Glow Plug Light Flashes When Cold
 

My glow plug light flashes (after the engine is started) when the outside temp is in the 40's or below. When the outside temp is higher the engine starts and the light turns off normally. I know the light flashes if there is a bad plug, but why would it only flash when the temp is low?

1980 300SD
Thanks in advance

I am not sure how the glow plug light knows that a glow plug is bad but when it checks your plugs might pass when warmer. This might be a wakeup call for next winter. Check out the resistance of each plug to see.