system    air    pressure.        Solenoid    valve    coil    (10)    is deenergized,    stopping    the    air    compressor    unit    from loading.        When    system    air    pressure    decreases    to approximately    3000    psig    (17225    kPa),    (10)    closes, energizing  (10)  and  allowing  the  air  compressor  unit  to load. 4-10. In   the   event   that   the   engine   should   overheat, cylinder  head  temperature  switch  (11)  will  close.    This will   cause   safety   relay   (10)   to   energize.      When   (10) energizes,  fuel  solenoid  valve  (13)  deenergizes.    Fuel flow   to   the   engine   ceases   and   the   engine   stops.      If engine oil pressure should fall below 8 ± 1 psig (55 ± 7 kPa), switch (12) will close.  This will cause safety relay (10) to energize.    If  compressor  oil  pressure  should  fall below 8 ± 1 psig (55 ± 7 kPa), switch (8) will open.  With the  LOAD/UNLOAD  switch  (7)  in  the  LOAD  position, unloader    valve    (10)    will    deenergize.        When    (10) deenergizes   the   air   compressor   unit   will   continue   to operate but will not load the air receiver. 4-11. OPERATING  CONTROLS  AND  INDICATORS. All controls and indicators necessary for operation of the air  compressor  unit  are  located  either  on  the  control panel or instrument panel.  Name plates are provided to identify  all  controls  and  indicators.    Identification  and function of each control and indicator is as follows (see Figure 42). 4-12. HOURMETER  (1,  Figure  4-2).    The  hourmeter indicates the total number of hours that the compressor has   been   operated.      A   pressure   switch   closes   the electrical circuit to the hourmeter when the compression cycle  begins.  Records  of  hourmeter  indications  will  be used  to  initiate  maintenance  schedules  for  performing service and maintenance functions. 4-13. AMMETER    (2,    Figure    42).        The    ammeter indicates  the  output  of  the  alternator  which  is  used  to change   the   battery.      Under   normal   operation,   the ammeter  will  indicate  a  changing  rate  of  the  alternator varying from zero (0) to a plus (+) 40 amps. 4-14. COMPRESSOR OUTPUT INDICATOR (3, Figure 4-2).  The COMPRESSOR OUTPUT INDICATOR will indicate the PSIG of the compressor output through the    mechanical    filter    (18,    Figure    8-2)    immediately downstream from the 4th stage of the compressor. 4-15. SERVICE  PRESSURE  GAUGE  (4,  Figure  4-2). This guage will give the  PSIG  pressure  available  at  the compressor unit service outlet (7). 4-16. COMPRESSOR   OIL   PRESSURE   GAUGE   (5, Figure   4-2).      The   COMPRESSOR   OIL   PRESSURE gauge indicates air compressor lubricating oil pressure in pounds per square inch (PSI).  Normal air compressor oil pressure is 25 ± 5 psi (173  ±  35  kPa).    If  the  compressor  oil  pressure  should drop  below  8  psi  +  1  psi  (55  kPa  ±  6.89  kPa)  while operating, the compressor will shut down and the cause of  low  oil  pressure  should  be  determined.      Refer   to troubleshooting, Table 5-4. 4-17. ENGINE OIL PRESSURE GAUGE (6, Figure 4- 2).    The  ENGINE  OIL  PRESSURE  gauge  indicates  the oil  pressure  present  in  the  engine  lubricating  system. Normal  operating  pressure  range  is  60  ±  10  psi  (414  ± 689 kPa).  If engine oil pressure should drop below 8 psi (55 KPa) while operating the engine will shut down. 4-18. SERVICE  AIR  OUTLET  (7,  Figure  4-2).    The SERVICE AIR OUTLET is the primary outlet.  A servicing hose  is  attached  to  the  outlet  and  storage  air  tanks  as needed. 4-19. START  SWITCH  (8,  Figure  4-2).    The  diesel engine  START  SWITCH  is  a  spring-loaded  switch  that controls  the  flow  of  the  electrical  current  to  the  starting motor.      When   the   START   SWITCH   is   pressed   it completes the circuit to the starting motor  which  in  turn completes  a  path  to  ground  for  the  battery  allowing  the starting motor to run.  When the start switch is released, it returns to the off position stopping the starting motor. 4-20. MASTER    SWITCH    (9,    Figure    4.2).        The MASTER  SWITCH  is  a  disconnect  switch  for  the  air compressor unit's electrical system. 4-21. THROTTLE  CONTROL  (10,  Figure  4-2).    The ENGINE    THROTTLE    is    connected    to    the    engine governor,  which  controls  the  engine  speed  from  idle  to 2,000   rpm   (2,000   r/min).      Pulling   the   THROTTLE CONTROL    out    raises    the    engine    speed.        Turn THROTTLE  CONTROL  handle  clockwise  to  hold  and lock engine speed.  Turning the handle counterclockwise unlocks  the  handle.    Push  handle  in  to  reduce  engine speed. 4-22. CLUTCH  OPERATING  ROD  (11,  Figure  4-3). The  clutch  which  connects  the  diesel  engine  to  the  air compressor, is operated by the CLUTCH operating rod. The clutch is disengaged when the clutch handle is away from   the   instrument   panel.      Push   clutch   rod   handle toward instrument panel to engage the clutch. 4-23. SERVICE    VALVE    (12,    Figure    4-2).        The SERVICE   VALVE   controls   the   flow   of   breathable   air through  the  SERVICE    OUTLET    (7).    The  SERVICE VAI.VE (1 2) should be fully open when a receiving unit is being   charged,   and   closed   when   the   compressor   is building up its working pressure. 4-4