My son has a '69 220 4dr sedan with a problem I thought would be as easy as replacing the alternator, but it has remained elusive to me. When the car is off, the alt/gen light stays on, then when you turn the key on or to start the light goes off (no spark to engine either) and the fuel gauge also goes to empty no matter how much is in the tank.
Only way to start the car is directly from battery disconnecting the alternator (either from the 3 wire harness or the red cable @ the rear of the unit). When car is running (and alt is connected), turning on anything electrical kills the engine, i.e. brake/backup lights, dome light, heater, etc. We have replaced the alternator twice and the voltage regulator once,(which is external on the wheel well) but fear there is something else that is killing those parts, so until we fix whatever it is I'm afraid we will just keep throwing cash away on new parts, an expensive process of elimination! We live up in the mountains 45mi west of Denver, so driving the car to a shop on battery power is risky and out of the question. What can I do to troubleshoot this? I have also read that on other Merc's it is necessary to polarize the alternator after it is installed and prior to starting the engine. Is this so? I have not read this before in the repair manuals I own. If this is true, is this part of my problem and how is this done?, and could I have just blow another alternator?!

Thanks for any Help!