1998 F150 Trailer Electrical Woes


#1

Okay folks, I’m tired of banging my head on the driveway (the bruise is getting painful).



I have a 1998 F150 XLT Supercab with the factory installed 6000 lb tow package. The last time I towed anything, the lights on the towed vehicle worked fine. I tried to tow a U-Haul last week, and I have running lights but no turn signals and brake lights. Using my trusty tester indicated that I’m only getting running lights and 12V back to the seven pin connector.



1. I’ve checked all the fuses in the cab and under the hood. They’re good.

2. Using my tester, I punctured the insulation of each wire behind the seven pin and got the same indication: running lights and 12 volts.

3. I got a wild hair today and decided I’d just not mess with things, purchased a “taillight trailer converter” to give me a four pin connector. After installing the tee and testing the four pin, SAME PROBLEM…running lights only.



I guess I’ve concluded that I’ve got a gremlin somewhere between the connector where I made the split and the front of the pickup.



Question: is there something OBVIOUS I’m overlooking? Is there a relay or possibly a third fuse box for the factory tow package that I’m missing? Has anyone had a similar problem?



I need to get this fixed so I can get a few tons of pellets to the house before cold weather hits.



Thanks in advance!!


#2

I just thought I’d float this one back up to the top to see if anyone has any ideas.

Time to head down to the library to see if I can find a wiring diagram to see how the fuses, relays, etc. are tied together.


#3

There should be a seperate fuse for the left/right turn & stop relays. Where it is I cant tell you.

The manual you need for this is the “1998 f-150 electrical & vacuum troubleshooting manual.” I have this manual for all 3 of my Fords, & it’s worth it’s weight in gold for figuring out ANY electrical problems.


#4

Here is a complete trailer harness wiring diagram. I hope it helps.
~Michael


#5

Thanks for the diagram Michael. I poked around for a couple of hours this morning with my trouble light and figured out the relays, but it’s nice seeing the confirmation via hard copy.

It looks like my problem is the 12V that comes from fuse 2 in the fuse box under the hood. That voltage at the relays intermittently when I move the wiring bundle going to the box with the relays. Voila. Oddly enough, I have the voltage at the other relays but not at the turn signal relays. It should be obvious once I break loose the trailer relay box and look at the bottom side.

Thanks again for the diagram.

Jim


#6

Looking at the diagram it looks like fuse 2 only feeds the turn signals.


#7

Jim:
No problem on the diagram. If you need to see a larger section or portions of another circuit let me know.
~Michael


#8

Many thanks to Michael for sending the electrical diagram in PDF format for me. I finally found this problem last weekend, fixed it, and have been towing trailers with impunity. It’s nice getting some much needed chores done.

Michael, you’re right, only fuse 2 fed the trailer turn signals. However, fuse two was actually in a THIRD fuse box on this vehicle. The owners manual only shows two. The third is called the “Engine Fuse Module” and is tucked to the very rear of the left fender near the fire wall. There are only two fuses in it, and good old fuse 2 was blown. If only the owners manual had mentioned this one…

Oddly enough since changing the fuse and getting my trailer signal circuit working, my intermittent check engine light has gone out and stayed out. Wish I had a code reader to read the fault, but it sure is coincidental. I guess if some cars throw a check engine for the gas cap being loose, my beast can throw one for trailer turn signals.

Again many thanks to all for helping me with this bug.

Jim


#9

Jim:
Glad to hear you got it fixed. Thanks for the update. If you get a chance swing into an Autozone, Schucks, etc… and get that code pulled. It is highly possible the BCM (Body Control Module) was sending fault information to the ECM about the blown fuse.
~Michael