Just to add a little more info from the service manual (more or less):

For Preglow system eninge 617.950, 1978 to 1980:
"If the glow current circuit is interrupted, e.g. by a defective 80 amps fuse or power relay..., the electronic system receives no current from terminal 30 and the preglow indicator lamp will flash for approx. 30 seconds after the preglow system has been switched on. If one or several pencil element glow plugs or lines are interrupted, the preglow indicator lamp will flash for approx. 30 seconds after starting."

For the system starting 1981 (617 eingine, USA), if there is a problem, the indicatior lamp will not light up when the key is actuated in position "2". The cause can be:
(1) no current to terminal 30.
(2) defective 80 amp fuse.
(3) defective power relay.
(4) one or more (but not all) defective glow plugs or wires to the plugs.

David