TRUCK SERVICE MANUAL TM 5-4210-230-14&P-1 BODIES AND CABS The refrigerant entering the evaporator expands and begins to vaporize.  Complete vaporization requires heat.  The refrigerant    absorbs    heat    from    the    air    surrounding    the evaporator coils and fins.  As heat is absorbed the refrigerant vaporizes. The    blower    draws    warm    air    from    the    cab    and circulates it past the evaporator where the heat is absorbed by the  refrigerant.    Air  exhausted  by  the  blower  is  cool  because the heat has been removed. From   the   evaporator,   the   refrigerant   is   drawn   by suction  into  the  compressor.    This  completes  the  refrigerant cycle  which  is  repeated  continuously  until  the  temperature  of the cab interior is lowered to the desired temperature. Cab     interior     temperature     is     controlled     by     the thermostatic   control   switch.      This   control   is   adjustable   to permit   the   vehicle   operator   to   select   the   temperature   level desired  in  the  cab.    The  blower  has  a  multi-speed  control which  permits  the  vehicle  operator  to  regulate  the  amount  of air circulation. OPERATION HEATING For proper cold weather operation the heat should be directed toward the floor level; therefore, close all instrument panel outlets and fully open both floor dump outlets. Adjust the temperature "HTR" (heat) lever as required to   give   the   desired   degree   of   heat.      The   full   right   "HOT" position    provides    the    maximum    heat.        Move    the    "AIR OUTLETS"  control  lever  either  to  the  full  cab  position  or  to any of the other three detented positions in order to obtain the desired   air   flow   distribution   between   cab   heat   and   defrost requirements.  For the maximum air flow, move the fan switch to the "HI" position.  The heater will also operate with the fan motor  in  the  "OFF"  position  due  to  the  ram  air  introduced during vehicle operation. Fig.  19   Heater Controls Set for Maximum Heat DEFROSTING To obtain maximum defrosting, move the "HTR" lever to the "HOT" position and place the "AIR OUTLETS" lever on "DEF".  Adjust the fan speed to provide the desired air flow. To clear system of humid air, operate blowers for 30 seconds  at  "HI"  speed  before  moving  the  "AIR  OUTLETS" lever to the "DEF" position.  This will minimize rapid fogging of glass,   which   can   occur   if   humid   air   is   blown   onto   a   cool windshield. To  improve  defroster  efficiency,  remove  ice  and/or snow from glass area. Fig.  20   Heater Controls Set for Defrosting FRESH AIR VENTILATION Fresh air enters the cab through either an independent  ventilation  system  or  through  the  heater  system itself. When  the  "VENT"  knob  is  turned  counterclockwise, air enters the cab from the hood scoop and flows through the instrument   panel   outlets   and   floor   dumps.      Except   for   the ventilation   mode,   the   knob   should   be   rotated   to   its   full clockwise  position  (vent  door  closed)  for  all  other  modes  of operation. To increase the quantity of air entering the cab in the ventilation  mode,  the  fan  may  be  used  to  power  ventilate. Adjust the fan speed and air outlets for the desired air flow. Fig.  21   Ventilator Control CTS-2731   Page 13 PRINTED IN UNITED STATES OF AMERICA