The preglow lamp in the instrument cluster doesn't work.
The preheating system is working OK, what I do is to wait around 20s before starting. My car, a 1981 300d, has the 5 preglow connections in parallel.
I checked the electric diagram in my Hanes book and found a blue-white cable going directly from the black relay box to the lamp. With my tester I checked the line continuity to ground and it's OK.
When I put in ignition the car, I can read 2v in this line. I suppose that it should be 12v. My electric diagram is not very detailed, but I am supposing that the transistor in the preglow circuit may be wrong. Does any body has experience repairing this circuit, or I should change all the black box.
By the way, this is my first post, but I been reading the diesel forum for some time and I find it very helpful and interesting. Thanks every body.

The dash GP light is just an indicator on how long you should wait for the GP to heat. If you can estimate how long to wait , you don't need the indicator.

Only thing you won't have is the bad GP indication.

P E H