On Mon, 2 Jul 2007 16:35:20 -0400, "Peter R."
wrote in :

I was taught that all the newer model C172s' Hobbs are wired to the
master,

Usually there's an oil pressure switch in series with the Hobbs meter,
so that it is only energized when the engine is running. I have no
idea if that is true of the newer aircraft, but it should be.