battery cutoff on negative wire

I've studied the M939-series wiring diagram and P2P program some more, and it appears that the 12V tap is only used to power the cab heater fan when in low speed mode. I think that a single battery disconnect switch between the negative battery terminals and the chassis should work just fine in this application.

Would something like this be safe to use? If I don’t drive the truck for a couple of weeks the batteries are dead and I want to take care of that before installing my new batteries.

https://www.amazon.com/dp/B01IH4JTSY/ref=sspa_mw_detail_0?psc=1

Thank you advance for your help.

I had one of those on one of my trucks and never got it to work right as far as making good contact, seems I had to really crank down on it.

Bill W said:

I had one of those on one of my trucks and never got it to work right as far as making good contact, seems I had to really crank down on it.

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Thank you for your reply. Did you find a better solution? I plan on re-wiring my truck eventually and doing it right with a proper switch but there are other issues I need to address first.

Sorry
Somehow I missed following up on this. I went with a Batt cut off switch

cat does it that way in there equipment I don't see a problem

Planning to this mod to my truck over the July 4th week.

There's one slight problem with a negative ground disconnect. Of course 12v and 24v pairs of batteries share the ground, but disconnecting the ground still leaves the 12v line and the 24 v lines connected to everything on the truck. That means when you start wrenching on the reverse polarity protector, for one example, you still have the full current of the 12v to 24v 'upper' pair of batteries 'looking at you. That is not safe.

IMHO the only safe way to have a safe battery disconnect on our dual voltage systems is to interrupt both the 12v positive line and the 24v positive line, meaning two switches, or a DPST switch to do both at once.