TM 1-1510-225-10 2-90 c.   Bleed  Air  Valve  Switches.     The  bleed  air flow control unit shutoff valve and pneumatic bleed air shutoff valves are controlled by two switches, placarded   LEFT   and   RIGHT   BLEED   AIR   VALVES OPEN /  ENVIR  OFF /  INSTR  &  ENVIR  OFF, located on  the  copilot's  subpanel.    When  set  to  the   OPEN position,    both    the    environmental    flow    control    unit shutoff   valve   and   the   pneumatic   shutoff   valve   are open.      When   set   to   the    ENVIR   OFF   position,  the environmental flow control unit shutoff valve is closed, and  the  pneumatic  bleed  air  valve  is  open.    In  the INSTR  &  ENVIR  OFF  position,  both  are  closed.    For maximum  cooling  on  the  ground,  place  the  bleed  air valve switches in the  ENVIR OFF position. d.  Cabin Temperature Mode Selector Switch.   A switch on the copilot's subpanel, placarded  CABIN TEMP   MODE   OFF  /   AUTO  /   MAN   COOL   /   MAN HEAT,   controls   cockpit   and   cabin   heating   and   air conditioning.      When   the   cabin   temperature   mode- selector switch is set to the AUTO position, the heating and air conditioning systems are automatically controlled.        Control    signals    from    the    temperature control    box    are    transmitted    to    the    bleed    air    heat exchanger  bypass  valves.    Here,  the  bypass  valves controlling    the    amount    of    air    bypassing    the    heat exchangers regulate the temperature of the air flowing to  the  cabin.    When  the  temperature  of  the  cabin  air has   reached   the   temperature   setting   of   the   cabin temperature control rheostat, the automatic temperature control allows hot air to bypass the air -to- air exchangers, admitting hot air into the cabin.  When the   bypass   valves   are   in   the   fully   closed   position, allowing no air to bypass the heat exchangers, the air conditioner    begins    to    operate,    providing    additional cooling. e.   Cabin  Temperature  Control  Rheostat.    A control    knob    on    the    copilot's    subpanel,    placarded CABIN   TEMP   INCR,   provides   regulation   of   cabin temperature     when     the     cabin     temperature     mode- selector    switch    is    set    to    the    AUTO    position.        A temperature  sensing  unit  in  the  cabin,  in  conjunction with    the    setting    of    the    cabin    temperature    control rheostat,   initiates   a   heat   or   cool   command   to   the temperature controller for the desired cockpit or cabin compartment environment. f.    Manual   Temperature   Control   Switch.    A switch on the copilot's subpanel, placarded  MANUAL  TEMP   INCR   /   DECR,    controls    cockpit    and    cabin compartment  temperature  with  the  cabin  temperature mode  selector  switch  set  to  the   MAN  HEAT or   MAN HEAT or MAN COOL     position.          The     manual temperature   control   switch   controls   the  cockpit  and cabin  temperature  by  providing  a  means  of  manually changing  the  amount  that  the  bleed  air  bypass  valves are   opened.      To   increase   cabin   temperature,   the switch is held to the  INCR position.  To decrease cabin temperature, the switch is held to the DECR position.   Approximately   30   seconds   per   valve   is   required   to drive the bypass valves to the fully open or fully closed position.  Only one valve moves at a time. g.  Forward Vent Blower Switch.   The forward vent blower is controlled by a switch placarded  VENT BLOWER HIGH / LO / AUTO, located on the copilot's subpanel.  In the  AUTO position, the fan will run at low speed.  The forward vent blower will not operate when the  CABIN TEMP MODE selector switch is set to the OFF position. h.  Aft Vent Blower Switch.  The aft vent blower is  controlled  by  the  switch  placarded   AFT  BLOWER ON  /   OFF,  located  on  the  copilot's  subpanel.    The blower    operates    continuously    when    the    switch    is placed   in   the    ON   position   with   the   air   conditioner compressor operating. (1)   Automatic Heating Mode. 1.   BLEED    AIR    VALVES     Switches    – OPEN, LEFT and RIGHT. 2.   CABIN TEMP MODE Switch – AUTO. 3.   CABIN TEMP Rheostat – As required. 4.   CABIN, PILOT, COPILOT, and DEFROST AIR Knobs – As required. (2)   Manual Heating Mode. 1.   BLEED    AIR    VALVES     Switches    – OPEN, LEFT, and  RIGHT. 2.   CABIN  TEMP  MODE Switch   –  MAN HEAT. 3.   VENT BLOWER Switches – As required. 4.   MANUAL TEMP Switch – As required. 5.   CABIN, PILOT, COPILOT, and DEFROST AIR Knobs – As required. 2-67.  AIR CONDITIONING SYSTEM. a.   Description.          Cabin     air     conditioning     is provided by a refrigerant-gas, vapor-cycle refrigeration system,  Figure 2-26.    The  system  consists  of  a  belt- driven   engine-mounted  compressor,  installed  on  the #2 engine accessory section, refrigerant plumbing, N₁ speed     switch,     high     and     low     pressure     protection