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Adjustment Procedure - TM-5-4210-230-14P-1_811
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Fig. 61  Modulator Test
ENGINE DIVISION SERVICE MANUAL TM 5-4210-230-14&P-1 NON-SCHEDULED MAINTENANCE OPERATIONS DECELERATION THROTTLE MODULATOR (DTM) SYSTEM Deceleration Throttle Modulator System Operation The   deceleration   throttle   modulator   system   consists basically of a vacuum operated throttle modulator unit on the carburetor, a solenoid vacuum valve and engine speed sensor unit. Normally  an  engine  will  emit  relatively  high  levels  of unburned hydro-carbons during "closed throttle" deceleration. This  is  because  the  intake  of  fuel/air  mixture  is  not  sufficient to   support   complete   combustion   and   the   engine   "misfires" resulting  in  fuel  being  passed  through  the  engine  unburned. The  deceleration  throttle  modulator  system  overcomes  this condition   by   maintaining   a   slightly   greater   throttle   opening (high  idle)  during  initial  deceleration  which  permits  intake  of just enough additional fuel/air mixture to promote combustion and eliminate misfire. The  engine  speed  sensor  is  calibrated  to  activate  or deactivate  (extend  or  retract)  the  throttle  modulator  unit  at approximately   1850   RPM   engine   speed.      Operation   of   the throttle modulator is illustrated in Fig.  60. Above  1850  RPM  engine  speed,  the  solenoid  vacuum valve  is  activated  (held  open)  by  electrical  current  from  the engine  speed  sensor  unit.    This  permits  manifold  vacuum  to act    upon    the    diaphragm    of    the    throttle    modulator    unit extending  the  modulator  unit  to  the  high  idle  position.    Upon deceleration,  the  returning  carburetor  throttle  level  contacts the  extended  modulator,  thus  holding  the  throttle  in  high  idle position. When engine speed drops below 1850 RPM, the speed sensor  unit  deactivates  the  solenoid  vacuum  valve  allowing the  valve  to  close  and  bleed  the  vacuum  from  the  throttle modulator  diaphragm  chamber.    This  permits  the  diaphragm spring to retract the modulator unit and allow the throttle lever to return to normal curb idle position. Check Deceleration Throttle Modulator Operation, replace if necessary. a. Check    modulator    for    free    operation    by    grasping modulator by large diameter of the plastic housing and pulling (without twisting) toward throttle lever.    Plastic  housing  should  move  freely  against internal spring force for about 6 mm (¼ ").  Modulator should be replaced if it does not move freely or does not return rapidly to fully retracted position (Fig.  60). b. Check throttle modulator speed setting and adjust, if necessary. (1) Connect tachometer to engine. (2) Start    and    operate    engine    until    normal    operating temperatures are reached. Fig.  60  Throttle Modulator Operation (3) Disconnect     vacuum     hose     from     modulator     and manually  apply  vacuum  to  modulator  (Fig.    61).    If modulator does not extend, unit is faulty and must be replaced.    Allow  throttle  to  close  against  extended modulator. (4) Observe   engine   speed.      For   proper   speed,   see Emission Control Tune-Up Specifications. CGES-215  Page 35 PRINTED IN UNITED STATES OF AMERICA

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