7-10 GOVERNOR CONTROL UNIT (MEP-115A). a. General. (1) The electric governing system Is a speed (frequency)  sensing  system  used  to  maintain  constant engine  speed  and  generator  output  frequency  during periods  of  unchanging  load  and  when  load  additions or deletions occur. The system consists of a control  unit,  magnetic  pickup  and  an  electric throttle actuating unit. The engine speed signal is  obtained  from  a  magnetic  pickup  mounted  in  the flywheel  housing  in  close  proximity  to  the  flywheel ring gear. The  frequency  of  the  pickup  signal  is proportional  to  engine  speed.  Figure  7-14  shows the  functional  theory  of  operation. (2) The control unit has four distinct cir- cuits; pickup signal amplifier, frequency reference oscillator,  phase  comparator  and  output  circuit. (a) The Frequency Reference Oscillator is voltage  controlled. The  frequency  setting  is adjusted  by  applying  zero  to  10  volts  at  the Frequency Reference Oscillator input. The internal frequency  adjust  provides  this  voltage  setting  when the control unit is in operation. The Reference Oscillator  does  not  maintain  a  constant  frequency. It  deviates  from  its  nominal  frequency  as  the  engine speed changes during load changes. The Reference Oscillator  is  forced  by  the  Phase  Comparator  to track  the  amplified  pickup  signal  representing engine  speed. The  voltage  representing  speed error is the amount of voltage required to drive the  Reference  Oscillator  off  frequency  in  propor- tion  to  the  engine  speed  deviation. (b) The Phase Comparator Circuit receives signals  from  the  Pickup  Signal  Amplifier  and  the Reference  Frequency  Oscillator  and  compares  the difference in frequency. The Phase Comparator measures the amount the engine signal is ahead or behind the Reference Oscillator signal. Its voltage output is used to force the Reference Oscillator  to  the  same  frequency  as  the  signal from the engine. The  Phase  Comparator  output is proportional to the speed error. The gain control is used to couple the Phase Comparator output  to  the  Reference  Oscillator. By  increas- ing  the  coupling,  a  small  voltage  change  from the  Phase  Comparator  represents  a  large  frequency change  and  vice-versa. (c) The Output Circuit allows governing by introducing  a  temporary  drop  during  a  load  change for  stability  purposes. It  has  an  adjustable  means to  control  the  magnitude  and  time  constant  of  the drop  to  match  the  dynamic  characteristics  of  the engine. The output current switching portion of the  circuit  provides  current  to  drive  the  actuator. The  output  transistor  is  switched  on  and  off  at  a frequency of 200 Hz. This is above the natural frequency of the actuator. The actuator responds to  the  average  current  from  the  transistor  and  moves in proportion to position the engine throttle. The output  transistor  is  switched  on  and  off  to  reduce power dissipation. b.  Malfunction. The  following  procedures  are  to  be  performed  with the  Governor  Control  Unit  in  the  generator  set. (1) Connect the breakout cable between wiring harness plug P17 (1, figure 7-15) and the electric  governor. (2) Using a digital voltmeter, make the voltage  readings  in  table  7-4  at  the  breakout cable  plug. All  readings  are  measured  between the terminal and ground. Terminals F, G, H and T are ground. c. Removal. (1) Disconnect electrical connector P17 (1, figure 7-15). (2) Remove 4 hex head screws (2), lock washers  (3)  and  nuts  (4). (3) Remove the control unit. d. Installation. Install  the  Governor  Control  Unit  in  reverse  order of  removal  procedures. e. Adjustment Procedure. (1) Disconnect the actuator linkage rod from the  engine  fuel  shutoff  lever. (2) Place the generator set START-RUN-STOP switch  (located  on  the  generator  set  control  panel in the RUN position. (3) Place the generator set BATTLE SHORT WITCH (located on the generator set control panel in  the  ON  (override)  position. (4) Beginning at the fully counterclockwise position,  rotate  the  engine  fuel  shutoff  lever  in a  clockwise  direction  until  a  slight  resistance  is felt; hold the lever in this position. NOTE This  resistance  is  the  fuel  injection pump’s  internal  governor  linkage  hook engaging  the  metering  valve  arm. (5) Move the actuator 1 ever to the “full fuel” position. This  is  the  direction  against  the  spring resistance  (fully  counterclockwise). Change    9 7-19