But in my travels the controller really doesn't go bad very often - most times it's bad connections because the controller is looking at voltage drops and bad connections will fool the controller into thinking that everything is heated up fully.
I am finding that out with mine, even though I have converted to a manual switch. When I had the controller, it was always the connections, either at the main harness plug or I found out the connectors to the glowplugs themselves. After bypassing the main harness plug, I still couldn't get it to work sometimes, and I found out my bad connections were on the glowplugs, since after I unplugged them to test them, the controller would work ok for a couple of weeks.
What I can tell now is my connections on the glowplugs are still sometimes giving me problems, because after using the manual switch, sometimes the engine is missing a few cylinders even though all the glowplugs are good. So I am figuring some of the bad connections on the glowplugs are keeping some of them from heating up properly. But it still starts with the manual switch, even with 3 bad glowplugs, and that's my priority, for it to start when I need it to start.
I am thinking of making my own connectors for the glowplugs. Right now I am thinking of using some small diameter brass rod(maybe 3/8") cut in short 1 inch pieces. Put one end in the lathe and drill it to accept the diameter of the glowplug, and have enough meat on the outside to drill and tap for a setscrew to hold it to the glowplug. Then drill the other side a smaller diameter to accept the wire, and solder it in place.