The answer is to read the spec for a given switch before you buy.
Normally 'make before break' is preferred as otherwise it can be
damaging to switch between batteries with the engine running. The
alternator output voltage can spike and damage electronics such as ECUs
and radios. This is a potential danger with using  most of these
switches since selecting the off position with the engine running can
damage things too. The boat supply store where I bought one of these
switches years ago also sold a device that attached to the alternator
and limited the output voltage in case someone mistakenly disconnected
the batteries with the engine running. Some switches now have an
alternator disable function but I have not seen how these are meant to
work I don't like or see the need to disturb the VW factory wiring so a
bigass relay is my preference. Don't have to remember to set anything
and no damage can occur. I like being able to use both batteries for
starting so I use bigass wires too.

Mark

Mike Collum wrote:

>>Yes, Another D*** Battery Question
>>This question regards the use of the typical marine battery selector
>>switch.  These usually have four possible positions: Batt 1, Batt 2,
>>Both, or Off.
>>Off is simple: cuts ALL juice from the circuit, on startup ECU must go
>>through 'learning' period and clock must be reset.  But what happens
>>when switching between the other modes?  Are these switches 'break
>>before make' (as I suspect) or 'make before break'?  If the former,
>>
>>
>that
>
>
>>means that there will be a momentary interruption of power as the
>>
>>
>switch
>
>
>>is being thrown leading to the question, how long does power need to
>>
>>
>be
>
>
>>interrupted to cause the reset of the ECU?
>>
>>
>>
>
>Loren, I really like your question .... and would really like to know
>the answer.
>
>If you find out off the list ... please post it.
>
>Thanks,
>
>Mike (who is considering the marine battery switch)
>
>
>