"Now what I don't understand is how could running them in ALL effect the Alternator 12vdc is 12vdc no matter where the source I would think that one may shutdown (thinking the batt was charged) and let the other do the work but I can't see how it could hurt it. I would think that you would have to have a high current overcharge situation to smoke the alternator."

This is a question I had asked in the PQ forum we were talking about the Perko batt switches. I am having a Alternator rebuilt and don't want to keep frying it. But I just don't understand the logic if a Alternator has a diode to prevent back feeds why would running the switches in ALL position hurt anything? Or is this just a Wives Tale.
I also have read that it could cause problems on a EFI system

Anyone have any facts??

Must be some of that voodoo stuff that Packinair was talking about

Running the Batt switch in "ALL" position won't hurt the alternator. What will hurt it is if you, by mistake, switch it to off while the engine is running. That'll bow diodes. The batt switch should be the "make before break" type. What that means is that when switching from one battery to the other both batteries will be briefly connected before disconnecting the first battery. This way there's no loss of voltage while switching.

I dont even have switches in mine Scott, all 3 are hooked together through an isolator. I dont see how running with the switch in the all position would hurt an alternator either. Used to run them that way in the formula.

If twins, running in the all position for both engines can cause problems for the alternator regulators. Basically what you have done is hotwire the two together. One regulator will fight the other. I run three batteries. "1" on each side is the respective battery. "2" is the center. I run one side on "1" and the other on "both" to assure I don't hot wire these. You also can buy a battery isolator.

If I am correct the diodes in the alternator will isolate them from each other. A battery isolator does just that it Isolates the batterys from each other. Sorry I need more info backing this up before I worry about not running them in ALL not that it much matters if I don't.

I found out today that my alternator was o.k. I have another problem possible breaker or fuse on the output ??? possible bad guage or wireing to it?? I will find out this weekend with a volt meter, a load/charge tester and some test clips. This and a oil pressure guage have been a problem for over a year now I must find it soon
BTW I have been running mine in ALL for three years.

I also think (not too sure) that the diodes we are talking about are used as rectifiers changing A.C. to D.C. so do they still isolate the input or is there another one for that.

I can't believe out of all the motor, electrical,engine, engineers,doctors,attorneys whatever no one on here used to own a alternator shop

Scott, 4 years of Electrical Engineering and with 3 years of shipboard systems experience count? I thought the same way as you until I fried 2 alternators. They guy at the alternator shop laughed me when I said I ran on "both" all the time. Maybe he was just giving me a hard time but I've never hard wired 2 alternators together again.

By the way if your at Kellys Island at the Casino, I be glad to talk in person (I'll buy you a beer).

I never go to the Casino
I respect what you have accomplished but even you admit I may be on the right track. It could of been other problems that knocked out your alternators. I need a schematic shoved in my face that proves this
All of this aside I will be happy to have a cold one with you and by all means thanks for the replies I am not trying to say you are wrong I just need to (want to) totally understand why he said that they said that we said that that must be right

WARNING, If anyone buys wire terminals from Napa/Belden. Look at them very closely, They are marketing a line of terminals that are not supposed to be crimped, just stick the wire in and heat shrink. Well, I used one on an alternator output wire earlier this winter and when I fired the engine I fought no charge all day Sunday until I finally isolated it to that terminal. Looked great, read 12.8 volts and zero ohms until I started the engine and it started to pass current and then that loose wire in the "no clamp" clamp broke the circuit. What a pile of trash those things are. I took them all back to Napa at least expecting my money back and the guy just says 'Oh well". Dont know if this could have anything to do with your issue Scott but I had to vent. BTW. I also have an EE degree and 12 years as a Merchant Marine Chief Engineer.