my 1994 mercedes C220 stalls at idle. the local mercedes dealer told me that it is caused by the alternator overcharging. can this be true???

An alternator that is over charging will damage your battery, but stalling at idle is usually another problem. Most likely caused by the throttle not closing enough to to position the TPS (throttle position sensor) to the idle position.

If the throttle doesn’t close enough, the computer thinks you are still on the gas so it doesn’t take over and control the idle speed. This could be due to a dirty throttle body, which you might notice by the throttle sticking a little as you first apply the gas, or someone may have adjusted the throttle stop screw thinking that it was like an idle adjustment on a carburetor.

You could also have a defective IAC motor (idle air control).

Alternator is putting out 16.9 volts at 400 RPM. Dealer says that it is confusing the computer & when the “exciter” wire is disconnected from the alternator, the car runs fine. Want’s to charge >$1000.00 to replace.
I still have a hard time believing this as my daughter was told by her dealer that her car (different) would die while driving because the tire pressures weren’t correct.