Ive noticed the time the glow plug light stays on is longer when it is very cold out.. and on the same day it will stay lit for half as long restarting a warm engine.It’s foolish to crank the engine while glow plug light is on because the shared load of glow plug and cranking can reduce voltage down to the point that damage to components can occur. However, know that just because the glow plug light is out does not mean that the plugs are cooling off and are not still being actively heated. They are. The light goes out signifying they are hot enough to start. Personally I wait for them to go out and give an extra 5 count then start, unless it’s already warm from driving and then I just wait for light out.
I therefore believe the indicator on the dash is actually showing us when current flows to glowplug. During starting, didn't you ever notice the headlights stay off? All available current goes to the starter. My lights come back a few moments on after the motor is running. Seriously doubt starter is sharing the current of the glow plug..
Been cycling the plugs 3x before starting on frigid days for 150,000 miles now..
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