I just bought a 1990 300E and put a new battery and alternator in it. The car dies even with a new alternator. Also, when I lock the car, the battery is draining. After getting in it, I can hear a motor running (sounds like it is somewhere in the back seat area). I think it is the motor trying to raise the windows after you lock the car. I am puzzled by the car dying while driving it with a new alternator. Any suggestions?

I think the motor under the back seat in the pump for the vacuum locks. Maybe you have a vacuum leak and the pump is constantly trying to make vacuum, it will do that. The pump is activated when the vacuum is insufficient, regardless of the car being on. I have head of some people draining their batteries, by locking and unlocking the car many time (usually more than 20) with out starting their cars. It might be a place to start.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired