The primary (zone)temperature indication system is used to display the compartment and duct temperatures for the flight deck zone, forward zone, and aft zone.
The temperature indication system for each zone consists of a compartment temperature indicator, a compartment temperature bulb, a duct temperature bulb.
The compartment temperature indicator is located in the flight compartment near the air conditioning control panel, and has three liquid-crystal-display (LCD) readouts to display the temperature (1 to 99-degrees) in each of the three zones.
Twenty-eight volt dc power operates the compartment temperature indicator. The indicator sends a reference voltage through each compartment temperature bulb. The resistance through each temperature bulb varies with temperature and the signal then returns to the indicator. Thus the voltage level returning to the indicator produces the temperature readout corresponding to the actual compartment temperature.
Following the schematic and wiring diagram of the unit
The following picture shows the temp indicator with power still on, but no signal from the sensors.
In the following picture I simulated a sensor using a 100 ohm resistor for each compartement which gives a readout of 22°Celsius
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