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Figure 24  Secondary Control Valve Operation at Governed Speed
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Figure 25  EVM Governor Operation
TRUCK GROUP SERVICE MANUAL TM 5-4210-230-14&P-1 GENERAL Electronic Vacuum Modulating Governor System (EVM) The    electronic    vacuum    modulating    governor    system consists  of  a  remote  mounted  electronic  control  unit  (ECU),  a threeway   solenoid   vacuum   valve   and   a   vacuum   diaphragm attached to the carburetor throttle plates.  The solenoid vacuum valve   and   vacuum   diaphragm   are   an   integral   part   of   the carburetor.  The ECU receives the engine speed signal from the negative  side  of  the  ignition  coil.    This  engine  speed  signal switches the ECU (when governed engine speed is reached) to allow  current  to  flow  through  the  solenoid  vacuum  valve.  The solenoid vacuum valve provides correct governor vacuum to the vacuum diaphragm.  The vacuum diaphragm acting against the governor spring thereby controls the throttle position to regulate engine speed (Fig. 25). Internal clean air is supplied to both the solenoid side of the solenoid valve and to the high pressure side of the governor diaphragm.        The    cone-tipped    side    of    the    solenoid    valve connects  the  venturi  and  intake  manifold  vacuum  to  the  low pressure side of the governor diaphragm. In an underspeed or current   off   condition,   the   solenoid   return   spring   closes   the source  vacuum  port,  thereby  venting  the  low  pressure  side  of the governor diaphragm directly to the clean air source. In this mode,  the  zero  or  near  zero  pressure  differential  across  the governor diaphragm prevents any governing action. As  the  engine  approaches  governed  speed,  current  is applied to the solenoid which pulls the valve against the return spring,  closes  the  port  to  clean  air  and  connects  the  governor diaphragm    to    source    vacuum.        Modulation    between    the extremes  of  solenoid  valve  full-off  and  full-on  is  accomplished by varying the percentage on-time of the current to the solenoid. This results in an average valve opening that is proportional to percent on-time and provides the desired governing action.  The speed is factory preset within the electronic control unit. Legend for Figure 25 1.  Clean Air Port Open 2.  Battery Supply 3.  To Negative Side of Coil 4.  Vacuum Port Closed 5.  Low Pressure Side 6.  Vacuum 7.  Throttle Control Governor Diaphragm 8.  High Pressure Side 9.  Internal Clean Air 10.  Clean Air Port Closed 11.  Vacuum Port Open Electronic Vacuum Modulating (EVM) Governor System Block Diagram CGES-125-T  Page 16 PRINTED IN UNITED STATES OF AMERICA

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