That is a series variable resistance with the glow plugs...
With that series 'heater' with the glow plugs... you can not give it too much time, because as it heats up, it's resistance goes up and that will reduce the current going through the system...
If you watch it closely you will see it glow bright and then dim down just a bit... At that time the glow plugs will be at their hottest... And that is going to be the optimum time for power to the GP's... KennyV
I have one Kubota engine with that system and even in the summer it will take about 10 to 15 seconds to properly heat...