Ignition System: Description and Operation

IGNITION SYSTEM

DESCRIPTION

All ignition systems include a battery, a distributor, an engine control switch, spark plugs, and the primary and secondary wiring. Information on the battery is located Battery. Refer to CAB ELECTRICAL for information on the engine control switch.

There are two different ignition systems used on these vehicles. If the vehicle has a carbureted engine, spark is controlled by the module inside the distributor with an integral coil. If the vehicle engine is equipped with throttle body injection (TBI), the ignition system includes a knock sensor, an electronic spark control module, a computer (ECM), and a distributor with a separate coil.

DISTRIBUTOR

Figure 42:

Figure 43:

Figure 44:

The distributor with an integral coil combines all ignition components in one unit (figures 42 and 43). The coil is in the cap and connects through a resistance brush to the rotor. On a distributor with a separate coil the coil connects to the rotor through a high tension wire (figure 44).

The distributor has an internal magnetic pick-up assembly which contains a permanent magnet, a pole piece with internal teeth and a pick-up coil. When the teeth of the timer core, rotating inside the pole piece, line up with the teeth of the pole piece, an induced voltage in the pick-up coil signals the electronic module to trigger the coil primary circuit. The primary current decreases and a high voltage of up to 35,000 volts is induced in the ignition coil secondary winding. The voltage is directed through the rotor and secondary leads to fire the spark plugs. The capacitor in the distributor is for radio noise suppression.

The magnetic pick-up assembly is mounted over the main bearing on the distributor housing. On carbureted engines without computers, the assembly is made to rotate by the vacuum control unit, thus providing vacuum advance. The distributor shaft is mounted on the camshaft at the rear of the engine, and rotates at one half the RPM of the engine. The force of rotation moves the advance weights against the springs, and provides cetrifugal advance to the timer core.

ENGINES WITH TBI

On engines with TBI, spark timing changes are determined electronically by the ECM (computer), which monitors information from various engine sensors, computes the desired spark timing, and signals the distributor.

The distributor does not contain centrifugal weights, springs, or a vacuum advance unit.

Figure 45:

Each TBI equipped engine also has a knock sensor mounted in the engine block and connected by a blue wire to a spark control module (called a "half-function box")(figure 45). In response to engine knock, a signal is sent from the sensor to the module and then to the ECM, which computes how much to retard spark timing to reduce knock. A retard command is then sent to the distributor.

Information on TBI and the computer controlled ignition system, system components and their locations, and diagnostic charts are located Computers and Control Systems.

IGNITION TIMING

Timing specifications for each engine are listed on the Vehicle Emissions Control Information label on the radiator support. Always follow Vehicle Emissions Control Information label procedures when adjusting timing. When using a timing light, connect an adapter between the number 1 spark plug and the number 1 spark plug wire, or use an inductive type pick-up. Do not pierce the plug lead. Once the insulation of the spark plug cable has been broken, voltage will jump to the nearest ground, and the spark plug will not fire properly.

Some engines incorporate a magnetic timing probe hole for use with special electronic timing equipment. Consult manufacture's instructions for use of this equipment.

SPARK PLUG WIRES

The spark plug wiring is a carbon impregnated cord conductor encased in an 8mm diameter rubber jacket. The silicone spark plug boots form a tight seal on the plugs.

SPARK PLUGS

Figure 46:

Figure 47:

Resistor type, tapered seat spark plugs are used on all engines. No gasket is used on these tapered seat plugs. Refer to figures 46 and 47 for an explanation of letter coding on spark plugs. Refer to the Vehicle Emissions Control Information label on the radiator support for correct gap information.

Normal or average service is assumed to be a mixture of idling, slow speed, and high speed operation with some of each making up the daily total driving. Occasional or intermittent high-speed driving is essential to good spark plug performance as it provides increased and sustained combustion heat that burns away any excess deposits of carbon or oxide that may have accumulated from frequent idling or continual stop-and-go or slow-speed driving. Spark plugs are protected by an insulating nipple made of special heat-resistant meterial which covers the spark plug terminal and extends downward over a portion of the plug insulator. These nipples prevent flash-over with resultant missing of engine, even though a film is allowed to accumulate on exposed portion of plug porcelains.

Do not mistake corona discharge for flash-over or a shorted insulator. Corona is a steady blue light appearing around the insulator, just above the shell crimp. It is the visible evidence of a high-tension field, and has no effect on ignition performance. Usually it can be detected only in darkness. This discharge may repel dust particles, leaving a clear ring on the insulator just above the shell. This ring is sometimes mistakenly regarded as evidence that combustion gases have blown out between shell and insulator.