4-24. SERVICE  HOSE  BLEED  VALVE  (13,  Figure  4- 3).  The SERVICE HOSE BLEED VALVE relieves all air pressure from the Service Hose.  Opening the valve, by turning  counterclockwise,  will  vent  the  air  in  the  service hose to atmosphere. 4-25. MECHANICAL     FILTER     DRAIN     VALVE     (14, Figure 4-3).  The MECHANICAL FILTER DRAIN VALVE relieves all air pressure from the air compressor through the  mechanical    filter.    Opening  the  valve,  by  turning counterclockwise, will vent the air to atmosphere. 4-26. DEHYDRATOR BLEED VALVE (15, Figure 4-3). The    DEHYDRATOR    BLEED    VALVE    relieves    all    air pressure from the mechanical filter through to the service outlet.    Opening  the  valve,  by  turning  counterclockwise, will vent the air in this line to atmosphere. 4-27. COMPRESSOR UNLOADER SWITCH (16, Figure  4-3).    The  COMPRESSOR  UNLOADER  toggle switch    serves    two    functions.        When    placed    in    the UNLOAD position the compressor  oil  pressure  switch  is bypassed,  enabling  the  engine  to  be  started.    Since  no current passes through the normally open solenoid valve, the    compressor    can    be    started    unloaded.        When compressor   oil   pressure   develops   after   engaging   the clutch, the UNLOADER SWITCH is placed in the LOAD position and the compressor begins to load. 4-28. PUSHBUTTON GLO-PLUG SWITCH (17, Figure 4-4-3).  The PUSHBUTTON GLO-PLUG switch is pressed  to  energize  the  glow  plug  in  the  engine’s  air intake  when  ambient  air  is  minus ^50F  (-150C)  or  below. This will preheat the intake air prior to the air entering the combustion chambers. 4-29 INDICATOR  GLO-PLUG  (18,  Figure  4-3).    The INDICATOR GLO-PLUG (13) is connected in series with the  glo  plug  in  the  engine  air  intake.    When  the  PUSH- BUTTON  GLO-PLUG  is  pressed  the  INDICATOR  GLO- PLUG (17) will heat-up and glow red to indicate that the glow plug is operating. 4-30. Air Cooler Doors (19, Figure 4-3).  The air cooler doors (26 and 30) control the amount of air being drawn through  the  air  cooler  by  the  fan.    The  doors  may  be adjusted  by  closing,  opening,  and  sliding  to  either  side for cold weather operation. 4-31. EXTRA   FUEL   BUTTON   (20,   Figure   4-3).      By pulling  and  releasing  the  extra  fuel  button,  extra  fuel  is injected    into    the    engine    for    easier    starting    in    cold weather. 4-32. OPERATION INSTRUCTIONS. 4-33. PRIOR  TO  OPERATION.    Prior  to  operation  of the air compressor unit, perform the following procedure: CAUTION The unit should be located in an area as free   of   dust   and   dirt   as   possible.      A highly  contaminated  atmosphere  places abnormal   load   on   the   air   cleaners,   oil filters,    dehydrators,    and    compressor, which can result in increased maintenance. a. Transport the unit to work site and select a location  as  nearly  level  as  possible.    Out-of-level  shall not exceed 150 in any direction. b. Perform all inspection, checks, and services, as required, refer to table 5-2. c. Open air cooler doors. WARNING Under  no  circumstances  should  the  air purification cartridges in cylinders No.  3, 4   and   5   (23,   24,   25,   Figure   8-2)   be allowed   to   become   contaminated   with moisture.        If    in    doubt,    change    the cartridges. NOTE When    dehydrator    and    air    purification cartridges are changed, make certain to record hourmeter readings. Approximate   time   elapse   for   cartridge change  is  as  follows.    Actual  frequency of cartridge change depends on governing weather conditions  at  a  given time or place: Dehydrators: Approximate hours Cylinders 1 and 2 8 hrs. Air purification Cylinders Cylinders 3, 4, & 5 96 hrs. d. Check    the    number     of     hours     that     the cartridges in the dehydrators and filters have been used. This  is  done  by  comparing  the  time  of  their  installation with the HOUR-METER (1, Figure 4-2) indication.  If the next period of 4-6