TM  9-2350-256-34-1 b. Starting System. The  generator,  which  is  coupled to the engine by a drive chain, also serves the dual role as a starter. When the preheat switch is held in the closed (ON) position, and the generator switch is open (OFF), one side of the preheat relay energizing coil on the  APU  engine  is  grounded  through  the  preheat switch and a set of APU generator switching relay con- tacts. This causes the preheat relay contacts to close since the opposite side of the energizing coil is con- nected directly to the vehicle bus voltage, Power is then supplied through the closed preheat relay con- tacts  to  the  preheater,  the  APU  generator/starter field winding, and to one side of the starter switch cir- cuit. When the starter switch is closed, it completes the APU starter relay coil circuit, and bus power is supplied to the APU generator/starter armature cir- cuit  through  these  relay  contacts,  thus  motorzing  the generator/starter  to  start  the  engine.  Therefore,  both the preheat and starter switches must be closed (ON), and the generator switch must be open (OFF), in order to start the API-J. Since the preheat relay energizing coil ground circuit is passed through the set of APU switching  relay  contacts  (as  mentioned  above),  the  pre- heat relay cannot activate if the generator switch is on.  The  reason  for  this  circuit  arrangement  is  to  pre- vent  vehicle  bus  voltage  from  being  applied  to  the  volt- age regulator field through the preheat relay contacts, thus  damaging  the  regulator  when  the  generator switch  is  on  (field  circuit  from  generator  to  regulator closed)  and  the  preheat  switch  is  activated  (bus  voltage applies  to  generator  field  winding). c. Lubrication.  A gear-type pump draws oil from the crankcase and delivers it through a replaceable oil filter to the engine. Normal oil pressure should be 25 psi or higher under normal operating conditions. An oil pressure switch will activate a low oil pressure warning light on the auxiliary power unit control box if the oil pressure drops below 14 ± 2 psi. A crankcase drain valve is provided with a handle so that the lubri- cating oil can be drained without removing the unit from  the  vehicle. d.  Governor.  A  constant  speed  governor  is  set  to maintain engine speed at 2000 ± 100 rpm. The gov- ernor uses a ball and cup mechanism on the camshaft gear as the sensing device. A yoke resting on the cup Legend for figure  4-2: Item Name 1 Base angle 2 Base angle 3 Support  angle 4 Support  angle 5 Support  angle 6 Support  angle 7 Gusset 8 Pad 9 Pad 10 Pad 11 Support  angle Qty 2 2 2 2 1 3 4 2 1 1 1 connects  to  an  arm  and  spring  mechanism  controlling the trottle lever. Any change in engine speed is trans- mitted from the cup to the yoke and to the throttle. An adjusting screw is provided on the front of the unit to adjust  the  speed  by  changing  the  spring  tension. e. Fuel system.  The  fuel  system  consists  basically  of a  primary  and  secondary  fuel  filter,  fuel  transfer pump,  fuel  injection  pump  and  two  fuel  injection  noz- zles.  The  transfer  pump  operates  off  the  camshaft, drawing fuel from the supply tank and delivering it through  the  two  filters  to  the  injection  pump.  The  in- jection pump meters and delivers the fuel at high pres- sure to the nozzles. f.   Electrical   System. The  generator/starter  is mounted to the chain case and engine housing and driven by the engine through a roller chain and drive sprocket. The output is controlled automatically by a voltage regulator and current limiting device. A de- tailed description of the auxiliary generating system is contained in paragraph 1-3g(2). g. Hydraulic System.  The pump is mounted to the chain case and driven by the engine. It supplies the pressure  for  the  auxiliary  hydraulic  system.  This  sys- tem powers the refuel pump and impact wrench and also supplies power for emergency operation of the boom, spade and winches. A detailed description of the auxiliary  hydraulic  system  is  contained  in  TM 9-2350-256-34-2. 4-2.   Repair a. General. (1) Current  limiter  calibration.   Whenever   the auxiliary  generator,  voltage  regulator,  APU  assembly, or  relay  box,  is  replaced  or  the  auxiliary  engine  is  re- placed or repaired, the rheostat in the switching relay box  must  be  adjusted  to  limit  the  auxiliary  generator output current to approximately 150 amperes with the APU in the vehicle. TM 9-2350-256-20 provides in- structions for calibrating the current limiter. I (2) Test  fixture.  The  auxiliary  engine  must  be placed  on  a  stand  to  operate  it  outside  the  vehicle (groundhopping). Refer to figure 4-2 for information on constructing a test fixture to support the APU dur- ing groundhopping tests. Size (inches) 2.00 x 2.00 x o.25 x 27.44lg 2.00 x 2.00 x 0.25 x 24.75 lg 2,00 x 2.00 x 0.25 x 23.06 lg 2.00 x 2.00 x 0.25 x 28.25 lg 2.00 x 2.00 x 0.25 x 19.38 lg 2.00 x 2.00 x 0.25 x 20.10 lg 0.25 x 1.75 x l.75 lg 0.75 x 3.00 x 2.88 lg 0.75 x 2.25 x 2.00 lg 0.75 x 2.25 x 3.00 lg 2.00 x 2.00 x 0.25 x 19.38 lg Material steel steel steel steel steel steel hr steel hr steel hr steel hr steel steel 4-6 Change 3