S-SERIES INSTRUMENTS This air core type fuel gauge consists of three (3) copper wire  coils  wound  around  a  plastic  bobbin  containing  a magnet  and  spindle  assembly.    Attached  to  the  magnet and  spindle  assembly  is  a  pointer  which  indicates  fuel level.  The fuel gauge requires a 0 to 90 ohm resistance sender  to  operate.    The  sender  is  the  tank  unit   and consists  of  a  float  and  arm  assembly  and  a  variable resistor.    The  sender’s  resistance  is  controlled  by  the position of float and arm assembly.  A full fuel tank raises float  to  its  highest  position.    At  this  position  the  variable resistor has a resistance of 88 ohms.  With an empty fuel tank  the  float  assumes  its  lowest  position,  creating  a sender resistance of less than 1 ohm. The  fuel  gauge  circuit  diagram  (Figure  18)  shows  that with   key   switch   "ON,"   current   flows   from   the   battery through a parallel circuit consisting of the empty coil and the   fixed   resistor   and   thence   through   another   circuit composed of: - The variable resistance fuel level sender, and the bucking coil and the full coil. - The fuel sender only when in its empty position. Variations in the position of the fuel level sender contact will vary the active portion of resistance element and thus control the amount of current flowing in the bucking and full coils  Maximum  current  in  the  bucking  and  full  coils occurs  with  the  tank  unit  in  the  full  "F"  position.    Under this  condition,  the  magnetic  field  of  the  bucking  and  full coils   is   at   maximum   and   the   pointer   and   armature assembly will align itself with the resultant magnetic field of the three coils - the "Full" position.  As the fuel is used, the fuel level sender contact position changes to reduce the current in the bucking and full coils and increases the current   in   the   empty   coil.      This   variation   in   current reduces  the  magnetic  field  strength  of  the  bucking  and full coils from a maximum at "full" to zero at the empty or "E"   position   at   which   time   the   armature   and   pointer assembly is aligned with the magnetic field of the empty coil.    Thus,  the  interaction  of  the  magnetic  field  of  the three   coils   produces   a   resultant   magnetic   field   which controls  the  rotation  and  position  of  the  armature  and pointer assembly. The sender and key switch terminals are connected by a calibrating resistor (Figure 18).  The other terminal is the ground terminal. The  sender  terminal  is  the  first  terminal  clockwise  from ground terminal when viewed from back side of gauge. NOTE  -The  gauge  Is  grounded  to  chassis  through the  ground  terminal  when  plugged  Into  Instrument cluster printed circuit. When   the   key   switch   is   turned   off,   pointer   will   not necessarily return to the empty position.  This is inherent in the instrument and does not indicate a faulty part. Testing 1. Disconnect wire at fuel tank sender unit. 2. Connect one lead of Gauge Tester to end of sender wire.  Connect second lead to ground.  (Gauge tester now substitutes for sender unit. 3. Turn  key  switch  "ON"  (Be  sure  there  is  power  to cluster. 4. Set  tester  for  88  ohms.    Fuel  gauge  should  read  at full. (Pointer within boundaries of ball. 5. Set  tester  for  44  ohms.    Fuel  gauge  should  read  at half scale.  (Pointer within boundaries of ball. 6. Set  tester  at  1  ohm.    Fuel  gauge  should  read  at empty.    (Pointer  within  boundaries  of  ball.    If  fuel gauge  responds  correctly  to  various  tester  settings, gauge  and  wiring  between  gauge  and  sender  unit are  OK.    Trouble  is  in  sender  unit  or  sender  unit  is not grounded. a. Check sender unit ground circuit.  Make sure circuit is grounded. b. If  ground  circuit  is  OK,  replace  sender  unit  If gauge does not respond to tester: a. Check continuity of gauge wiring circuits.  Make sure connector terminals are clean and tight. b. Check  gauge  cluster  ground  circuit.    Make  sure circuit is grounded. c. If wiring is OK, replace gauge. WATER TEMPERATURE GAUGE Operation The   water   temperature   gauge   circuit   consists   of   two basic  components  -  the  cluster-mounted  gauge  and  the thermistor sending unit.  The sender controls the  gauge reading which indicates the water temperature.  The two units are connected electrically as shown in The   operating   principle   of   the   temperature   indicating system     can     be     understood     by     reference     to     the temperature gauge circuit diagram (Figure 19).  With the ignition  switch  closed,  current  will  flow  from  the  battery through   the   bucking   and   "cold"   coils   and   the   fixed resistor  to  ground,  and  through  the  "Hot"  coil  and  the variable  resistance  temperature  sender  to  ground.    The temperature sender consists of a thermistor en-closed in a   sealed   threaded   shell   containing   a   heat   transfer medium  and  equipped  with  an  insulated  terminal.    With the    temperature    sender    immersed    in    a    cold    liquid (1000F),  its  resistance  is  high  and  the  current  flowing through the "hot" coil is small; therefore, the CTS-4140Y - Page 10