[Blue-S]

I realized that I do have a scan tool that is capable of communicating with the DME in SAE J1978 "Generic scan tool" mode. One of the data lines available in the Generic scan tool mode is ECT, so off I went to compare temp gauge readings, HVAC diagnostic mode coolant temp readings and scan tool ECT readings.

Key on, engine off:
HVAC = 31 deg C (88 deg F), Scan Tool = 91 deg F

Gauge needle exactly on the "tic" above the "8" in "180':
HVAC = 78 deg C (172 deg F), Tool = 181 deg F

10 miles on highway at 75 mph indicated, 83 deg F ambient temp & gauge needle straight up between "8" and "0":
HVAC = 91 deg C (196 deg F), Tool = 194 deg F

Idling:
HVAC = 100 deg C (212 deg F), Tool = 208 deg F
HVAC = 101 deg C (214 deg F), Tool = 210 deg F
HVAC = 102 deg C (216 deg F), Tool = 212 deg F (gauge needle near right edge of "0" in the "180" label)

Given these results, I can only conclude that (on my car, anyway) the HVAC panel diagnostic mode is reasonably accurate at typical cruise temps, and that when the gauge needle is "straight up" between the "8" and the "0" my coolant temp is 190 - 195 deg F. It is a bit odd that the HVAC diagnostic display ECT is cooler than the scan tool displayed ECT until approximately 190 degrees F coolant temp, then the situation reverses -- the HVAC display reads slightly higher than the scan tool reported ECT. The Bentley manual hints that the HVAC panel receives engine coolant temp from the instrument cluster, so maybe the cluster is re-scaling the ECT data to effect a slight change...or maybe not.

These results make a lot of sense to me, because a 10-11 degree C (18-20 deg F) coolant temperature gain through the engine is quite plausible under the conditions noted above. If the inlet side thermostat is 80 deg C, and the coolant exiting the engine is 90-91 deg C, then I'm quite happy with that.

BTW -- I noticed an anomaly: at highway speed, my cluster was showing 83 deg F ambient air temp but my Intake Air Temp (on the scan tool) was showing 66 deg F! Maybe this gives me a wee bit more spark advance than I otherwise would have...