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Old 04-02-2014, 08:56 PM   #1
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Need to change 4 Pin to 7 Pin Trailer Plug/Brake controller

I am trying to figure out this process. I have already have the Prodigy brake controller installed and the Ford harness for it, but I have a few questions. It is a 2003 E350 7.3 PSD. Help is very much appreciated.

1. The Van has the four pin already installed, but I need to switch it to 7 pin. I read about the connection above the rear wheel on the frame rail, but it only has 4 pins in it. Does the Ford Harness correct this for the 7 pin? Am I missing something? What do I need?

2. When I plug in my Prodigy the right blinker arrow stays lit and no lights on the controller. Is this because I do not have the system complete? The Hazards will not bling either when it is plugged in.

Thank you for any help. I did price the factory 7 pin from Ford ($140). I was hoping to do it my self and save some money , but might be willing to pay if it is too much hassle.

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Old 04-02-2014, 09:41 PM   #2
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Re: Need to change 4 Pin to 7 Pin Trailer Plug/Brake control

Ouch, I wouldn't pay that for it! Go to Etrailer.com

My 02 Ford had the 4 pin under the frame. I got a harness from Etrailer that had the 4 pin plus 3 additional wires. I'm almost positive that the additional 3 are ground, constant or keyed 12v (not sure which) and a wire you have to run all the way from the brake controller. I think you can find all of this info on Etrailer.com and if you buy a harness from them they will most certainly help you with the wiring. They did for me.

I just looked and can't find the one I got on their website, but give them a call.

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Old 04-02-2014, 09:50 PM   #3
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Re: Need to change 4 Pin to 7 Pin Trailer Plug/Brake control

86Scotty,

Thanks. It seems that the factory harness is not available any longer. I may have to go with etrailer.

TexGX
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Old 04-03-2014, 12:46 AM   #4
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Re: Need to change 4 Pin to 7 Pin Trailer Plug/Brake control

Are you sure you don't have the other three trailer tow wires? They terminate above the rear axle, with their own connector. There is no 7-pin connector. The Ford 7-way harness simply connects to both connectors, or you can extend the other 3 wires yourself.

I'm not sure that the non-wagons got the extra wires however. If not, you will have to run the brake controller wire from the dash, the B+ from the battery, and the reverse light from your own added relay. E-trailer does have a universal wiring kit to add those circuits.
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Old 04-03-2014, 08:40 AM   #5
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Re: Need to change 4 Pin to 7 Pin Trailer Plug/Brake control

I found the one with 4 pins. I will look again. That one did branch off to the rear diff. It looks more and more like I will need to run wires. Thanks for info.

Anyone have a 7.3 that has done this?

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Old 04-06-2014, 12:10 AM   #6
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Re: Need to change 4 Pin to 7 Pin Trailer Plug/Brake control

I've done this on a v10. It took me a while to find all the correct wires, especially the correct plug up front. I already had all the correct fuses in place.

If you have the harness up front (btw, there are two identical plugs, only one has the right color wires, make sure you get the right one -- on my van the correct one was much more well hidden than the other one), I'd be surprised if they didn't run to the back. In that case, the back is, as already mentioned, on the drivers side frame rails by the diff.

The extra wires there are in the same bundle as the four plug ones that run further back. Just follow those forward until you (hopefully) find the extra wires. Test for continuity against the other end in the front plug to make sure you have the correct ones.

Running wires back from where they terminate is trivial. I used automotive crimp connectors with a ratcheting crimp tool. I think it was an hour or two of looking for all the correct wires the night before, and then with the help of a friend, I mounted a hitch and wired up the plug in another two hours or so.

The biggest snag I ran into was the random second plug up front. It has the exact same connector on it and was in fact closer to where all the instructions told me to look than the correct one. Test for correct power and brake signals.
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Old 04-06-2014, 10:43 AM   #7
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Re: Need to change 4 Pin to 7 Pin Trailer Plug/Brake control

Thanks MadScience,

I did get the project done. I had everything wired, but it did not seem to be working. I took it to my RV guy and he pointed out I had my brake controller plugged into the wrong plug. Why would ford put two of the same shape plug in the same location? After we fixed that, the location I plugged into for the brake signal was wrong. I ended up running the wire inside and hooking directly to the controller wire. Everything works now, but I wanted the power to be only on when the truck was running. I need to find a different source for power so that I do not drain my battery if trailer stays hooked up.

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Old 04-06-2014, 12:23 PM   #8
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Re: Need to change 4 Pin to 7 Pin Trailer Plug/Brake control

Yeah, that other plug would ideally have been keyed differently. Probably tied up in either conventions or standards.

I was also nervous about battery drain from the brake controller, but in reality (I've measured battery drop with it connected and disconnected while chasing down a parasitic draw) it seems to be less of an issue than an alarm. I have a Tekonsha P3 and it seems to fall asleep properly.

One way to do this would be to put a correctly specced relay in that's switched from the ignition current, but even that's not great. I'd want my trailer brakes to keep on working if the ignition somehow turned off.

I've also considered a switch, but you really would never want to be able to bump that. If I went that route, I'd spring for a switch with a latching cover.
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Old 04-06-2014, 01:34 PM   #9
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Re: Need to change 4 Pin to 7 Pin Trailer Plug/Brake control

I also have a Prodigy P3 and it does not draw enough to worry about draining the battery.

The trailer battery charge circuit on my van is keyed.
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Old 04-06-2014, 05:23 PM   #10
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Re: Need to change 4 Pin to 7 Pin Trailer Plug/Brake control

I am not worry about the controller, because like MadScience I have a Prodigy P3 and it seems to sleep properly. It is the Power straight from battery to the trailer. I am worried about the trailer battery draining the starting battery on my truck. Any suggestions on that?

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