Thanks Midnight! Back 5-6 years ago I had a nice 1991 F250 parts truck (wish I now still had it) so I went and looked at some photos....... you're correct, no MFS. Just a plain old turn signal switch. The '94 F250 we fixed with it did have a MFS. Principle is the same as far as the wiring but thanks for the correction. Would the 90 F150 have had a MFS? Is that why some of my resources are showing a MFS? My 93-12 resources are fantastic but they don't support 92-back any more.

Been a WHILE since I've worked on 80'-'90 Fords. I just got my '94 on 2/11/12.

 

Just had to say that you guys are awesome!

2000 F250 7.3L SD
Been trying to trace my problem with rear brake lights (flashers work, turn signals work, cab brake light works, but rear brake lights don't work) for about a week.

Decided that problem is either with 1) splice between exit from brake switch and entrance to multifunction switch (but OK from exit from brake switch to cab brake light etc, or 2) open inside the multiswitch itself (internal MFS wire from LG wire entrance to turn signal switches. Everything you guys said reinforces that, so I'm going to dig in to the column wiring tomorrow to see if it's a break in the wiring or the switch itself. Will let you all know...

Thanks.

 

Coz here reminded me about this thread.

Basically once I got the dash off this truck we discovered that the wiring underneath was a total rats nest where Bubba had added in wiring nuts and apparently attempted at one time to install an amp along with God-knows-what-else he did.

My response was to, rather than attempt to run down all the problems, build a new wiring harness. I did this and ran new wires to the brake lights.

This did not fix the problem, but it did get me power running through the MFS and to the lights. I then found that the ground connections were corroded to the point that they looked pretty normal but would not transfer electricity.

I cut the original grounding wires, drilled a hole for each in the metal skin of the truck bed behind the taillight assembly and attached them with a terminal and stainless sheet metal screw.

After that, everything worked fine. All I had to do was rewire the whole dang truck!

 

OK, after I found out where Multi-Switch was (on steering column), I opened it up and read LG wire (MFS input) while pumping brake pedal. OK (12V w/pedal down, 0V w/pedal up). No voltage at MFS outputs (neither LG/O or O/LB). I noticed that I could see metal on the LG input wire, like it was pulled out of the connector some. Wiggling the wire made contact and brake lights would work. I pressed the pin into the connector as hard as I could and then released pressure. I taped the connector wires tightly and repeated the wiggle with no failure. Don't know if this is a permanent fix, or not, but it seems to be working OK now, after reassembly. If it fails again, I may need to purchase a new MFS, but will wait and see.

Thanks for leading me in the right direction!!

 

Hello strychnine08. Was reading your forum from 2012 and was wondering if you found the problem. I have the same problem and received a fix it ticket from a bord cop who had nothing better to do in Oakland. About to dive in to it and see what I can find. Any help much appreciated. Thanks

 

1990 Ford Super-Duty 7.3 Diesel.

I got the similar problem too. I've noticed so many symptoms that I'm lost on how to tackle it.
-On/Off switch was replaced earlier(had melted marks on old one, but new one fixed it temporarily).

-Hazards, turn signals, Front running and cargo inside will work all the time.

-Find the sweet spot for headlights, dash and running lights to work at once.

-Foot button helps with getting all lights to work, but has to be set on high beam.

-No brake lights for the most part at any time.

-Plugging trailer harness in seemed to startle the whole shebang.

-Some luck at times with pressing the brake and then tinkering with the ****.

-Lately, after a bit of driving they all start to flicker.

-Speed Odometer bounces, and battery gauge twitches with turn signal blinks.

-With low beam setting: Only running lights, turn signals work(no headlights or brake).

All fuses seem good, don't know how to test multi-meter, and really have no clue how someone could explain it to me. I've read everything I could find and have a mechanic trying to get time to check it out, but I could rant and rave about this goofy problem and scenarios I've seen for pages.

I think it's a multiple of things, but I'm trying to narrow the search with checking the brake pedal switch and/or the low-high beam switch(discontinued part) first.

Any word is a good word with this disaster. Thanks

 

high low dimmer switch is not a discontinued part, they are an in store stock item at any napa store.

 

1990 Ford Super-Duty 7.3 Diesel IDI.

I narrowed down the lighting problem a bit.
-New brake pedal switch(fixed brake light problem)
-New Low/High beam floor switch(fixed that issue)
-New inner light switch wiring harness eased the question on connectivity.
Outcome: Still no rear(only) running lights. Plus, not getting power to all connection near trailer light wiring harness.

My questions are:
What's a good way to run new wires for rear running light problem?
Could my trailer light harness cause the issue with the running lights?
And Last, Don't it sound like I should just replace the whole shabang?

 

did you inspect the wiring going along the frame from the firewall to the back of the truck? i have seen cut and chaffed wires from failing cab mounts that let the cab sit on t e frame shorting out the wires.
also, if you suspect the trailer harness, remove it from the system and see if that is the cause. if it is, the running lights will start to work once it is removed.

 

Thanks for the reply.
I didn't do a complete trace due to the weather and time I had already spent. Plus, the frustration set in.
I'll do what you advise and see what I find.
I like to ask the questions to motivate me and have someone(like yourself) give me a sense of direction to follow. Very appreciative, tjc.

 

Trailer wiring has caused hair loss to many. Do you have the factory wiring, a Hoppy/U-Haul harness or a home made harness?

 

When troubleshooting electrical problems it pays to be methodical. Work through whatever subsystem you are troubleshooting from one end to the other, don't jump around. A side benefit of this is that you may find iffy areas that may cause problems later on.

 

It's a cluster to me. I had a guy who worked for Ford spend 2 hours with the previous issues. He was even a bit puzzled.

It's a box truck(U-Haul type), with the wiring harness going to its own lead. It has a lead that branches to the lights and back up beeper. Everything is wrapped with electrical tape. It's very hard to describe or picture as on your guy's side.

It has a trailer brake module near the stick shift that lights up when I hit the brakes, blinks with flashers and lights up when I move over the slide button.

The trailer light harness is a smaller round(assuming older style 8 pin) that takes an adaptor for 4-way plug.

The mechanic was saying, "I'm not getting power to running light wire from switch". Relay worked(I think he said).

In the middle of truck (on frame), it has a 4 way something connector that has me all confused. I don't know what all it runs back there.

I will try to take some photos if It helps, but I understand these problems are hard to diagnose over the computer.

Thanks for everything

 

Very true. And that's what I need to do with this situation. I want to learn these kinda problems as well. You guys might not know how much these posts help me out, but I assure you, they give me motivation to go back out and try.
Thanks a lot.

Extra: What really gets me on troubleshooting this...Test lighting the problem. I have no idea when the wire should be hot, when ignition should be on/off, or what to do afterward.

 

I think you might get valuable use out of the EVTM, Electrical, Vacuum Troubleshooting Manual. You can find them on eBay or at Bishko Books, Repair Manuals, Owners Manuals, Parts Books & Sales Brochures. It really makes trying to figure out the various subsystems of your vehicle easier. If you want to get really detailed, pick up a copy of the wiring diagram at either of the above two sites.