I am working on this unit, and I found the glow plug fuse blown. I replaced it and tested. the glow plugs stay on all the time. When I pull the relay, the power goes out for the glow plugs. So my question is, is what powers up the relay, is there a timer, or does it work off the sensor that is in the thermostat housing? the engine is a Z15 diesel.

Thanks so much,
John