'69 220 electrical....Help!

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!

[stevebfl]

I am having trouble putting all your symptoms together. For the alternator light to be on with the key off you almost have to have a bad alternator, BUT! the alternator has nothing to do with starting the car, SO, maybe you have an ignition switch problem.

Its possible that you have a ground problem either with the alternator or engine also. Your running problem with accessories and a good battery almost confirms a high resistance circuit in either power or ground. Get a voltmeter and take voltage drops acroos the circuits you are working with.

Thanks for the reply! The ignition switch had crossed my mind as well. Is it also a possiblity that it is some sort of nuetral safety switch that is bad?, or does a '69 column shift manual even have this item?

[stevebfl]

No NS switch on column shift manual trans cars. Besides it would only affect starting; your symptom package has the circuit opening with a load. It would not be in the circuit after starting. I think a visual inspection of the ignition switch is called for. The back part is crimped into the pot metal front. If it is loose it should be replaced. Based upon a list of $59 for ign switch # 116 462 00 93 you can probably get one for 30-40 bucks.

Thanks Steve, I'll give it a look see, It sure can't do anything but help!