aimed  at  the  timing  mark  and  gradually  increasing speed.   If   the   advance   mechanism   is   operating,   the timing mark should move away from the pointer. If the timing mark fails to move as the speed increases or it hesitates  and  then  suddenly  jumps,  the  advance mechanism  is  faulty  and  should  either  be  repaired  or replaced. Replace   the   distributor   vacuum   line   and   see   if timing   still   conforms   to   the   manufacturer’s specifications. If the timing is NOT advanced when the vacuum  line  is  connected  and  the  throttle  is  opened slightly,   the   vacuum   advance   unit   or   tubing   is defective. Most  computer-controlled  ignition  systems  have no  provision  for  timing  adjustment.  A  few,  however, have  a  tiny  screw  or  lever  on  the  computer  for  small ignition timing changes. A computer-controlled ignition system has what is known as BASE TIMING. Base timing is the ignition timing   without   computer-controlled   advance.   Base timing is checked by disconnecting a wire connector in the computer wiring harness. This wire connector may be found on or near the engine or sometimes next to the distributor.  When  in  the  base  timing  mode,  a conventional   timing   light   can   be   used   to   measure ignition  timing.  If  ignition  timing  is  not  correct,  you can rotate the distributor, in some cases, or move the mounting  for  the  engine  speed  or  crank  position sensor.   If   base   timing   cannot   be   adjusted,   the electronic  control  unit  or  other  components  will  have to  be  replaced.  Always  refer  to  the  manufacturer’s service   manual   when   timing   a   computer-controlled ignition system. REVIEW  3  QUESTIONS Q1. Of  the  two  circuits  within  the  ignition  circuit, which one uses conventional wiring? Q2. What component of the ignition circuit acts as a pulse  type  transformer  for  producing  a  short burst of high voltage? Q3. What are the two type of sparkplugs? Q4. What factors determine the heat range of a spark plug? Q5. The smaller the number designator for a spark plug, the hotter the sparkplug. (T/F) Q6. The amount of time in degrees that the contact points  remain  closed  between  each  opening  is known by what term? Q7. Q8. Q9. Q10. Q11. Q12. Q13. In   an   electronic   ignition   system,   the   trigger wheel  replaces  what  contact  point  ignition component? In  a  computerized  advance,  which  sensor  allows the computer to retard timing when the engine knocks or pings? The  condition  that  exists  when  an  electric  arc passes  through  the  insulation  of  a  spark  plug wire is known by what term? What number of degrees can dwell vary before a distributor is either rebuilt or replaced? What tool is used to set the air gap of the pickup coil? What  ignition  timing  condition  adversely  affects engine fuel economy and power? In  a  computer-controlled  ignition  system,  the ignition timing without the computer-controlled advance is known by what term? LIGHTING  CIRCUIT Learning   Objective:  Identify   lighting-circuit components,  their  functions,  and  maintenance procedures. The  lighting  circuit  (fig.  2-54)  includes  the  battery, vehicle frame, all the lights, and various switches that control  their  use.  The  lighting  circuit  is  known  as  a single-wire system since it uses the vehicle frame for the  return. The  complete  lighting  circuit  of  a  vehicle  can  be broken down into individual circuits, each having one or  more  lights  and  switches.  In  each  separate  circuit, the lights are connected in parallel, and the controlling switch is in series between the group of lights and the battery. The  marker  lights,  for  example,  are  connected  in parallel and are controlled by a single switch. In some installations,  one  switch  controls  the  connections  to  the battery, while a selector switch determines which of two circuits is energized. The headlights, with their high and low beams, are an example of this type of circuit. In  some  instances,  such  as  the  courtesy  lights, several switches may be connected in parallel so that any switch may be used to turn on the light. When a wiring diagram is being studied, all light circuits  can  be  traced  from  the  battery  through  the ammeter to the switch (or switches) to the individual light. 2-43