TM 5-6115-593-12 TO 35C2-3-463-1 instrument reaches approximately 120F     5 (50°C     2). (d) As the water begins to cool, note that heater automatically   comes   on   at   approximately 100°F    5 (38C    2). (e) If  switch  fails  to  meet  above  test,  it  should be replaced. (f) Open both shutoff cocks.' (2) Heater  H101,  H102  and  H103  (18  and  53,  figure 4-30). (a) Place  EXTERNAL  POWER  BREAKER  and PREHEAT switch to the off position.  (figure 2-10). (b) Locate terminal board within heater electrical   enclosure   (40   figure   4-30)   and remove    wires    X173H    and    X174H    from terminal  points  6  and  7.    Isolate  12  gauge and 14 gauge wires from terminal point 7. (c) Using  an  ohmmeter,  check  resistance  be- tween  terminal  point  4  and  12  gauge  wire isolated  from  terminal  point  7.    Resistance should   be   8.0   ohms.   Resistance   between terminal  point  4  and  6  should  also  be  8.0 ohms.  Resistance between terminal point 4 and 14 gauge wire isolated from terminal point   7   should   be   50   ohms.      (Refer   to Engine Accessories Wiring Diagram FO-11) (d) If the above tests are not met, one or more of   the   heater   elements   are   defective   and must be replaced. c. Replacement. CAUTION Shut   off   engine/generator   set   and   allow sufficient  time  for  engine  coolant  to  cool before  handling  hoses  and  fittings.    Use suitable    containers    to    prevent    loss    of coolant.      Switch   off   EXTERNAL   POWER BREAKER. (1) Close  both  shutoff  cocks  (4),  drain  coolant from     preheat     system     by     disconnecting heater hose going to highest point on engine and carefully lowering into suitable container. (2) To remove coolant heater assembly, remove  the  wires  going  to  heater  element (18)   and   (53).      Remove   heater   coolant hoses. Remove four bolts, nuts and washers (34). (3) To   remove   oil   heater,   oil   must   first   be drained  from  oil  sump.    Remove  bell  cap (38)   then   disconnect   wires   from   end   of heater element. Refill engine crankcase with oil once heating element has been replaced. (4) Open    both    shutoff    cocks    when    coolant heater   repair   or   replacement   have   been accomplished. 4-83