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In the middle of fitting a second battery so I bought a new change over switch,when I removed the old one inside the box that the switch is mounted in there is a diode it's all wired up,but what is it for.



Cheers

Taza

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1997 Fourwinns 205 Sundowner
Volvo Penta 5.7 GLI

If my guess is right. Its to charge the 2nd battery and the diode will act as a one way valve. It will let the charge voltage in but no voltage out.

It appears to be a low amperage circuit (likely ~10A max) to give a source of battery voltage from either bank. Most likely for your stereo memory supply or bilge pump in order to power the pump from either battery, but not be able to backfeed one battery to the other. Make sure there's a marine rating or "ignition protected" stamp of approval on this diode, because as we all know, if it doesn't have one, your boat will blow up.

guys
The boat has only one battery and the selector switch is for a single battery,the selector switch is mounted inside a metal black box also fitted is a fuse holder for the bilge pump,its the origanal as the wiring is part of the loome and is on plug n sockets.

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1997 Fourwinns 205 Sundowner
Volvo Penta 5.7 GLI

It doesn't look OEM, with the clear plastic part and the double sided tape holding it on ( also what are the 2 holes for ?). My 1999 Sundowner, with one battery didn't have that diode. The later dual battery set up had 2 diodes connecting the two battery terminals at the switch together, that provide power to the bilge pump (so always powered by both batteries, but one duff battery could not flatten the other).

Maybe what you have is a remnant of a non OEM dual battery set up ( but not using a dual battery selector switch) that was removed before you got the boat ; maybe connecting a leisure type battery not used for starting, or a small 12v domestic alarm type battery for radio memory retention; the diode would still allow them to be charged. I have such a diode set up but using a small domestic alarm 12V battery at the helm for my chartplotter, which is very sensitive to the very voltage drop when the engine is started . The small battery maintains the voltage to the chartplotter when the engine is started but the starter cannot draw any current from it because of the diode. Despite the voltage drop across the diode, the small battery maintains a pretty high level of charge from the alternator.

If there is only one battery, why would it be a "selector switch"... now I'm real confused! (not hard to do)

On a single battery set up, where FW have fitted a selector switch either as an extra or standard, (that depend on the model), you can select "on or "off" !!

So then, it is not a selector switch, but just a plain old switch.
Now I know why I was confused. Thanks!

Now that I know that, I look at this a little differently. It would appear to me that this has nothing to do with the battery lead whatsoever. Neither of the leads going to the diode come from the either terminal on the switch. It would appear there's a panel mount fuse that the leads are tie-wrapped to, which may or may not have anything to do with the circuit. I'd trace the wires back to where they're connected for clues as to what the mystery diode is for.

It's defo OEM fitted,found another drawing in the Fourwinns manual and it's all to do with the bilge pump.

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1997 Fourwinns 205 Sundowner
Volvo Penta 5.7 GLI

In that case, my guess would be that it is to make sure power is to the bilge pump regardless of whether or not the main switch is on. To do that, they have to isolate the circuit through this diode so that if the main switch is off and the bilge pump switch is on, the power is not fed back into the de-energized side of the battery switch. So, likely the current runs from the bilge pump switch through this diode, but there's also a fused path from the battery side of the disconnect switch directly to the bilge pump circuit (likely to the float switch).

Picture from Fourwinns manual but this is or dual battery you can see the diode.
At least I know what it's for now.

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1997 Fourwinns 205 Sundowner
Volvo Penta 5.7 GLI