[Steeveedee]

On points ignition GM vehicles, there are two terminals besides the big one for the main starter cable from the battery positive. One is labeled "S", and it is on the frame side of the starter (away from the engine block). It should have the 8 or 10 Gauge purple wire from the ignition switch attached to it. The other terminal is labeled "R", and has a smaller yellow wire attached to it that supplies power to the coil while cranking.

"S" stands for "Start" and "R" stands for "Resistance", btw. Resistance, because the resistance across the solenoid's many, many windings provides voltage drop so that the coil isn't hit with full battery voltage during cranking. Power to the coil comes from the solenoid during cranking.

If by "cranking", you mean that the starter spins, but the truck does not turn over, the Bendix spring in the starter is bad, or bound on the shaft.

If it is still "cranking" after you took the key out, the purple wire is still getting power. You will have to chase that problem down. How did you stop it from doing that? Remove a battery cable?