1991 F-350 Centurion (Started life as a crew cab).
Wiring should be same as Crew Cab or Chassis Cab.
460 (7.5L) gas engine, E4OD xmission, w/ Air Cond.

I was working on a friend's trailer that I borrowed, and accidentally shorted out the running lights circuit.

The lights on the trailer went out, but the lights on the truck remained.

I tried all of the fuses in the main fuse box under the dash near the steering column.

I looked for another fuse box under the hood... what I initially though was one turned out to be a bunch of relays (including for trailer lights). That explains why I couldn't figure out how to open it!

I checked the fusible links on the RHS near the battery. According to the wiring diagram in my Haynes manual, there should be a wire coming off of the starter solenoid and going through a fusible link to the relay.

I checked voltage with a DVM at the plug, and had a wierd floating 10.8 volts at the trailer connector. But it isn't enough power to light an incandescent bulb.

I checked the poles of the relay... assuming I understand the wiring diagram, I showed the 10.8 volts at the terminal that the voltage used to switch the relay should be. I saw no voltage on the power in and power out terminals of the relay. When I put 12V directly onto the terminal that I think is the power out from the relay going towards the trailer connector plug, it didn't show up at the other end.

There is a trailer brake controller, but I can't remember offhand if the running and turn signal lights go through it or not (I wouldn't think so).

Keep in mind that this is mostly by memory from when I last tried to fix it, about 2 months ago (I have made several attempts, and It was painful enough that I remember most of it pretty clearly).

Is there another fusebox somewhere? Are there other places where the circuit could be protected (e.g. under the rear bumper?)

I'm starting to suspect that someone may have hacked the circuit long ago (I'm the 3rd or 4th owner). One of the fusible links is missing about 2" and is corroded, and it does match the description of the one I should be looking for. But there are several that match the description for the color of the wire and the fusible link coming from the starter relay supply terminal.

I may end up having to build a new circuit, but I'd prefer to fix the existing one. At a minimum I hope I can use the existing trailer running lights relay.

If anyone knows how to identify the most likely places that circuit protection for the trailer running lights (on the tow vehicle) might have protection, I'd sure appreciate any likely places to start my search.

Thanks

 

Isn't there a special fuse box for the trailer/camper package under the hood?

Do you have an owner's manual to look in ?

I remember there being a fuse box that contained 5 Circuit Breakers as well as numerous fuses.

 

On my '92, in the underhood fuse box, there is a fuse for trailer running/backup lights (25A, mini fuse position #4, fourth one from the bottom on the left hand side) and also a relay for 'trailer tow', whatever that means (relay position #4, second one down from the top, on the right hand side).

There are also two 5A mini fuses in position #'s 6 and 7 for 'trailer stop lights', one for the right and one for the left.

There is a 30A Maxi fuse in position #21 (second down from the top, in the center column) for 'trailer brake', which I assume that to be for the brake controller.

I don't know if it would be the same on a '91 or not.

 

I've looked pretty extensively, and all I found was a black box that had a bunch of connectors plugged into it. It has some designations on it, but I don't remember what they were exactly... they say something like trailer tow, fuel pump, etc.

Once I get dressed and start my day, I'll go out there and record what it says exactly, and maybe post some pictures.

I have the owner's manual, a Chilton and a Haynes manual. The Haynes manual does say that "late models" have a second fuse box on one of the fenders.

I though that the black box I found might be a fuse box, but I couldn't get the cover off. I ended up removing the mounting scews so that I could look all over and around the box to see if there were some sort of latch clips that hold the top on... I couldn't even find a seam. I found that there were nothing but relays in it, with a harnes loaded with connectors terminating into it (plugging into each relay). I even broke the mounting for the trailer light relay, which is okay because it let me pull it out and check voltages at the terminals on the relay.

But so far, I haven't found another fuse box yet, and I don't see any possibility that the relay box could contain fuses. According to the wiring diagram in the Haynes manual the circuit (the power going into the relay) is protected by a fusible link. I checked the fusible links with a Digital Multimeter, and there was voltage downstream on all of the ones of the correct color.

I'll see if I ever downloaded the pictures I took last time I worked on it from the digital camera to the computer. If so, I'll post those.

It's possible that there is a fuse that a prior owner may have added, or even that the circuit has been hacked, and didn't have a fuse (but I doubt that because there were no sparks when I screwed up and shoreted the circuit out in the first place).

 

I'm having some problems posting images... maybe you can see them?

