As to what to do when it doesn't work as intended, and you are stranded somewhere without a block heater? Simple. Get out a pair of jumper cables, and use just one of the clamps to short together the two big poles on the contactor to manually energize the glow plugs that still remain operational. Want to avoid warm start glow plug operation? Find an ambient air thermostat like you would use on a furnace, and route the power feed or ground wire to the GPC through it. As long as the thermostat is warm, the glow plugs won't come on.
This is simple enough for the turbo folks with the GPC hanging off the valve cover, however you know how easy it is to access it on a n/a truck. Not only does jumping the contactor terminals with a jumper cable get a bit dicey there (gotta make sure the other end cannot touch anything metal, a leather or winter glove actually works pretty good for that), but there is also the time it takes for the person jumping the contactor to get off the engine and run into the cab (or jump the starter relay on the fender) to start the engine - that allows plugs to cool down and when you're not running on all 8 to begin with it could require multiple jump-glow cycles to get the ice-cold engine fired up. And if you have a helper to hit the start the moment you're done with the plugs, you're now sitting on top of a running engine racing at high idle with that B17-propeller-sized fan ready to chop down any appendages you may feed its way - not a very safe situation considering when people get cold they also tend to move a bit more awkward...
As for the air thermostat, I think the cold-advance sensor would be more suitable - air temperature in the intake of a shut off engine can be a dicey thing, and it's the coolant (well, engine block) temperature that makes it easy or hard to get combustion going. So the solution, at least mine and one more person's on here, was to cut key-on power to the controller from the harness, and instead feed it thru the cold-advance temp sensor by tapping off the fast-idle or cold-advance solenoid wires.
Don't take me wrong, I'm not advocating removal of the factory system in favor of full manual switch just for the heck of it. A 3-position switch would be much better - manual glow on one end, system full off in the middle, and full-auto on the other end. Then you can have you cake and eat it too - if you drop a few plugs for whatever reason you can use the manual position to get her fired up, and if the controller hangs up like ours did you can still shut it off without affecting anything else on the truck (and technically you can still fire up the truck since the plugs were just active). No climbing in the engine bay on top of fast-moving potential meat-tenderizers, and everything is at the fingertips of just one person.
