a dead trailer battery may have caused the problem but if everything was working right the trailer battery should not have went dead, unless it's time was up and a cell opened in the battery.

If the relay turns out to be toast. Replace it and try this, if the relay isn't activated by the key, it may be activated by the factory brake controller if it has an on and off switch.
 
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Thanks PopRichie77, I'm going to run one last test today and then call it and run the wire as you suggest. What I find funny is that the +12V wire heading into the trailer adapter is tiny. #14 or #16 maybe? The ground looks like it might be #12 or #10.

Problem with my new plan is that I'll end up splicing that tiny wire (the 7 pin receiver is a one-piece rubbery mold - I'm not sure that it opens) so anything heavy gauge going in is almost pointless.
 
MooseMan said:
Thanks PopRichie77, I'm going to run one last test today and then call it and run the wire as you suggest. What I find funny is that the +12V wire heading into the trailer adapter is tiny. #14 or #16 maybe? The ground looks like it might be #12 or #10.

Problem with my new plan is that I'll end up splicing that tiny wire (the 7 pin receiver is a one-piece rubbery mold - I'm not sure that it opens) so anything heavy gauge going in is almost pointless.
Click to expand...

You can buy replacement sockets that come apart for wiring. I'd do that if your +12V supply wire is too small.

- Jack
 
Good job, I would say that as long as you turn the key off first, you won't have to unplug the trailer to raise the top, as the relay disconnects it. Also when using battery power in the trailer it won't drain the TV battery.

With the trailer plugged in and the Expedition running, put the fridge on DC and check the voltage at the trailer battery should be a least 13 volts and at the fridge, should be over 12 volts. This will tell if the wiring is heavy enough.
 
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