I'm sorry if this question has been previously answered, I looked through the archives (I admit not every single one) but I couldn't find this particular problem.
I bought a 1984 300SD last week and everything seems to be fine except for the glow plug light in the instrument cluster. It doesn't come on when I turn the key for preglow. If I leave the key on and wait for the glow plug relay to click off then turn the key off and back on again, the glow plug light works as normal. It stays on for some seconds then turns off. If I wait for a few minutes and try again the light does not come on. So far the car starts very well, but its the middle of summer in Los Angeles.

Thanks for the help. Also thank you to all the people who answer all these questions. Sometimes I feel guilty that I get all this help for free. Its as if I have a Factory Service Manual that can talk back when you ask it a question.

Gilbert