Sit in the drivers seat.
Notice big round metal cap with chain attached next to your right leg?
That is where you plug in a diagnostic machine that tells you things like exact voltage from each alternator and amps, whether you have fuel pressure or vacuum at the fuel filter....this is the only use of the fuel vacuum switch. Only the Military vesion of the GM Diesel light trucks have the fuel pressure switch.
It also lets you know if the heater on the fuel filter is working and the water sensor. Water temperature of engine, oil pressure, operation of glow plug system (when engine is ice cold aka: room temperature) All in all a handy little device to have for preventative maintenace but I am drawing a blank...can't remember the model of the diagnostic machine. Last I heard they go for about $100 at Military fleamarkets.