Hi, I have a 1987 300e with about 150k on it and this morning the car wouldnt start. It has been working fine for months but for some reason it wouldnt start today. I did have to replace the OVP relay once though. I jump started it with another car and the engine would turn off as soon as one of the cables were removed. Does anyone know what the problem is or what I should do? your help would be greatly appreciated. thanks

If you have a voltmeter check the battery should be around 12V with the car off. Batteries die in a couple of ways. They are usually made of 6 cells in series to get to 12V. The cells can die open or short. If a cell dies the voltage will drop 2V per cell. If you get 12V this indicates the altenator is the problem. They can become a resistive short across the battery and drain it overnight. Since you said that the car dies when you disconnect the jumper I think the altenator has gone bad but it is a lot easier to pull the battery and have it checked out first if you cannot do this yourself with the voltmeter.

Since you have replace the OVP check the fuse on top of it. Strange things happen when that fuse blows.

Sounds a lot like the alternator may be the culprit.