Battery switch question

[Rich35758]

I'm attempting to troubleshoot an alternator problem on a newly acquired boat. Prior to removing the alternator, I set the battery selector to OFF, yet still had 12V on the wire running from the switch common to the alternator. Is there any reason why the switch would send juice to the alternator "+" terminal when the battery switch is OFF? Thanks!

[Tim B]

It shouldn't do that. Check the resistance between the switch contacts in each position.

Cheers,

Tim B.

[bntii]

Isolate the circuit- That is determine if the source of voltage you are seeing is perhaps getting to you test point by some other route. Is there a paralleling switch? Is there a hot wire going to the ignition circuit that is common to the alternator cable and a starting battery?
Also- are you testing the correct pole on the switch?
Also- is the boat wired so that the alternator cable is in fact not switched but is just a 'pass though' cable on the rotary switch lugs?
Many boats suffer from some small circuit which serves to parallel the house and starting cells quite by accident.

[CDK]

This depends on the concept the boat builder had in mind when the wiring was done. If you see the starter motor, alternator and battery as a unit (that is normally the case), setting the battery switch to off simply means that no current is drawn externally. The advantage is that you do not need a 500 Amp switch but can use a much smaller one.
In a twin engine installation you can switch between 2 or more batteries to allow starting from battery A or B or both. In that case you need a heavy switch that can handle the starting current.

[sr71]

That sounds tricky... I would check with a professional first.

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