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Trailer light problems. Help!

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Old 05-27-2014, 07:48 AM   #1
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Default Trailer light problems. Help!

Having trailer light problems with my 2008 F150 XLT. I believe the trailer wiring is all factory, but i am not 100% sure as I am the second owner. I have checked all the lights on my trailer and they seem to test out fine. I bought a tester for the truck lights and here is what it's saying: When the truck headlights are OFF, the left and right trailer tail lights work but the trailer running lights do not. When the truck headlights are turned ON, the left and right tail lights do Not work but the running lights do. I have checked the obvious fuses under the passenger side glove box and they seem fine. The wiring itself seems fine everywhere I can check. The connectors and connections seem fine. The grounding seems fine. I found a small black box with some fuses and such under the hood on the drivers side. There are two empty fuse spots in this box. I can't find anything about this box in my owners manual and when I called my local ford dealer they couldn't tell me what it was. Anyone else have a problem like this? I'm not sure how long this has been going on as we may not have noticed it until now (as the lights do "sort of" work!)
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Old 05-27-2014, 11:09 AM   #2
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get a tester with the pointed pick end on it; with the alligator clip on the other end. Start testing the actual plug end on your truck. The test has a light in it that will show whether you have power. You need to eliminate your truck as a source of problem first. you should see the tester light up when you turn on your tail lights, signals, stop light, etc. you should also have a constant source of power in one of the openings. If your truck turns out to be fine (likely), then the trailer is the problem.

https://www.google.com/search?q=7+pi...x-a&channel=sb

This link shows the 7 pin connector map.

Also, be darn sure of your ground. A bad ground can cause some very strange problems.
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Old 05-27-2014, 11:27 AM   #3
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Thanks!

The trailer Tests out fine. I have tested the truck and the results are:
When truck lights are on- no trailer turn signals or brake lights
When truck lights are off- no trailer running lights.

I'm an electrician, so I can tell what's working and what isn't... I just don't know vehicles too well so I can't tell WHY. Lol
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Old 05-27-2014, 11:29 AM   #4
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4 pin or your 7 pin connector ?? and the little black box with a couple fuse's under the hood is for your A/C
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Old 05-27-2014, 12:45 PM   #5
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Four pin connector.
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Old 05-28-2014, 10:25 AM   #6
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Sometimes there is a relay that is not dealer installed but supplied with the truck(new) that needs to be put in to work your TT lights and braking system. Not sure if that holds true for just a 4 pin....Refer to the owner manual for location
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Old 05-28-2014, 10:43 AM   #7
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Doesn't say anything about that in the owners manual. I've read everything on there on towing and trailer lights.
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Old 05-28-2014, 12:05 PM   #8
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Quote:
Originally Posted by emmarose007 View Post
Doesn't say anything about that in the owners manual. I've read everything on there on towing and trailer lights.
Somewhere in the manual there will be a diagram for the fuse relay box..

PDF Pg 301-304: F10, 42

http://www.google.ca/url?sa=t&rct=j&...,d.aWw&cad=rjt
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Old 05-28-2014, 12:27 PM   #9
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Everything seems to be there and all fuses are intact.
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Old 05-28-2014, 07:39 PM   #10
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Here is EXACTLY what is happening:

When headlights are ON and I put the 4way flashers on I get 14 volts from ground to the running light pin (reading my meter). I also get approx 14 volts (flashing) on my other two pins BUT if I apply any type of load to the running light pin (an LED, my trailer's lights etc) I no longer get any voltage on my other two pins.

So I read 14 volts on all three pins individually when the brake/blinkers and running lights are all on... but as soon as I apply a load to the running lights, the brake/blinkers have no voltage. The "load" I'm applying for testing purposes is a single LED.

I have checked the ground pin grounding connection and it reads approx 2 Ohms between where it seems to be connected to the frame and the ground pin on my four pin trailer connection. I also tested the resistance between the negative battery terminal and my ground pin on my trailer connection and it reads fine (less than 2 Ohms).

Is it worth re-doing all of my ground connections? Both the battery negative terminal to truck frame and the wiring harness (big mass of wires that has three different ground connections under one lug) to frame? They seem to be tight and the ring lugs themselves seem to have continuity, but the bolt heads don't read as well.

I am seriously at a loss here... I don't know why a load on the running lights would cause the blinker/brake lights not to work. I'm an electrician (that's my day job)...NOT a mechanic.
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