Driving to work everything was fine. Came out to go home 11 hrs later and on engine start the ABS light was blinking. Discovered I had no brake lights; fuse blown. Replaced fuse pressed brake pedal and fuse blew with a bright flash and loud pop.
Suspected the trailer plug wiring since the gooseneck hitch installer piggybacked a 6 pin round to the back of the factory 7 pin round and then the RV 5th wheel hitch installer piggybacked the 7 pin flat to the 6 pin round and wrapped it all with about 2 rolls of electrical tape. Disconnected all that and still have a dead short in the brake lamp circuit. Disconnected the main harness plug at the back of the truck; still have short. It's somewhere between the output leg of the fuse box and the main harness plug at the back of the truck.
Any ideas what I should look at? Are there any parts (besides wire)that could fail and cause a short?
Thanks

 

The brake light circuit goes through the multi-function switch...
Do the turn signals and/or hazard lights work?

 

Turn signals yes
Hazards no

 

Is fuse #13 the one that blows?

If the turn signals work that indicates the problem is not in the wiring harness from
the multifunction switch back to the brake light/turn signal bulbs

You said that the fuse blows when you pressed on the brake pedal.

If you replace the fuse (do not press the brake pedal) and turn on the hazards does the fuse blow?

If the fuse does blow the problem is probably a bad multifunction switch.

If the hazards work do the following. At the brake pedal switch should be a Light Green/Red
wire which is hot all the time. There should also be a Light Green wire. This is the wire that
goes to the multifunction switch and then passes the 12 volts to the brake/turn signal bulbs.

With the rear wiring harness disconnected.

If you use a ohm meter, with the brake pedal switch plug disconnected from the
brake switch measure the resistance from the Light Green wire to ground. Should not
read zero ohms. If you do that would indicate the multifunction switch is bad or something is
wrong with the Light Green wire that goes up the steering column to the multifunction switch.

In a 96 schematic it shows the Light Green wire also goes to the Engine Controls. Doesn't say for
what purpose. I don't know if that circuit is used on your 93.

 

That would be the brake signal to the PCM, so it knows you're using the brakes and it'll disengage the torque converter clutch and start gradually closing the idle air valve.

 

Yes, fuse 13

Good point. Turn signals do work.

Further investigation last night. With good fuse hazards work and do not blow the fuse.

I'll try these steps tonight and report back. THANKS!!!!!

 

I have 7.3 diesel and manual trans

 

Disconnected the brake switch and put the dvom on the light green wire to ground. 0 ohms, dead short. Unplugged the harness from the multifunction switch and rechecked light green wire to ground. Still shorted.

With the brake switch disconnected and a fuse in slot 13, the ABS light no longer comes on, but ABS system doesn't function. (Assume it needs input signal from brake lamp circuit?)
Since this short occurred suddenly I have a hard time believing the wire is the culprit. I haven't done any modifications to any wiring and jiggling the harness around didn't change the dvom reading. I would think a short in the wire would be intermittent.

I ran out of day light so that's where I stopped. Tonight I'll try running a by pass wire from the brake switch to the multifunction switch and see if I get brake lights and ABS function back.

 

Appears fuse #13 blowing is a common problem. Ford issued a Technical Service Bulletin
for 94 and newer vehicles. I'm guessing 93's could have the same problem.

Scroll down to (O). It shows what are the likely causes of a blown fuse #13.
Looks like solution 10 is the only one that could apply to your situation.

1983 Ford Bronco TSBs & FSAs (Recalls) for '83-96 Broncos & F150s picture | SuperMotors.net

Here's a link (Not a 93 F350) where the guy just cut the Light Green wire and did point
to point wiring. He never found the short.

FSB Forums - View Single Post - Fuse 13 blowing, no cruise control installed - definitly a short.. but where?

 

Thanks F150xlt. That diagram and the link to the thread were a huge help. I now have a better idea where that wire goes. Tonight I'll unplug the cruise harness, the ABS harness and the trailer brake controller and see if the short goes away. I will also inspect the harness where the TSB indicates a possible short. Now that the weekend is here I can tear things apart tonight and not worry about getting back together for work the next morning. If don't find the short by Sunday I'll run a bypass.

 

Just to clarify, your 93 has the old style cruise control that uses a mechanical
servo. Alot of the posts refer to the brake pedal pressure switch used for
the cruise control. It's mounted on the front of the brake master cylinder. That
switch was a fire hazard and Ford had a recall to replace the defective switch and
added a fuse. You do not have that type of cruise control.

 

Thanks. I got the truck in 98 and the cruise has never worked.

Update,thanks to the wiring diagrams in the thread you linked me to, I learned all the other places the light green wire ties into.
When I got home I hooked up my dvom and it was still shorted. In trying to narrow down the search the diagram indicated the light green wire goes to the trailer brake controller, the RABS controller and the cruise module. I unplugged the trailer controller, no change. Unplugged the cruise model, no change. Then I grabbed a two wire plug that just connects to a dummy plug on the firewall behind the master cylinder. I unplugged then replugged it in. Went back to my dvom and it was showing resistance. What in the world? I jumped across the brake switch and yea I had brake lights. No blown fuse and I'm even using a smaller 10A since I ran out of 15A fuses. I left the brake switch jumper-ed, wiggled all the harness, pushed all the pedals, cycled the key on and off and could not get the fuse to blow. I'm glad it's working but I still don't know what caused the short or if it will happen again. Tomorrow I'll go for a drive and see if the ABS works.

 

I was also looking at the other places the Light Green wire went too.
The diagram below is really blurry. Says for 93 F350 but I don't think 93 F series trucks had a high mounted brake light.

1) High mounted brake light which you don't have.
2) RABS controller.
3) Trailer controller.
4) Can't read it but something about clutch interlock sw out
clutch interlock sw jumpered out
cruise control servo amp.
Can't read the last line. The last word appears to be lightning or lighting.
5) PCM.

Also if you ever get to trouble shooting your cruise control and can
find the schematic, here's a detailed trouble shooting procedure.

http://www.justanswer.com/ford/4serj...rd-pickup.html