When ignition switches fail, occasionally they won't immediately break the circuit which keeps the coil energized when you turn the key off. This will cause a condition which almost exactly mimics dieseling because usually the engine will run for another 5 to 10 seconds before the ignition switch contacts can "let go."

The 250 sixes had HEI ignition...or at least I know for a fact some of them did. If a relay wasn't added between the heavy gauge wire needed to supply the coil then that could definitely cause the ignition switch to fail. And I am a shining example of this oversight. I recently added HEI to my 235 and forgot all about the high amp demands of the HEI coil.

So now I have to wire at least a 20 amp relay (preferably 30) connecting the 10 gauge coil wire straight to the battery (the ignition switch will then be operating that relay in the same manner it operates the starter solenoid). Otherwise my engine will continue to run every time I turn it off until the ignition switch finally burns out.

As I recall the original switch was designed to handle a maximum of 15 amps and with the HEI coil that isn't enough. And yes, I am irritated that I made such a boneheaded error, but I did and now I know. Good luck on your search of the problem. One way to check for this is to (a) find the coil (some were mounted on the engine) and (b) have somebody turn the engine off at the same time you disconnect the plug from the coil. If it stops without continuing to run, you've found the culprit.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end