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Originally Posted by Blue-S
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.
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Based upon the 986/987 wiring schematics, I would have suspected that would be the case, which unfortunately means that the inaccuracy and non linearity of the dash gauge is transposed onto the HVAC display. Standard diagnostics for faulty dash displays always start with measuring the same data via the OBD II port, which is an indication that the port sees the sensors directly without the influence of the dash display. This is why I tend to trust my diagnostic instrumentation over what the dash is telling me…………….
"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..."
Be aware that these measurements come from two different places, one is in the front grill, the other is inside the MAF; so cooling effects of the relative air movement as well as exposure to ambient sunlight are different…….