Ignition System Description. GMC Truck GMC P42 Commercial Chassis 1998

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC P42 Commercial Chassis 🢒 Engine 🢒 Engine Electrical 🢒 Ignition System Description

General Description

The ignition system includes the following components:

•	The battery

•	The distributor

•	The ignition switch

•	The spark plugs

•	The primary and secondary wiring

For information on the battery, refer to Battery Description . For information on the ignition switch, refer to Engine Controls.

The material used to construct the spark plug wires is very pliable and soft. This wire will withstand high heat and carry a high voltage. Due to the more pliable wire, scuffing and cutting become easier. Route the spark plug wires correctly in order to prevent chafing or cutting. When removing a spark plug wire from a spark plug, twist the boot on the spark plug and pull on the boot in order to remove the wire.

Distributor Ignition (DI) System

The distributor ignition (DI) system includes the following components:

•	The distributor

•	A hall-effect switch

•	The ignition coil

•	The secondary wires

•	The spark plugs

•	The knock sensor

•	A crankshaft position (CKP) sensor

The vehicle control module (VCM) controls the ignition system. The VCM performs the following functions:

•	Monitors information from various engine sensors

•	Computes the desired spark timing

•	Controls the dwell and firing of the ignition coil (through an ignition control line to the coil driver)

For information on the diagnostics of these ignition systems, refer to Engine Controls.

Distributor (4.3L)

Distributor (5.7L and 7.4L)

The distributor connects the ignition coil and spark plugs through secondary wires. The rotor directs the spark to the appropriate cylinder. The distributor has a hall-effect switch that connects to the VCM through the primary engine harness. This switch provides cylinder identification information.

For bench repair of the distributor, refer to Distributor Overhaul .

Identification

The part number is pin stamped into the plastic base, directly below the high tension terminals and next to the three pin camshaft sensor connection.

Ignition Coil (4.3L and 5.7L)

Ignition Coil (7.4L)

When signaled by the VCM, the coil driver controls the current flow in the primary winding of the ignition coil. The primary engine harness makes the interconnects. The ignition coil can generate up to 35,000 volts and connects to the distributor through a secondary wire.

Secondary Wires

The secondary wires carry voltage between the ignition coil, the distributor, and the spark plugs. The secondary wire connections form a tight seal that prevents voltage arcing.

Hall-Effect Switch

The hall-effect switch connects to the VCM through the primary engine harness. The hall-effect switch provides cylinder identification information. The hall-effect switch is under the distributor cap.

Crankshaft Position Sensor

The crankshaft position (CKP) sensor connects to the VCM through the primary engine harness. The crankshaft position sensor provides crankshaft reference information to the VCM for spark and fuel delivery. The crankshaft position sensor is in the front cover behind the crankshaft balancer.

Ignition Base Timing

Notice: The distributor used with the OBD II ignition system is located in a fixed, non-adjustable position. Do not adjust the engine base timing by rotating this distributor. Otherwise, crossfiring and damage to the distributor will result.

For ignition base timing information on the 4.3, 5.7L and the 7.4L engines, refer to Engine Controls.

Spark Plugs

These engines use resistor-type, tapered-seat spark plugs. These plugs do not have a gasket. These spark plugs are High Efficiency (HE) spark plugs that have a fine wire electrode, a nickel plated shell, and a platinum pad across from the electrode. These features give the HE spark plug long life and durability. For correct spark plug gap information, refer to Spark Plug Usage or the Vehicle Emissions Control Information label on the engine cooling fan shroud.

If the spark plug shell hex is not fully engaged in the spark plug socket wrench, the socket may cock at an angle and cause insulator cracking or breakage during plug installation or removal.

Normal or average service is assumed to be a mixture of idling, low speed, and high speed operation with some of each making up the daily total driving. Occasional or intermittent high speed driving is essential for good spark plug performance. High speed driving provides increased and sustained combustion heat that burns away any excess deposits of carbon, or oxide, that may accumulate from frequent idling, continual stop-and-go, or slow speed driving. An insulating boot made of special heat-resistant material that covers the spark plug terminal, and extends downward over a portion of the plug insulator, protects the spark plugs. These boots prevent flash-over with resultant misfire of the engine, even though a film is allowed to accumulate on the exposed portion of the plug porcelains.

Do not mistake corona discharge for flash-over or a shorted insulator. Corona discharge is a steady blue light appearing around the insulator, just above the shell crimp. This is the visible evidence of a high-tension field, and has no effect on the ignition performance. You usually can only detect corona discharge 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 the shell and the insulator.