I wrote earlier (see below) w/regard to my son's 69 220/8 electrical gremlin. And have some additional info as it pertains to the ongoing diagnosis.
Original Post 4/20/01:
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 to the starter). 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?!
Since that post I have determined that the ignition switch is not at fault and that by disconnecting the red wire from the starter selenoid, (with the other end connected to the alt) also results in turning the alt. light out on the dash.
My only other thought is that it may be a bad starter/selenoid, as I recall now that this whole thing started as an intermittent problem, where you had to try starting the car several times before the charge light came on when you turned on the ignition and there was a spark to the engine.
Any other thoughts?