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 D.  When the selector knob is moved to AUTO the third switch closes and a circuit is completed to the temperature regulator. Setting the knob pointer for a particular cabin temperature adjusts a potentiometer fixed to the knob shaft. This potentiometer serves as a reference resistance in the regulator temperature control bridge. The cabin temperature sensor provides the resistance in the other leg of the bridge. If cabin temperature is already the same as that asked for by the selector, the controller will prevent any current passing on to the mix valve. At a cabin temperature other than that selected the temperature sensor will provide a resistance either higher or lower in the other leg of the control bridge. As a result the controller will move the mix valve either toward hot or cold, as required, to bring cabin temperature to the required air temperature to slow down changes requested by the controller and prevent duct overheat. The controller moves the mix valve so that cabin temperature changes without sudden blasts of cold or hot air and without raising duct temperature above limits. The same system overheat protection described under manual control is in effect during automatic control.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

TEMPERATURE CONTROLLER TEMP SENSOR
Right Temperature Control Circuit (Left Control Similar) Figure 4
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 CONDITIONED AIR TEMPERATURE CONTROL SYSTEM -
__________________________________________ TROUBLE SHOOTING
________________
 1.  General
_______
 A.  Temperature control trouble shooting provides a method for isolating faulty regulating components of automatic temperature control and failure or malfunctions in the manual temperature control system.
 B.  Each component used during manual control is also used during automatic control. It is practical to check for problems in the manual system, therefore, before checking the automatic system.
 C.  Trouble lights and a temperature indication selector system on the overhead panel provides assistance in trouble shooting the temperature control system. For example, the mix valve, whose position is set by temperature control, governs cabin air temperature. The 190-degree duct overheat thermal switch cuts out temperature control and drives the valve to full cold when a duct overheat condition exists. Should the thermal switch fail closed, temperature control will be lost. At the same time, the duct overheat light will come on. Since the duct overheat switch and supply duct temperature bulb are located in the same duct, a supply duct temperature selection when the overheat light comes on will confirm that a duct overheat condition exists or will call attention to a faulty duct overheat circuit.
 D.  Lack of temperature control may be caused by malfunction in the mix valve, duct overheat relay, pack overheat relays, or in either of the relay circuits as well as in the automatic temperature control regulating components. Accurate cabin temperature changes may not be provided by the cabin temperature sensor and bulb if airflow is restricted by an excessively dirty air grille or duct and outlet tubing.
 E.  Trouble shooting, therefore, consists of two steps. First check operation of manual temperature control then check the automatic system. If it is determined by watching airplane instruments that manual control is functioning properly only the regulating portion of automatic control need be checked.
 2.  Equipment and Materials
_______________________
 A.  Ohmmeter
 CAUTION: AN OHMMETER CONTINUITY CHECK WILL DAMAGE THE FOLLOWING
_______
 TEMPERATURE SENSORS BEYOND REPAIR.

 (1)
Cabin Temperature Sensor

 (2)
Passenger Cabin Duct Anticipator

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