My stop/hazard 15A fuse blew a while back and blows everytime I put a new one in... last night at some point i noticed my turn signals weren't working. I look at the fuse (also 15A) and it's blown now too.

Where's the best place to look for a bad circuit? also, what/where is this MFS switch I've been told to look at?

 

Since your stop / hazard circuit has something in common with your turn signal circuit (the brighter filament in your front / parking lights and the same filament in your tail lights), I'd suggest that's a likely place to start. I'd also check to see if you have brake lights since they share the same circuit. If the brake lights work, I'd suggest that the problem is at the front of the truck with one of your parking lights or their wiring. Look for shorts. Try pulling one bulb out, then the other and putting a new fuse in each time. This will help you isolate the problem area.

As for the MFS, I see by your signature that you've got a pre-1997 truck. If memory serves me correctly, they didn't have the problem switch.

 

I had a similar problem on my '89 F150. Mine was a short in the trailer wiring that ran underneath the rear bumper between the tailights.

 

The trailer wiring is gonna get a checkup... mine is wrapped through a hole in the bumper and zip tied on... etc so it might be fudged. also I'll check the bulbs and stuff when I get 10 minutes.

 

I checked for any damaged wires for as far up as I could.

I don't have brake lights at all (3 lights).

My hazards work in the front only, and this part is strange to me... when I hit the brakes, the hazard lights freeze on. That is to say when the hazard lights are flashing (whether the truck has no key in it, on ACC or started) they stay on steadily when i press the brake, running lights are off.

I tried blowing the fuse to see when it blows, but I can't think of anything else: hit the brake, turn left, turn right, started it put it in R, N, D etc and it's still intact

also, my red brake light on the idiot gauge doesn't come on, even with parking brake on

 

Hmmm, you do have some problems there! One thing that I don't think is a problem is your hazard (4 way) lights freezing when you hit the brakes. That was what happened in older vehicles.

As for your continuing problem, these things are tough to diagnose from afar. Hopefully, SCULLY will be able to chime in here because I don't have a wiring diagram for a 1991 truck handy. The problem definitely sounds like it's at the rear of the truck. Did you pull out any bulbs before putting new fuses in? Did you disconnect the harness (trailer or otherwise) at the rear of the truck to see if that keeps the fuse from blowing? You might have a dead short right in the socket. In a truck that old, it's not at all unusual.

 

I pulled the right bulb out before I replaced a fuse... the left one I haven't tried yet cause there's a fiberglass container thing screwed into that side and makes it hard to get to in 38 degrees in the dark D: I'll try pulling the trailer plug out (I assume it just disconnects somewhere between the lines in the back? I haven't seen how it works yet, but I'll figure it out)

edit: I think I forgot to mention that my turn signals work in the back... don't they use the same filament in the bulb as the hazards?


I found an ungrounded wire behind the cd player and the two connecters that the ground wire comes from were disconnected from something attached to the front of the computer.... but that wasn't it

I pulled the bulb out of one side then the other and got no results either way.

disconnected the trailer hitch wiring and plugged the two sides into each other and still nothing.

Do I have a 3rd brake light? I opened what I thought was the third brake light (the one that lights up when you turn the light switch to the left and there's just the one bulb in there.

My dome light wasn't attached, so I put that back on, still no results.

Is there any way this could be tied to my front gas tank recently not showing how much fuel is in it? I noticed they share the same wire jacket when I was tracing the lines up to the cab, although I doubt they share the same circuit or anything...

 

I found an ungrounded wire behind the cd player and the two connecters that the ground wire comes from were disconnected from something attached to the front of the computer.... but that wasn't it

I pulled the bulb out of one side then the other and got no results either way.

disconnected the trailer hitch wiring and plugged the two sides into each other and still nothing.

Do I have a 3rd brake light? I opened what I thought was the third brake light (the one that lights up when you turn the light switch to the left and there's just the one bulb in there.

My dome light wasn't attached, so I put that back on, still no results.

Is there any way this could be tied to my front gas tank recently not showing how much fuel is in it? I noticed they share the same wire jacket when I was tracing the lines up to the cab, although I doubt they share the same circuit or anything...

 

I figured out what is blowing the fuse. I was making a right hand turn and heard what sounded like celphane crumpling (or sparks) and the fuse blew (I had my hazards on and waited to see when they don't work anymore)

I'm going to get into the steering wheel tomorrow to see if i can see anything... would this possibly be a good place for the brakes to not work too? the brakes don't come on even when the fuse is in.

 

I'm in the middle of putting a new turn signal switch in....

I'm to the point where I'm almost ready to put the steering wheel back on, but everytime I put the key in the ignition, it honks the horn... I'm certain I didn't cross any wires putting the clip back on the wires. (so I wouldn't have to take the shift lever off, etc)

Anyone got any ideas? Something too tight? What is it under the steering wheel that makes the horn do it's thing?

 

I figured out the horn thing... but now I've got a sitiation... When I turn the ignition, my back right blinker starts to blink, but not the front.

Also...

With the stop/hazard fuse OUT of the fusebox, pulling the hazard switch makes my front hazards go on along with the dash lights... and my back right light still blinks at the same time.

 

Ah, the joys of working on older vehicles. My guess is that you still have a short in there someplace. Seeing as you're getting power to stuff with the fuse out, I might also suspect a bad ground. Bad grounds lead to current trying to find a ground path any way it can - which leads to all sorts of weird stuff happening. Keep looking and good luck!

 

This is getting strange.

Without the Brake/Hazard fuse in, putting the key to ACC makes the back right taillight blink. Left turn does a normal left turn and right turn makes all the blinkers stop, as if the blinker lever were in the "neutral" position, including the dash lights for each one.

The hazards make the front ones go and the back right light continues to blink along with it.

This is the part that SHOULD give me some clue as to where the problem is... When I put the fuse in the Brake/Hazard Slot with or without the key in the ignition, the right rear taillight keeps blinking and a second "click" that goes along with the blinker is coming from behind a steel plate screwed onto the passenger side right under the dash, except this one sounds Metallic(A).

If I have time after my wife's shindig at red lobster I'm gonna get into this panel and see if I can better locate this noise

 

Hazaa... fixed it... Turns out it must've been the turn signal switch causing the original problem, and get this... the switch I got from autozone had the top half of the wires in the harness reversed. how the funk does that happen??

I pulled the wires, put them in the order my old switch had them in and voila, everything works...

I still find it strange though I have no 3rd brake light... I always thought I did.