The ubiquitous 12 VDC hour meters used in the light aircraft market are
'Hobbs' hour meters.

See: http://content.honeywell.com/sensing...scorp/cat1.asp

Available at Aircraft Spruce, Sporty's etc.

My Vetus (marine diesel) control panel came with a standard Hobbs hour
meter; maybe it's the same as Volvo...?

MW