TM 9-2320-363-20-1 On-and-off  operation  of  the  expansion  valve  means  that the   receiver-drier   is   not   removing   moisture   from   the system.  These contaminants should cause the moisture indicator's element to turn white and then pink. REFRIGERANT COMPRESSOR Compressor problems usually show in one of four ways: abnormal   noise;   seizure;   leakage;   or   low   suction   and discharge pressures. Resonant  compressor  noises  are  not  causes  for  alarm; irregular   noise   or   rattles   are   likely   to   be   caused   by broken parts. EVAPORATOR The   evaporator   coils   are   basically   trouble-free   when airflow  over  the  fins  is  not  blocked.    External  or,  less often, internal blockages will cause low suction pressure as well as little or no cooling. If  a  leak  exists  in  the  system,  and  it  cannot  be  traced  to other   parts   or   fittings,   suspect   damage   to   one   of   the evaporator coils.  Notify your supervisor. CONDENSER The   condenser   is   usually   trouble-free.      Normally,   the temperature   of   the   condenser   outlet   line   is   noticeably cooler  than  the  inlet  line.    However,  when  road  debris (such  as  leaves  or  dirt  buildup)  cakes  up,  airflow  over the  condenser  fins  is  blocked;  air  is  not  able  to  absorb enough  heat  to  turn  the  hot  refrigerant  gas  into  a  liquid. High head pressures will result.  In these cases, carefully clean   off   the   outer   surfaces   of   the   condenser   with compressed air or a soap and water solution; be careful not to bend the fins. High  head  pressures  will  also  occur  if  the  condenser's tubing is abnormally bent, blocking the flow of refrigerant.  Frost will appear at the point where the flow is restricted. Less common internal blockages (bits of foreign material or metallic grit build-up) will stop the flow of refrigerant. A  quick  test  to  check  that  poor  system  performance  is caused  by  the  condenser  is  to  direct  a  spray  of  water onto  the  condenser  while  the  system  is  running.    If  the air conditioner cools better because of the assist provided by the water, it is a sign that the condenser is not working. When  troubleshooting  a  suspected  condenser  problem, remember   that   the   problem   may   be   caused   by   the radiator transferring high levels of heat to the condenser. THERMOSTATIC SWITCH IMPORTANT:    Before  troubleshooting  the  thermostatic switch, notify your supervisor to check for a full charge of refrigerant   in   the   system.      The   compressor   will   not operate,  or  will  cycle  too  often,  if  there  is  not  enough refrigerant in the system. Quick   or   delayed   cycling   of   the   compressor   may   be caused  by  a  thermostatic  switch  that  is  working,  but  is out  of  adjustment.    If,  after  doing  the  tests  below,  the switch  seems  to  be  out  of  adjustment,  replace  it  (the thermostatic switch cannot be recalibrated). 1.   Be    sure    the    compressor    clutch    is    operating correctly. 2.   Expose the evaporator coil. 3.   Start  the  engine.    Place  the  air  conditioner  control at  its  coldest  setting;  turn  on  the  air  conditioner and the fan. 4.   Place  an  accurate  thermometer  in  contact  with  a tube    on    the    evaporator    coil.        Be    sure    the thermometer  is  in  good  contact  with  the  tube,  or you will get a wrong reading. When the temperature drops below 31 to 36F (- 1 to     2C), the compressor clutch should disengage and remain this way until the temperature rises to 39 to 44F (4 to 7C). 5.   If    the    compressor    did    not    engage    when    the temperature  was  above  the  accepted  high  range, do the following test: 5.1      Connect  a  voltmeter  or  a  test  light  from  one of the terminals on the thermostatic switch to ground.      Repeat   this   test   with   the   other terminal on the switch. 3-92.2  Change 3