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When the engine is cold or only slightly warm the glow plugs need to cycle on to preheat the pre- chamber where the fuel is injected. If the glow plug light does not come on, then there is a fault you will have to locate. I am not sure if the transmission neutral switch is part of this circuit, but as you have described, it seems like it is. The switch is cheep in price and easy to replace on the transmission. Could also be the linkage needs adjustment, but I would think probably not.
If all checks out with the switch, you need to check the glow plug relay and wiring and ALL the glow plugs.
Mercedessource have a manual kit if all else fails. If I remember right, it sells for about 100 USD
But as the glow plugs cycle on and the engine starts well in neutral, I would think the neutral switch on the side of the transmission is faulty.

Quote:

Originally Posted by djgonzo (Post 114604)

So I spoke with the previous owner and he suggested that I put the car in neutral, and then turn the ignition on. I did so and the glow plug light came on then went off, put the car in park and it fired right up! It seems like the glow plug light won't come on while the car is in park but will when shifted to neutral. I repeated theses steps a few times and the car fires up immediately. BTW, it is cold outside.

The previous owner remembers this happening on a 300d he had a few years back but says it hasn't happened on this car before.

I test drove the car twice, both times in cold weather and didn't have to do this before nor did I have to do this Saturday when I bought the car and drove it all over town turning it on then off 6-7 times while in park.

Any ideas as to why the glow plugs go on/off in neutral bu not in park?