Unsavoury language and voltage drop

How many amps are you drawing?

Is there any heat build up in the joints or the switch.

Stab in the dark but are you talking about one of those useless red key isolation switches by any chance?
Cheers Craig...............

And the answer to Notso's key question? How much current was flowing through the switch when you took your measurements?

A better way to measure the voltage drop across the switch is to connect the meter directly across the switch, not measure two voltages to earth. The voltage directly across the switch will depend on the current going through it, hence Notso's question. With a few amps running through it and by measuring right at the switch then moving progressively outward you should be able to narrow it down to the switch itself, or the crimped (?) terminals, the connecting wire, battery terminals..... I'd check for any voltage between the battery terminal post and the terminal that's clamped onto it - might look like a good tight metal to metal bond, but can corrode in there and cause losses. With a few amps flowing you shouldn't see more than a few millivolts drop anywhere. (Of course with no amps flowing you should see no voltage drops at all.)

Cheers

John

Good timing. I was about to set off yesterday and noticed the voltmeter reading low on the second battery which has one of those red isolator switches on it. Same 0.9 voltage drop across it, so I simply bypassed the switch and back to full power. This must have been a gradual decline in its operation over the past few months as I monitor the voltage regularly.
I also need to replace it with better quality unit. Any suggestions appreciated.
Thanks,
Michael