My gf's 98 Blazer has been acting real weird lately. Notably, the temp guage will show 220*F or more, but the engine will feel cold. I need to narrow it down between the gauge, the wiring, the temp sender; but I do not know what type of values I should be seeing from the sensor itself. Does anybody know the resistance to temp scale for these units?

The temp sender on a 98 blazer should be a two wire sensor in the drivers side head. Its actually the CTS sensor, the PCM uses that sensor for the fuel injection and the gauge. Your best bet would be to hook up a scan tool and see what the reading is. More than likely the gauge cluster is bad.

The gauge cluster being bad would be a good assumption, but the heat also never warms up, even on a 20min drive. I'm worried their is a deeper issue.