The  basic  methods  to  control  ignition  system timing are as follows: CENTRIFUGAL   ADVANCE   (controlled   by engine speed) VACUUM   ADVANCE   (controlled   by   intake manifold vacuum and engine load) COMPUTERIZED  ADVANCE  (controlled  by various   sensors—speed,   temperature,   intake, vacuum, throttle position, etc.) Centrifugal  Advance Centrifugal  advance  makes  the  ignition  coil  and spark   plugs   fire   sooner   as   engine   speed   increases, using   spring-loaded   weights,   centrifugal   force,   and lever  action  to  rotate  the  distributor  cam  or  trigger wheel.   Spark   timing   is   advanced   by   rotating   the distributor   cam   or   trigger   wheel   against   distributor shaft rotation. This action helps correct ignition timing for  maximum  engine  power.  Basically  the  centrifugal advance consists of two advance weights, two springs, and a advance lever. During  periods  of  low  engine  speed,  the  springs hold   the   advance   weights   inward   towards   the distributor  cam  or  trigger  wheel.  At  this  time  there  is not  enough  centrifugal  force  to  push  the  weights outward. Timing stays at its normal initial setting. As  speed  increases,  centrifugal  force  on  the weights  moves  them  outwards  against  spring  tension. This  movement  causes  the  distributor  cam  or  trigger wheel to move ahead. With this design, the higher the engine speed, the faster the distributor shaft turns, the farther  out  the  advance  weights  move,  and  the  farther ahead  the  cam  or  trigger  wheel  is  moved  forward  or advanced. At a preset engine speed, the lever strikes a stop  and  centrifugal  advance  reaches  maximum. The  action  of  the  centrifugal  advance  causes  the contact points to open sooner, or the trigger wheel and pickup  coil  turn  off  the  ECU  sooner.  This  causes  the ignition coil to fire with the engine pistons not as far up in the cylinders. Vacuum  Advance The   vacuum   advance   provides   additional   spark advance  when  engine  load  is  low  at  part  throttle position.  It  is  a  method  of  matching  ignition  timing with  engine  load.  The  vacuum  advance  increases FUEL ECONOMY because it helps maintain idle fuel spark advance at all times. A vacuum advance consists of   a   vacuum   diaphragm,   link,   movable   distributor plate, and a vacuum supply hose. At  idle,  the  vacuum  port  from  the  carburetor  or throttle  body  to  the  distributor  advance  is  covered, thereby   NO   vacuum   is   applied   to   the   vacuum diaphragm, and spark timing is NOT advanced. At part throttle,  the  throttle  valve  uncovers  the  vacuum  port and the port is exposed to engine vacuum. The vacuum pulls the diaphragm outward against spring force. The diaphragm  is  linked  to  a  movable  distributor  plate, which  is  rotated  against  distributor  shaft  rotation  and spark timing is advanced. The  vacuum  advance  does  not  produce  any advance at full throttle. When the throttle valve is wide open,  vacuum  is  almost  zero.  Thus  vacuum  is  NOT applied  to  the  distributor  diaphragm  and  the  vacuum advance does NOT operate. Computerized  Advance The  computerized  advance,  also  known  as  an electronic  spark  advance  system,  uses  various  engine sensors and a computer to control ignition timing. The engine  sensors check various operating conditions and sends  electrical  data  to  the  computer.  The  computer can   change  ignition  timing  for  maximum  engine efficiency. Ignition   system   engine   sensors   include   the following: ENGINE   SPEED   SENSOR   (reports   engine speed to the computer) CRANKSHAFT  POSITION  SENSOR  (reports piston position) THROTTLE  POSITION  SWITCH  (notes  the position of the throttle) INLET   AIR   TEMPERATURE   SENSOR (checks the temperature of the air entering the engine) ENGINE   COOLANT   TEMPERATURE SENSOR  (measures  the  operating  temperature of the engine) DETONATION SENSOR (allows the computer to  retard  timing  when  the  engine  knocks  or pings) INTAKE   VACUUM   SENSOR   (measures engine vacuum, an indicator of load) The computer receives different current or voltage levels   (input   signals)   from   these   sensors.   It   is 2-37