Building on the earlier blog post showing the common diagram for wiring a solenoid control relay often seen on the 39MT Delco starters, we have expanded to show the diagram using the optional over crank protection thermostat (OCP). This is the small two pin black plastic plug on the comm end of the starter.
The purpose of the OCP is to open when it gets too warm - usually caused by the operator cranking the engine for too long. The two pins are soldered to each other inside the rear housing of the starter - when it gets too warm the solder will melt causing the pins to no longer be connected and the solenoid control relay will lose its ground connection disengaging the starter. Once the starter cools, the solder will become solid again allowing the ground connection of the solenoid control relay and the starter should operate.