Kevin:

The temp sender is called either a HPCA (housing pressure cold advance) switch or a glow plug inhibit switch depending on what parts book your looking at. The unit is simply a heat activated switch that turns off at 130 degrees F. Normally when they go, they stay on. Should be around 20 bucks at any auto parts store.

On the older glow systems, this switch also prevent the glowplugs from coming on when the engine is warm. If it is disconnected, then probably the glows aren't working unless the PO hotwired the controller. If you convert to the newer controller, it can be wired to turn on the glows, regardless of engine temp. This is a good idea as any of my 6.2s are real grumpy about starting without the glows at any temp other then full operating.

Testing the switch is simple. Put a meter across the two terminals and when cold, it will read a dead short then fire up the engine and get the temps over 130F and your meter should read an open circuit.

Bill