I'll create a User Gallery in my "Profile" and link to it later.



http://www.imagestation.com/picture/...7/f96d4e3b.jpg

http://www.imagestation.com/picture/...e/f96d4e30.jpg

http://www.imagestation.com/picture/...c/f96d4e21.jpg

http://www.imagestation.com/picture/...9/f96d4e27.jpg

http://www.imagestation.com/picture/...0/f96d4e34.jpg

 

http://www.imagestation.com/picture/sraid106/pa820a31c17706a93c637ee08dc2e4097/f96d4e3b.jpg

http://www.imagestation.com/picture/sraid106/p78a9c6d94e93e7cca258746da24f64fe/f96d4e30.jpg

http://www.imagestation.com/picture/sraid106/pf9d347a2f070a886fc7ec27f51cd16bc/f96d4e21.jpg

http://www.imagestation.com/picture/sraid106/pfba70fd83a5d983e640160e750f98e49/f96d4e27.jpg

http://www.imagestation.com/picture/sraid106/pc12539009d38d1594066051c3a2596d0/f96d4e34.jpg

 

https://www.ford-trucks.com/user_gal...4&albumid=8961

http://www.maximum-suzuki.com/ibf/in...ST&f=4&t=28505

If you want to see what a Centurion is:
https://www.ford-trucks.com/user_gal...4&albumid=8962

 

7 Pin connector?
Do you have the standard 7 pin rv connector? Do you have electric brakes on the trailer? If so , does the brake controller show a connection to the trailer? Do the brakes function?
I had a similar problem , and found a simple solution . I inspected and found the feeds to the female connector on the truck were badly corroded. Also the trailer ground for 12v was corroded and not getting a good ground. I would try to clean the female connector , 1 screw / wire at a time , then check the connector with your DVM , check for any open voltage. Then check for brake power ( have someone or something hold the brake pedal down) . Then check for turn signals , backup (if so wired) and running lights . If no go , check the ground from the 7 way to its` grounding point on both trailer/truck. These are simple things , but sometimes the solution isn`t as complicated as we think!


Let us know how you fix it.

Good luck!

Sty

 

I have both the 6 ond 7 pin round connectors, wired in parrallel.

The trailer running lights use a relay that is triggered by the voltage
from the main running lights on the truck. This allows the trailer lights
to be on their own dedicated circuit. But, on my truck, the previous
owner had abandoned the factory wiring and tapped into the running
lights on the truck directly in order to power the lights on the trailer.
Some trailers have a lot of marker / running lights, which is why the Ford
designed circuit uses the relay to isolate the trailer circuit. Also, that way
if you short out the circuit on the trailer, you will still have running lights
on the truck.

When I shorted out the trailer, I knew I had done it because I was filing
the contacts in the bulb socket (D'oh... Forgot to turn of the lights)
So, I assumed I must have blown a fuse when I shorted it, but to my surprise,
the truck's running lights were still working. After looking at the wiring diagram
I determined that I must have blown out a fuse on the power supply
side of the trailer lights circuit. I couldn't find any blown fuses anywhere...
not in the main fuse box under the dasboard... not in the wires that had
been added (all over the place) by the previous owner(s)... no where.
I looked for a second fuse box under the hood, but there was none.
I did find the relay box, and the correct relay for the trailer lights. I re-checked
the wiring diagrams and found that the trailer light circuit originates
from the hot terminal of the starter relay, then goes through a fusible
link, and then through the relay and out to the wire harness, then aft to
the trailer light connector socket.

There was one fusible link that had fried, and was missing about an inch, but
it had corrosion that made it appear that it had been fried a long time ago.
Still, I ran a jumper across it to see if the trailer lights would come back on if
I energized the circuit. What I didn't know was that the wire had been cut
near the rear bumper and abandoned. So, I checked all of the other
fusible links that were connected to a yellow wire originating at the starter
relay (solenoid). They all had voltage at the point after the fusible links, so
it couldn't be any of those. I then went and put 12V on the outgoing
side of the relay (brown / white wire), but the railer lights still didn't come on.
When I checked voltage, I had 10.8V at the trailer plug, and also at the trigger
terminal of the trailer light relay. I assumed that it was some sort of floating
voltage that lacked the amperage to light the trailer lights.

(to be continued)

 

So, I decided to attack it from the rear bumper end of the truck.

I made a test light by using a trailer connector plug and connecting
a marker light assembly to the ground and running light terminals.
When I plugged it in and turned on the running lights, it lit up. I figured
one bulb must be low enough wattage that the 10.8V floating was enough
to light just one bulb.

I went to the back of the trailer connector jack and traced the supply
wire for the trailer running lights... it went to a thin (16-18 Ga) wire that
supplies voltage to the tail lights of the truck. It was spliced into the wire
with one of those crimp splices that come with a lot of trailer wire plugs...
the type that doesn't require cutting or stripping the wire. It had quite
a bit of corrosion, and the plastic part of the splice crumbled when I
touched it. I could see that there was no way it could have made a good
connection... and then I realized that when I shorted it out, it should have
blown the fuse for my running lights on the truck, but the connection
was so corroded that it just lost the connection (probably from a spark)
when I shorted it out.

