S210, Glow plug light on for 40 secs, even when warm.

Hi,

I did try search.

1999, E300td. Car starts fine, glow plug light goes out ofter 40 secs from cold which i think is normal. When car is stopped and restarted the glow plug light is on, then goes out again after 40 secs even when the car has reached temperature. ?, is that normal ?