[MBZ] SD Glow Plug Light Blinking

[beswisck]

The dash light for the glow plug in my 1980 SD blinks for 15-30 seconds after 
the car starts.  I don't know when it started doing this, since the car is now 
being driven by my son.  Does not seem to have any effect on the car, but I'm 
wondering if it's some kind of code.  Anyone know?
TIA