I've been having a strange problem with my ignition on my 78 F-250. It all started with the truck not starting until I let off the key to the "run" mode. No big deal, I replaced the ignition switch. When I went to fire it up, I heard a "pop" from under the dash. After that, the truck would start sometimes, and other times it wouldn't. After going through a couple of switches like that, I found that the stator ground wire was coming out of it's crimp in the harness, which explains why it would start about half the time. I replaced the stator, and put in a new ignition switch for good measure. Went to fire it up and "pop" again. After some investigation, I found the "pop" to be the little brass ground strap on the switch body, and the little brass piece is blowing in half. Since I replaced the stator, the truck starts every time now, but there has to be a reason that ground strap is blowing apart. It is only doing this when the key is in the "start" position. Has anyone run across this before? Any help is appreciated. Sorry this is so long winded, but I am trying to give as much info as I can.

 

I would think that there is a bad gorund somewhere and that little brass ground strap is taking the whole load and it cannot handle it.

 

I learn something from you guys every day. I read your problem and was wondering why in the world the ignition switch even have a ground? If it's just switching hot wires like I thought, there would be no need for it. But I looked at some of my diagrams(a little bit later years than your 78) and discovered indeed, the switch has many poles in it, and two of them are grounded internally. I looked at the 1980-81 diagrams, and believe there is an error in the drawing, because it doesn't make sense. But I looked at the 1982-83 drawings, and they show one of the grounded parts of the switch is used for the brake warning indicator.

The brake warning indicator in this instance is a light in the dash, and is fed 12 volts when the key is in run. The other side of the bulb is not grounded, but is wired to the switch inside the proportioning valve on the brake system. If there is a problem in the brake system, the switch grounds the wire which makes the light bulb come on. But also tied to this same wire that grounds when there is a problem, is another wire that goes to the ignition switch.

Everytime you turn the key to start, it grounds the wire momentarily, so you can verify the bulb is still good.

Like I said, I am finding all this out in a later diagram. If someone has a 78 diagram, maybe they can verify it is wired this way or not that year. The grounding wire that they use is a pink/white wire going to the switch. I would be curious if you cut this wire, and re-soldered the ground strap back together, would it still pop in two? If it didn't, then you know where the problem circuit is.

As for what the problem could be, one thing that comes to mind is the wiring may have been changed, and the bulb no longer exists, or somehow the pink/white wire has become a hot wire. When the key is turned to start, it would be a dead short to ground for this wire, and in normal circumstances it should blow a fuse before it melts the ground strap, but not not blowing any fuses is suspicous to me that there may be some modfied wiring somewhere.