78 Suburban starter will not shut off after engine starts. Turned off engine and removed key and starter kept going. Had to disconnect battery to stop starter from from turning.
Disconnect the activation terminal (wire from ignition switch) from the solenoid. When you now reconnect the battery and if the starter cranks, the problem is in the starter. Either the solenoid is stuck or the bendix drive is holding in. If the starter does not chank, measure the voltage at the disconnected terminal. If there is voltage, trouble shoot back to the ignition switch. If there is a starter relay in this model, consider changing that.
Hope the above helps.