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
Replace the distributor vacuum line and see if
timing still conforms to the manufacturers
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
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 manufacturers
service manual when timing a computer-controlled
REVIEW 3 QUESTIONS
Of the two circuits within the ignition circuit,
which one uses conventional wiring?
What component of the ignition circuit acts as a
pulse type transformer for producing a short
burst of high voltage?
What are the two type of sparkplugs?
What factors determine the heat range of a spark
The smaller the number designator for a spark
plug, the hotter the sparkplug. (T/F)
The amount of time in degrees that the contact
points remain closed between each opening is
known by what term?
In an electronic ignition system, the trigger
wheel replaces what contact point ignition
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
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?
Learning Objective: Identify lighting-circuit
components, their functions, and maintenance
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 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
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