Well, at this point the mystery was solved, and I decided to take the bad
splice off, and see if I could reclaim the factory wiring... with the relay to
isolate the trailer as intended by the factory.

I went into the harness just in front of the fuel tank, where the splice into
the running lights circuit had been made. I opened the corrugated
plastic sheath and found the Brown/White wire that I figured would
probably go back to the outgoing power terminal of the relay. I traced
it all the way back to the rear bumper, and just after a 4-terminal connector
in the wire harness it had been cut off. So, I went to the relay, and put
12V on the Brown/White wire of the harness, and checked the other end
of the wire back at the rear bumper... VOILA! 12V.

I then went and put 12V on the wire with the fried out fusible link by the
starter (solenoid) relay, and picked it up at the ingoing power lead of
the trailer light relay.

So, now I knew all I had to do was put a fuse in place of the burned out
fusible link, and splice the trailer connector back into the original (brown/white)
wire back at the rear bumper. It was getting cold and dark, so I called it
a day. It was Saturday, and since we had at least one sick kid, I knew I was
going to miss church anyways, so I could finish up the next day. Now that
I knew what was going on, I could rest easy, and go back to it knowing
what I was dealing with. I'd taken a load off of my mind by sorting out
several mysteries, so it was time to go in and get some rest... It took me
the whole day Saturday to work this all out, including making the test light.

(to be cont'd)

 

Sunday morning was (of course) wet and drizzling. I pulled as much of
the truck as I could into the garage door, and started underneath the truck.

The wire that had previously been tapped into was corroded in the area
where the splice had been, so I cut about 1/4" away and put in a butt splice.
I found some really nice butt-spliced that are crimp type, but have heat shrink
on them instead of the semi hard plastic. With a heat gun, the temperature
required to shrink the thick walled shrink tubing had softened the wire insulation
which made the shrink tubing and the wire insulation melt together
and bond very nicely to creat a hermetically sealed joint, as if it still had
continuous insulation protecting the wire. I found another wire nearby
that looked like it had also had a splice in it at one time, complete with the
same signs of corrosion, so I cut the corrosion out of it in the same way.
It also went through a gap between frame members (it was run outside the
wire harness) that looked like a place for chafing and a short circuit to
eventually occur, so I slipped some vacuum hose over to protect it.

The more I looked around under there, the worse things looked. Most of
the wire harness was not attached to the frame, so I spent quite some time
tie-wrapping it in place. Then I noticed that most of the heat shield on
the exhaust pipes where they go over the axle and near the floorboards
were barely attached. I put large stainless steel hose clamps on them to
secure them to the exhaust tubes.

Now I was nice and damp from rolling around under the truck in the rain,
but I was making progress, even though it was on issues other than the
one I had set out to repair.

Finally, sometime in the mid to late afternoon, I was ready to repair the
actual trailer connector running light circuit. I had to insert two wires into
one end of the butt splice, so after I crimped it but before shrinking
the tubing I shot a little silicone into the end with two wire and then
shrunk the tubing... it really looked like a good seal, which I couldn't
have gotten by just shrinking the tubing over the doubled wires.

I also put silicone on several other existing doubled up wires in the old
style crimp splices (the double wires are there in order to have a 6 pin and
a 7 pin connector hooked up in parallel).

Well, finally the work underneath the truck was done, so I turned it around
to put the nose in the garage door, and I disconnected the negative
wite at the battery so that I wouldn't short anything out when I put yet
another ring terminal under the nut on the large post of the starter solenoid.
I wired in a fuse, and tied in to the existing wire going to the trailer
light relay. I put the battery cable back on, and now all I had left to do
was hope that the realy was working. I plugged in my test lamp, said a prayer,
and turned on the running lights.

Lo, and behold... there was light... and it was good!

In hindsight, all I can figure is that there must be some sort of resistor
that reduces the voltage to the tail lights from 12V to 10.8V. At the wire
running to the truck's tail lights (previously used to supply the trailer
running lights) and at the trigger terminal of the trailer lights relay there
is still only 10.8V. The corroded connection at the old (blade type) splice must
have been intermittant, and when I shorted the trailer lights initially, it
must have made it go open circuit for awhile, or weakened the electrical
connection enough that I could read the 10.8V, and I could power my test
light, but not power the marker lights of a trailer.

Well, that's the way it is when you buy a used vehicle... you can't ever be
sure that repairs and modifications were done correctly in the past.

Incidentally, while I was under the back end tracing wires to the bumper
I found several other abandoned wires. It's too bad people won't use a wiring
diagram and some diagnostic techniques to avoid having to run a new wire.
I suspect that the old wires are fine, but someone didn't want to take the
time to find the real trouble, and just patched togeth something that "works".

Now if I can just figure out what that noise at 2000 RPM is...