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. In an electronic ignition system, the trigger wheel replaces what contact point ignition component?

Q8. In a computerized advance, which sensor allows the computer to retard timing when the engine knocks or pings?

Q9. The condition that exists when an electric arc passes through the insulation of a spark plug wire is known by what term?

Q10. What number of degrees can dwell vary before a distributor is either rebuilt or replaced?

Q11. What tool is used to set the air gap of the pickup coil?

Q12. What ignition timing condition adversely affects engine fuel economy and power?

Q13. In a computer-controlled ignition system, the ignition timing without the computer-controlled advance is known by what term?

IGHTING 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.