Last weekend I had a small electrical fire on my 71 F250. Basically about three feet of the violet wire that runs from the ignition key to the dual brake warning switch burned. The brake warning light on the dash stays on now when the engine is running which makes sense since the wire to the ignition is not providing the necessary current. I can replace that wire and any others that were damaged but what I'm trying to figure out is why it happened so I can prevent it in the future.

I'm thinking that for some reason the differential pressure valve became activated grounding the switch and drawing current through the burned wire. I have equal braking front and rear and the master cylinders are full with no leaks. At the time I noticed the smoke I may have been trying to back up with the emergency brake on. Could this cause the proportioning valve to not be centered? Was this just a bad wire? One other thing to mention is about 30 minutes before this happened I had been towing a boat with the truck for the first time. I'm not sure if this had anything to do with it.

Sorry for the long post, any help would be appreciated.

 

Electrically speaking, a short is a runaway current situation (no resistance) so I figure the powered wire illuminating the brake warning light grounded itself somehow.

... the sacrificial component in this case, like a fuse, was the actual wire that powered the light.

 

Ford Engineering design was a big screw up.

The Brake warning light was hooked into the main power in fuse block,
that is a 20 amp fuse that was used. Now since the Brake warning light wire is like a 18-22 wire happens too short out any where it "will not pop" the main 20 amp fuse since the wire size is so small of gage, but it just becomes red hot wire burning the insulations off of it.

I also had it happen to me to when pulling away from the curb with out any waning other then a lot of smoke clouding up my view of the windshield along with the smell of burning wires. And know why mine burnt, no bulb in the socket for 20yrs or so when the hot socket wire finally had grounded.

I've since then added a 2amp fuse inline coming off the main 20amp fuse at the top of the block top coming to the brake warning light.

I think everyone should add a fuse to their warning light so this don't happen too them.

Now why did yours short out is what you'll have to investigate an find out why.
Even if P-valve grounding switch shorted that wire it would only turn on the bulb warning light, not short the wire to burn.

Only the hot wire coming off the 20amp main fuse block to the warning light wire if that gets shorted will burn your wiring. Since the rest of the violet wire is a grounding wire.
So I'd pull the Brake warning light down an inspect that hot wire for any cuts in the insulation.
Orich

 

I don't know specifically what caused the wires to burn but, this is how the circuit is wired.

http://www.fordification.com/tech/wi...rninglight.jpg

Outside of a short circuit, and assuming the pressure differential valve is centered, the warning light (on '68-'72 model Bumpsides) should only momentarily come on in the moment that the key switch is in the Start position. Once the key moves to the Run position, the warning light should go off --IF the valve spool is in fact centered.

If the pressure differential warning switch on the brake valve didn't get burned/melted/damaged, it is possible to isolate it from the circuit to check it and to determine if the pressure differential valve is actually centered or not.

Here's how you can check the valve spool position without the switch even being wired into the electrical system of your truck --you can do this test with the valve/switch still on the truck without removing them. If this test comes back good, you know that it's an electrical short circuit problem and not a problem of the valve being shifted.

https://www.ford-trucks.com/forums/1...l#post16470989

 

2X Orich's comments. When you replace the wiring put a 2 amp fuse inline for the power to the brake warning light

 

Thanks for the comments they have been very helpful.

Based on the drawing of how the circuit is wired the violet wire should be connected to a ground terminal on the ignition switch but it was connected to a hot terminal. That would explain why the wire burned when the brake warning switch grounded. I can fix that and will add the 2 amp fuse to the wire powering the dash light.

I did the continuity test on the proportioning valve and did get a resistance reading, so the valve spool is not centered. This makes sense to me because with the ground wire burned and the dash light on with the ignition in the start position it must be grounding through the proportioning valve.

Now I need to figure out why the valve isn't centered. I'm thinking that because I towed my boat for the first time that day and applied the brakes very hard several times this highlighted a problem with the brakes. I've recently completely rebuilt the front brakes. Could the solution to the problem be as simple as adjusting the rear brake pads to equalize the front and rear brakes?

Does the above make sense or am I off track?

 

There are only two reasons why a pressure differential spool would shift:

1. There was a failure of a brake component seal or, a line or line fitting is leaking and caused a loss of pressure in either the primary or the secondary brake circuits.

2. The brakes were bled at some point and a differential in system pressures was created on one side, compared to the other side, causing the spool to shift.

Applying the brakes heavily will not cause the spool to shift --unless reason #1 happens at the same time.

Brake system pressures will be the same on the front (primary) brake circuit as it is on the rear (secondary) brake circuit. If they weren't, the brake warning light would trip the very first time you stepped on the brake pedal.

 

If I understand you correctly if it was a differential in pressure due to bleeding the brakes that would have shown up right away. So it sounds like I have a loss of pressure somewhere in the braking system. I will work on tracking that down.

Thanks for your help.

 

I would look the system over thoroughly and make sure there are no leaks in the hard lines, flexible lines, at the fittings or at any of the wheel cylinders. If you have manual brakes, look up under the dash on the inside of the truck and make sure the back end of the master cylinder isn't wet with brake fluid. If all this is checked and there are no leaks, you can proceed with trying to recenter the pressure differential valve.

If it was caused during the time the brakes were bled, most likely the brake valve spool is shifted towards the secondary side (the side of the brake valve that would supply fluid to the rear brakes), since the rear brakes would be the first circuit to be bled, it's pressure would have been lower and the primary side would have been higher.

 

One last question. Is it possible to get air into the brake line during the recentering process?

 

If you don't have Speed Bleeders installed, you'll have to proceed just like you would if you were bleeding the system with regular wheel cylinder bleeders. --open bleeder, push on brake pedal, close bleeder if light goes off (or you get to the bottom of the pedal stroke) before you let the pedal back up so you don't draw air into the system.

If you have Speed Bleeders, you don't have to close them before letting the pedal back up.

IF the spool is shifted towards the side for the rear brakes, you would have to open a bleeder on the opposite side of the system --the fronts.

 

No as the p valve spool is o-ringed at each end. not any fluid should be in the switch area threaded hole.

This is what I used to keep the spool center while bleeding filled with JB weld epoxy

 

Okay thanks everyone.

 

I would also look close at the back of the ignition switch and it's wiring. Other wires there burnt, bare copper, melted terminals on the switch, etc.

 

Regs require the brake lamps to always have power they could not be fused with a fuse that would not reset. Same as headlamps.

Later on they were fused with a CB such as the guy below.