Electronic Climate Control (and Coolant Temperature Sensor)

[Marc Boucher]

Looking further into various temperature sensors, I noticed that the electronic climate control has a number of outputs for temperature readings when set into diagnostic mode.   One of these, the value on 'channel 06' displays the following 'value' (according to Bentley): "Coolant temp..phased in production 255 - open circuit."

Sure enough, mine reads 255.  

I'm wondering...on the cars that DID have the coolant temperature already phased in, from which sensor did they get the coolant temperature?  Was an additional sensor added to accommodate this, or did they read the value from the MFTS, or from the ECU coolant temperature sensor?