Technically on a diesel
engine there is no "ignition" circuit. There is usually a panel circuit that powers the panel, buzzers, instruments, glow plugs and starter.
Gord's distinction is important.
1 - Wired to the panel - will count hours any time the panel is powered
2 - Wired to a normally open oil pressure switch - Will count hours when there is oil pressure. Note the normal low oil pressure switch works opposite of this - i.e. it is open when there is oil pressure. You likely will have to add this switch if it doesn't exist.
There are switches wired to the alternator
output - parallel to the RPM
gauge. Because this is a variable output the counting is variable. The meter is usually calibrated at a specific RPM
- like cruising RPM.
I personally like option 2.