My "new to me" 1998 F150 had what I thought was a bad trailer plug, cover was broken and very corroded, had running lights on trailer but no brake and turn lights. All lights worked fine on truck, and on trailer when plugged into my Excursion. So got a new 7 pin and replaced it, and have exact same problem. Running lights - no brake or turn lights. Have checked fuses (kick panel) 18 and 23, and (engine) 1, 16 - #1 was blown replaced no change. Pulled connector on frame rail drivers side where rear lights plug in before it T's off to truck lights and trailer plug, cleaned connector. Checked for power at the connector and have power to running, brake and both turn lamps, plug it back in and still nothing at plug.

Stumped because the turn and brake lights work on truck, know the 7 pin is wired correctly, same problem before and after new plug. I seem to have no power in the brake, turn wires (Yellow, Green) after it T's off from the main harness. Pic below is of the old plug.

Thoughts, am I missing another fuse or a relay that might be bad?

 

I would look closely at where the trailer plug wiring connects to the factory harness . I see many issues in that area .Poor splice jobs , bad connectors , etc.

 

Thanks I will pull the wrap off that entire T connection in the morning and see if I can find anything. I'm rock hard stupid with wiring but that was kind of where I ended up, if the truck lights are fine it has to be from that junction down to the harness.

 

Okay, get me on the same page. How are you verifying power at the seven pin on your truck?

Steve

 

Using a light tester, get light with the running light (brown), and reverse (purple I think), nothing on the right or left turn/brake (green and yellow). Get light at the connector on the frame rail in the pins for running, brake, reverse and (flashing light on the tester) for turn lights.

 

Checking on the back side or front side of the pins?

 

Pins at the harness - front the pins with the ground of the tester connected the frame rail, and back side of the harness right where each wire connects at the set screw.

 

you know the trailer lights are not the same as the tail lights on the truck ? Different fuses. I have not followed your logic completely, but there is a connector on the truck frame , drivers side by the spare tire. You can check it there for power to the back. After that, it has to be the 7 prong plug or the wire attachment.

 

So you have eliminated the seven pin connections as a problem and you have verified power to the point where the wires to the seven pin are fed. There is nothing else in that circuit other than the branch feeding the seven pin. Broken wires due to age possibly. You can verify that hypothesis by making a short jumper between the connection and the seven pin as a temporary power source. If that works, it would seem you need to make or purchase the harness from the connection to your seven pin. When I work on this stuff I assume nothing and work it through section by section as you have been doing.

Steve

 

I did check that connector and all of them had power, I don't think it's a bad connection at the actual 7 pin, what are the odds of two connectors have the same two wires not working. I guess it has to be at the connection where it splits off the main frame rail wires to the 7 pin. I'm going to have to open that up and look tomorrow.

 

Her truck is old enough that the trailer lights aren't fused separately from the truck lights. You had the first thought that I did, until I pulled the manual for her truck. Only fuses are for running lights, brakes, and battery charge...

 

Electrics are like water ... They follow the easiest path . Any break , you loose something . Poor ground , poor tap, connection , you loose something . Just like leaks in a water pipe , electricity bleeds away from leaks . Not only positive but ground needs to be examined closely ! Best of luck !

 

All lights on the trailer use the same ground.The running lights work, just nothing else. Break has to be on hot side, not ground. OP also is testing using a chassis ground and it makes no difference at the seven pin leads.

One possible concern is if someone wired the seven pin according to color rather than function, power is there, but not on the correct pin, but I am guessing all pins were checked by OP when a function was initiated and not just the intended pin. I see folks make that mistake fairly often where they check the pin where they think power should be rather than checking all the leads for power. The seven pin wiring diagrams on the Internet show a front and rear view for seven pin wiring and it is easy to use the wrong one. I have rewired dozens of seven pins due to that mistake.

To have this problem requires a loss of power down three separate leads, if I am understanding this post correctly. In my experience that has to be a plug or connection issue and should be easy to resolve. I would, however, verify that there is no power on any lead when a function is initiated, before going further given it is the easiest thing to do.

Using a toner from the seven pin back on the leads in questions should locate the problem very quickly.

Steve

 

If the '98 came from the factory with the trailer tow lights installed, they were fused separately from the truck with their own relays.

 

Perhaps I am misreading what the OP is stating as I interpret her post to mean she has power back to the juncture feeding the leads to the seven pin at the rear of the truck. If that is not the case, the OP has not established power to the leads to the seven pin.

Steve