TRUCK SERVICE MANUAL TM 5-4210-230-14&P-1 BODIES AND CABS is of fin-tube construction.  Refrigerant entering the evaporator through the expansion valve vaporizes and absorbs heat from the walls of the evaporator tubes and fins which, in turn, draw heat from the air forced through the evaporator by the blower. This  absorbing  of  heat  by  the  refrigerant  results  in  a  flow  of cool air from the system.  Moisture from the air condensing on the  evaporator  is  drained  to  the  exterior  of  the  vehicle  via  a drain tube. Air Filter On     vehicles     equipped     with     air     conditioning,     a replaceable  air  filter  is  located  between  the  blower  and  the evaporator    to    keep    the    evaporator    fins    clean    to    assure efficient heat transfer. Fig.  12 Thermostatic Temperature Control Switch Thermostatic Temperature Control Switch The   thermostatic   temperature   control   switch   (Fig. 12)  is  located  in  the  bottom  of  the  combination  unit.    This switch performs these functions: 1. Turns air conditioning system "ON" and "OFF". 2. Engages   and   disengages   magnetic   clutch   to   start and  stop  operation  of  the  refrigerant  compressor  as needed to maintain desired cab cooling. 3. Overrides   the   engine's   radiator   shutter   control   to open  the  shutters  whenever  air  conditioning  system is turned on. 4. Supplies   current   to   the   engine   fan   drive   override switch (where equipped). The thermostatic switch is controlled through a cable connected    to    the    top    (A/C)    lever    of    the    control    panel assembly.    With  the  control  lever  in  the  "OFF"  position,  the magnetic  clutch  will  not  engage  (air  conditioning  system  will not operate). As the lever moves from the "OFF" position, two sets of contacts close.  The smaller set of contacts, which remain closed    through    all    remaining    lever    travel,    connect    feed terminal  "2"  to  terminal  "3"  supplying  current  to  the  shutter override   solenoid   valve   and   the   fan   drive   override   switch (where equipped).  (See wiring circuit diagram, Fig.  17.) The larger set of contacts connect feed terminal "2" to terminal "1" to supply current, via terminals "1" and "2" of the low pressure switch  relay,  to  engage  the  magnetic  clutch.    (See  Fig.    17.) Compressor cycling (engaging and disengaging the magnetic clutch) is accomplished by making and breaking the larger set of contacts. Cycling temperature setting is determined by position of the air conditioning control (A/C) lever.  Moving the lever to the  right  decreases  the  control  temperature  with  maximum cooling obtained when lever is in "COLD" position. Making and breaking of the larger set of contacts (to engage and disengage the magnetic clutch) is controlled by a bellows connected to a capillary tube filled with refrigerant. The   other   end   of   the   capillary   tube   is   located   in evaporator core where it senses evaporator fin temperature. When the fin temperature in the evaporator is higher than   the   control   temperature   setting,   the   refrigerant   in   the capillary tube expands moving the bellows, overcoming spring pressure    and    closes    the    contacts    to    engage    the    clutch (operate  the  compressor).    When  temperature  of  the  air  flow is  decreased  sufficiently  the  refrigerant  in  the  capillary  tube contracts  allowing  spring  pressure  to  open  the  contacts  and disengage the clutch. This compressor on-compressor off cycle repeats as often as necessary to maintain the desired cab temperature. Low Pressure Switch A   low   pressure   switch   (Fig.      13),   located   in   the evaporator   outlet   tube,   is   used   to   protect   the   refrigerant compressor from damage in the event of refrigerant loss or if ambient     temperature     is     too     low     to     provide     sufficient evaporation of refrigerant. CTS-2731   Page 8 PRINTED IN UNITED STATES OF AMERICA