Check Engine light, 300E 1990
 

The last couple of days My check engine light has been comming on after I remove the key from the ignition. Last night it was on all night and ran the battery down. After I start the car it goes out. The only thing I have done to the car in the last week is replace the fuel pumps. They shouldnot be powered with the key off. Any ideas?

This is odd with the light on when the switch is off and key removed.
When I have done work that causes a regular "check eng." lite I just "erase" the codes by doing the following which I got from a mechanic: disconnect the negative lead on the battery, turn the ignition switch on all the way (not start), step on the brake pedal, and reconnect the ground cable. I have never had the light stay on or come back on so I assume that as long as the trouble is fixed the light will stay off. I don't know if this will help in your case.
It sounds like you may have a short or a cross some where if "erasing" doesn't help.

I found the problem. When I was checking the engine compartment for a short, I touched the OVP and felt that it was too warm, hot even. Even though I had replaced the OVP a while back when I was trouble shooting another problem. I pulled the OVP and found that it was full of water and corroded badly. Luckily, I had not thrown the old OVP away. I reinstalled it and the light went out and all works well. Side benefit is that the car now starts on the first rev. This is a inherent problem with this model as I have seen in it other posts. Water seems to drain directly on the top of the OVP. I suggest covering it with some tape or plastic to keep it dry.