trailer plug not working

Discussion in '3rd Gen - 2007 - 2017' started by Gary Waugh, Apr 20, 2019.

  1. Gary Waugh

    Gary Waugh Full Access Members

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    I have a 2008 Limited Expedition EL with the ford heavy duty tow package. I just collected a new trailer today and discovered the Auxilary 12V (pin 4) pin on the 7 pin trailer plug is not providing 12 volts. I looked in the owners manual and it shows fuse 42 (on the passenger foot well fuse panel) should control this pin, but the fuse is perfect, I then looked in the main fuse panel above the radiator (under the hood) and it listed fuse 12 and relay 56 as controlling this pin. The fuse was fine and I checked the relay and swapped it with a known working relay and none of this has allowed 12 volts to appear at pin 4. I know the ground pin is good, so I am trying to follow the red wire from pin 4 of the 7 pin plug up to the front of the car to see if there is a break in the cable but am having a lot of trouble following the wiring. I believe the ignition has to be turned on before pin 4 becomes live, but does the engine also have to be running? Can anyone tell me where I can find a wiring diagram for the trailer electrics so I have a better chance of finding the issue?

    Thanks for all suggestions.

    Gary
     
  2. Hamfisted

    Hamfisted Full Access Members

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    You won't find 12v on the 4 pin unless you turn your parking lights on, step on thte brake pedal, or turn on a turn signal. You will find it over on the 7 pin. If you have a multi meter, test the contacts on the 7 pin and you'll find 12v .


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  3. chuck s

    chuck s Full Access Members

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    On my 2007 there was a fuse and relay that needed to be plugged into the under-hood fuse box to activate 12v power to the trailer connector.

    07 Expedition fuse box.jpg 07 Expedition fuse box photo.jpg
     
  4. Gary Waugh

    Gary Waugh Full Access Members

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    Thanks Chuck, that was what I was looking for. Looks like it’s the orange wire in your diagram, it goes to a relay and then a fuse. I was confused as the manual shows 2 fuses for the plug, one under the hood and the other fuse in the passenger footwell, I couldn’t understand why there would be 2 fuses, but your diagram makes good sense and only a single fuse. Will check the wiring this week.

    Gary
     
  5. chuck s

    chuck s Full Access Members

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    Again from 12-year old recollections I think the fuse here:

    -- Chuck

    07 Expedition trailer fuse.jpg
     
  6. Gary Waugh

    Gary Waugh Full Access Members

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    my manual shows the fuse you point to and the pink one to the left of it, but the one you point to is listed as optional, so not sure what that means, but I will check both locations.!! Thanks for the help.

    Gary
     
  7. chuck s

    chuck s Full Access Members

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    Stick a fuse in there! I'm guessing the fuse is for the "optional" 12v trailer battery charge line. I had no aftermarket circuits on that Expedition. It's been gone for two years. The photos I took for a travel trailer forum 12 years.

    -- Chuck
     
  8. Gary Waugh

    Gary Waugh Full Access Members

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    going to try it tonight, but if there is no fuse there, a relay is probably also missing!! :)
     
  9. chuck s

    chuck s Full Access Members

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    So you haven't bothered to look yet... :) Takes 5 seconds including opening the hood.

    -- Chuck
     
  10. Gary Waugh

    Gary Waugh Full Access Members

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    Okay well I still don't understand it! My last trailer had an internal light which worked when the ignition was turned on, but the internal light on the new trailer doesn't work. I confirmed the new trailer uses the auxiliary pin (pin 4 of 7) and assumed the previous trailer used the same pin. Anyway after all your suggestions I went and added a $5 relay and a fuse to the trailer auxiliary slots (55 & 13 in the fuse panel under the hood)) and the new trailer interior light now works with the car ignition key!! I guess the old trailer must have used a different pin to get its power for the internal light, still don't understand why ford sell this truck with a tow package and don't install that relay and fuse as standard!! So for $5 it is all fixed, thanks for all the suggestions..

    Gary

    Chuck I see you also added the relay (its the small black relay in between the larger grey relays in your fuse box!!), mine is identical. What messed me up was the light working on the previous trailer, convinced me all the relays and fuses had to already be in place and I was looking at a connectivity problem!! Was much simpler as you pointed out, the relay and fuse where missing!!
     
    Last edited: Apr 24, 2019

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