How does my battery selector switch work

[witchy woman]

I have a basic Guest battery selector switch on my boat. Simple 1/2/both/off. Has always worked great and as far as I can tell, it still does. But I'm confused to the point that I pulled it all the way off the boat and bench tested it with the multimeter. With no batteries hooked to it, it functions as it should. Continuity to the feed post from either position 1 OR 2 OR both. No continuity from 1 to feed when in position 2 and vice versa. Works great. On the boat and hooked up like this: Battery 1 to post one. Battery 2 to post 2. Feed to starter and to helm (two cables). Both bilge pumps and trim pump are wired to post 2 at the switch which means they are wired directly to battery 2 no matter what position the switch is in. No common ground between the batteries. Both battery grounds are wired to engine ground. Now my confusion. With negative probe connected to engine ground, I get battery voltage on the meter no matter what position the switch is in or which battery I probe. In other words, negative probe to engine ground, positive probe to positive terminal on battery, I read battery voltage regardless of which position the selector switch is in, including OFF. True for both batteries. BUT, with switch in OFF position, I get no power to helm. No accessories work, which is the way it should be. Can anyone explain this to me? Where is the connection I'm missing?

 

[Scott Danforth]

With negative probe connected to engine ground, I get battery voltage on the meter no matter what position the switch is in or which battery I probe. In other words, negative probe to engine ground, positive probe to positive terminal on battery, ?

your reading battery voltage of the battery with the probe on the battery. move the positive probe to the engine starter and you will be reading system voltage.

 

[bruceb58]

With negative probe connected to engine ground, I get battery voltage on the meter no matter what position the switch is in or which battery I probe. In other words, negative probe to engine ground, positive probe to positive terminal on battery, I read battery voltage regardless of which position the selector switch is in, including OFF.

You measure battery voltage because you are not switching the negative. The way you are measuring is equivalent of measuring directly across the battery.

 

[Silvertip]

Measure from the COM terminal to engine ground in each switch position. Should read: Off = 0. 1, 2,and both should read 12.6 volts.

 

[gm280]

Here is a little pictorial diagram of how the battery switch is setup. Seem you were reading from the common terminal to the ground. So in every position but the "off" position you were reading 12.6 volts. Hope this helps you see it a little better. :noidea: