I had an 89 300SEL that did the same thing. With out the light coming on the car started shutting down. So I am not sure why it didn't work or the one in the diesel. I know when I turn the key on the light comes on so I know it's not burned out.

So far I have been driving around with out a problem. I am not letting it idle very long with a lot of electrical stuff on. It has started fine every time, it’s not dragging at all, but I know it’s not right. When I had it looked at they said a diode is open. My friend's garage is warranting the alternator. I will check out the glow plug relay as I said earlier it stays on a long time.

I have pulled the regulator and brushes out on each one and they had some wear on them but worked fine.

As for the car it’s an amazing animal with the miles. With the exception of the battery/alternator thing it’s been a great car.

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Mike

90 300D 511k
95 C280 133k
14 BMW 528i