I have an old f-150 that I'm gettin running again. Right now, when I turn the key, the engine starts as normal, but as soon as I let go of the key the engine shuts off.

I asked about the problem in a couple other places and they pointed the problem to either a shot ballast resistor, or ignition switch.
Now the next thing I need to figure out is where is the ballast resistor and where do I get a new one.

If fixing that doesn't work then I'll try the ignition switch

good dirt,

 

I thought that Ford used a ballist resistor built into the coil instead of a separate external ballist resistor. I do know that older Mopars had a dual element ballist resistor, 0.5 ohms for start and 5.0 ohms for run. And what you describe is a classic failure mode for them, as the run side always failed first, so they started easy but would not run.

If there is a ballist resistor on your truck it will be wired in series with the points, coil and switch. Just follow the points' wire from the distributor to the coil. If you don't find it there, then it may be in the wire from the other side of the coil to the ignition switch. If you have a resistor it will be in one of these two wires. It will be a ceramic block about 3 inches long. If you don't find one, then it is built into the coil, and your problem is the ignition switch.

Hope this helps.

 

what year is this truck?