its because with the gas engine computers, the sensors and signals from those sensors are similar to all gm gauges(oil pressure, coolant temp, vss in some applications), and can be wired straight up. Or, the computer is compatable where you can change the signal to a different pulse(tach only). But with the diesels, they work on a completely different signal, voltage, hertz whatever it is, cant think of it right now. Bottom line, you cant just whip out a autometer catalog and get the right gauges because they dont make any that are compatable to my knowledge. But......it might be possible to take apart the late model gm cluster and mod them to fit into an older style cluster like we have in the older gm trucks.......just a thought.
Regards,
Remington