Truck Repair: Diesel wont shut down, push button starter switch, bottom position

Question
Hi Mike , I have a 1973 fire engine with a 6n71 engine.There is a 3 way toggle switch on the dash for starting and shutting down the engine. the bottom position gives power to the push button starter switch, middle is neutral no power and the top position is a spring type position that you have to push and hold to shut down the engine. My problem is , when I try to shut down the engine it won't shut down. I have to use the emergency shut down knob to stop it. It just happened this morning. I checked the switch and the contacts are tight. I don,t know where else to look. I dont even know the principles of how a diesel shuts down. Can you help ?
Thanks!

Answer
The stop part of the switch should supply power to the stop solenoid on top of the pump.  When it gets power it should activate a plunger to push the stop lever. Older Diesels run on there own power, once you get them going they will run forever without any 12v (or 24v) power.  The way they can be shut down is to either cut off the air, or the fuel.  The stop solenoid just simply cuts the fuel flow off.