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Have the diagram for two alternator three battery system. Easy to understand. What I do not understand is how to make it work with my Steyrs. My engines start, charge and run with just one cable going from battery to on/off switch straight to starter. The wire diagram for isolator has included wires from alternator, which I do not have, to the switch/isolator. I know this is much simpler than I am making it out to be but don't understand how I can crank the engines through the isolator. Just trying to install the isolator in the existing set up so I can charge the third battery. What am I missing besides logic here?

Kevin,
Do not hook the main starting cable to the isolator. It will not work.

Take a wire size big enough to handle the alt output. Attach it to the starter battery stud and then to the alt terminal on the isolator.

Do this for each engine. Then take a wire from the isolator, same size and run to each battery.

Bruce,
Ahhh, grashoppa understand now. I went back to the boat before I got your response to pontificate my wiring dilema. I was on the right track and did not think I would have to unbolt any of the existing set up. Instead just add to it, only I was not sure how to tap into it. So in the end, out of the three output studs on the isolator, two of them will be vacant due to the simplicity of the engine wiring. Thanks for help.

Kevin,
You could leave the two wires off but if the stb alt quits, the only way the port alt would charge the stb battery is to have a paralell switch.

Hook all the wires up.