ScanGauge D

If you buy an adapter on Ebay (inline 5, about $290), you can hook into the engine's ECM and see the engine hours and much, much more. Cummins has free software available to owners / operators called PowerSpec. It's fully supported and will even let you change parameters of the engine (but not the dangerous stuff like axle ratios and horsepower).

The software can even run on your laptop while you are traveling and give you tons of useful information -- all without voiding any warranty you may have.

Cummins also sells a separate hardware device called "Road Relay" that's a souped up version of the ScanGuage.

I agree-- I have and like the ScanGaugeD, but do not use that function, nor do I recall hours as one of the parameters that could be tracked.

Here is the list of what IS available:

http://www.scangauge.com/products/scangauged/

I have one and it does not record engine hours. I go by time and/or mileage per the engine manufacturers specs.