Be careful with the glowplug light never going out though. I've had this problem and decided to ignore it. After a day I couldn't start the car - battery drained. Brought it over to a mechanic (before I started working on car myself) - he claimed that the alternator was burned out and I didn't need to worry about the relay.

He replaced the alternator and battery and off I go - guess what - next day I can't start the car again.

So he had to replace the glow plug relay, alternator and battery again.

I guess, the glow plug relay will pump insane amount of electricity to glowplugs to get the hot and is supposed to shut off after a certain period of time. And if the relay is bad and constantly pumping amps to glow plugs the current drain will be too much for the alt. and battery.

My $.02

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1992 W140 300SD
1982 W123 240D
and others non MB vehicles...