Trying to help my nephew again, on his '66 289 Ranchero. The problem he's having is that the engine begins cranking the instant the key is turned even slightly towards the 'Run' position...nowhere near over to the 'Crank' position...and then it won't stop cranking, even when the key is moved to 'Off'!
What's really puzzling is that everything worked fine for several weeks, and then just appeared one day. The starter (Denso hi-torque mini) was new, as were the battery, cables (including solenoid to starter), solenoid, alternator, regulator and ignition switch. The battery has been kept fully charged. The ignition system is a Pertronix III module in a Ford 289HP distributor, and the recommended Pertronix low-ohm coil. A Pertronix relay routes full 12V power to the coil, just using the factory wiring to activate the relay.
We've tried disconnecting the battery ground cable when this starter run-on happens, and of course the cranking stops immediately. It also stops when the small red/blue wire is removed from the 'S' terminal on the starter solenoid. If we 'jump' the solenoid from the 12V input terminal over to the 'S' terminal...whether with the red/blue wire connected or not...the engine cranks. If the key is in the 'Run' position, the engine will start.
A lot of reading on forums seems to indicate that many people have this same issue, on a multitude of cars and trucks of all years. There does not seem to be any real consensus on solving this problem either. Frequent recommendations are to replace the starter, solenoid, or ignition switch. Yet many posters say the problem has persisted even with these new parts.
With the red/blue wire disconnected from the solenoid 'S' terminal, the engine can't be cranked with the ignition key. With that wire back on, and the brown 'I' terminal wire (ignition) disconnected, the engine won't crank until the key is moved all the way to the 'Crank' position. It stops cranking when the key is moved back to 'Off'. When the red/blue wire and the brown wire are swapped on the solenoid, the engine starts cranking in the run position, and won't stop.
The ignition switch has been replaced with yet another new one, but no difference. All wiring connected to it other than the factory 3-wire pigtail connector were removed, but still the starter run-on persists. I'm fresh out of ideas here, and would appreciate some input! Thanks if you got this far, and can help.