Tractor glow plugs get hot when battery voltage is directed to them, either a simple circuit though a fuse and Ignition switch position, or though the use of a timer control unit and coolant sensor. My tractor has only a ignition switch position just before the start position that turns the plugs on, and it's up to me to time it. So the power would come from the battery, through a fuse, through the ign sw to the glow plugs. The voltage is grounded throught the plug's resistor core into the block. Troubleshooting would involve a test light or volt meter probing at the bulb feed and at the glow plugs.You can also eyeball the bulb if it's easy to remove. You can compare resistance values for each glow plug with the wires off. Then check for battery voltage with glow plugs energized. A quick check of the fuse might be good starting point though. A more complex system with a timer or computor control might need a peek at the wire diagram. For instance, some modern diesel cars cycle the glow plugs when the driver's door is opened, so that there is no "wait to start" . Of course the system is computor controlled, and depends on many sensors to decide if glow plugs are needed and for how long.
