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Fig. 61  Modulator Test
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TM-5-4210-230-14P-1 Aerial Ladder Fire Fighting Truck Manual
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Fig.  64  Checking Belt Tension With SE-2312 Gauge
ENGINE DIVISION SERVICE MANUAL TM 5-4210-230-14&P-1 (2) Slowly accelerate the engine and observe the speed at which the test lamp lights.  Test lamp should light above 1850 + 50 RPM. (3) Slowly  decelerate  the  engine  and  observe  speed  at which the test lamp goes out.  Test lamp should go out below 1850 + 50 RPM. (4) If test lamp does not function properly, check speed sensor  wiring  for  proper  routing  and  for  clean  and tight  connections.    If  wiring  is  correct,  speed  sensor is  faulty  and  must  be  replaced.    (The  speed  sensor and solenoid valve are sold as one assembly.) AIR INJECTION SYSTEM (U.S.A.  Only) Air Injection System Operation The   air   injection   system   forces   air   into   the   cylinder head    exhaust    ports    to    promote    "after-burning"    of    any combustibles remaining in the exhaust gases flowing from the combustion  chamber.    The  components  of  the  air  injection system are illustrated in Figure 70. The  air  pump  used  in  the  air  injection  system  is  a rotary  vane  type  which  is  belt  driven  from  the  engine.    A centrifugal type air filter located behind the pump pulley filters the  air  entering  the  pump  to  prevent  entrance  of  dirt  and abrasives (Fig.  66). The air manifold distributes air under pressure from the pump  to  the  individual  exhaust  ports  in  the  cylinder  heads. Figure 70 shows the location of the air manifolds on the small V8 engines.  The air manifold is an integral part of the MV and large V8 engine cylinder heads. A one-way check valve is used on each air manifold to protect   the   pump   from   a   back   flow   of   high   temperature exhaust   gas.      The   diverter   valve   prevents   backfire   in   the exhaust  system  by  shutting  off  air  flow  during  the  first  few seconds of deceleration.  The diverter valve also incorporates a pressure relief valve which exhausts excessive pump outlet air  pressure  during  high  speed  operation.    Operation  of  the diverter valve is illustrated in Figure 69. In normal operation (cruising) the air pump delivers air through the air manifolds into the cylinder head exhaust ports. With the addition of air from the air injection system, an over- rich  mixture  can  cause  backfire  in  the  exhaust  system.    To prevent backfire, the diverter valve momentarily stops the flow of air to the cylinder head exhaust ports during deceleration. The   diverter   valve   is   controlled   by   intake   manifold vacuum.  At constant manifold vacuum spring pressure holds the valve in normal delivery position.  This permits air to flow from  the  air  pump  to  the  air  manifolds.    During  deceleration high manifold vacuum overcomes spring pressure and moves the valve to bypass position.  This closes the outlet port and bypasses   the   air   flow   through   the   internal   silencer   to   the atmosphere.    The  valve  returns  to  normal  delivery  position when the vacuum being applied to the diaphragm is balanced through  the  orifice  to  the  chamber  on  the  other  side  of  the diaphragm.          Operating     cycle     time     for     the     valve     is approximately six seconds.  In cases of high pump output air pressure  (due  to  high  engine  speed  etc.),  the  pressure  relief valve   opens   and   exhausts   excessive   air   flow   through   the internal silencer to the atmosphere. Check Air Injection System and replace, if necessary. Inspect Air Pump Drive Belt(s) Air  injection  pump  drive  belt  tension  can  be  checked with belt tension gauge SE-2312 (Fig.  64).  Check tension at mid-point  between  pulleys.    Apply  gauge  to  the  longest  belt span.  Instructions for checking are as follows: (1) Grasp  gauge  and  depress  ball  handle  all  the  way down.    Make  certain  hook  extends  beyond  legs  to pick up the belt. (2) Apply   gauge   to   belt.      Be   sure   belt   is   positioned between   nose   piece   and   hook   and   that   the   nose piece is centered on belt. (3) Release  ball  handle  quickly.    A  slow  release  may present   full   return   of   hook,   thus   giving   a   false reading. (4) Observe the area of gauge face indicated at the index mark.      If   index   mark   does   not   indicate   a   NEW reading on a new belt or a USED reading on a used belt, it CGES-215  Page 37 PRINTED IN UNITED STATES OF AMERICA

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