Chrysler Full-Size Trucks 1967-1988 Repair Guide

Description and Operation


1972-83 MODELS

See Figures 1 and 2

Electronic ignition was optional in 1972-73 and became standard in 1974. This unit functions basically the same as a breaker point distributor, although the parts used are different.

The distributor housing, cap, rotor and advance mechanism are the same as on the conventional distributor; both systems also use the same spark plugs and ignition coil. A magnetic pickup and control (reluctor) replace the breaker points and rotor. A condenser is no longer necessary. The only maintenance required on electronic ignition systems is inspection of the wiring, as well as cleaning and changing of the spark plugs.

As a result of the elimination of breaker points, the dwell on electronic ignition units is non-adjustable. It can be read on a dwell meter, but cannot be adjusted.

Don't fool with the reluctor. The reluctor teeth may appear ragged at the edges, but no attempt should be made to clean them. A sharp edge is needed to quickly decrease the magnetic field and induce negative voltage in the pickup coil. If the teeth are rounded, the voltage signal to the control unit may be erratic.

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Fig. Fig. 1: Common electronic ignition system

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Fig. Fig. 2: Electronic ignition schematic-6-cylinder engine with dual pickup distributor


The system consists of the battery, ignition switch, ignition resistor control unit, coil, dual pickup distributor with vacuum advance mechanism, dual pickup start/run relay, spark plugs and necessary components for the routing of primary and secondary current.

During engine cranking, the dual pickup start/run relay is energized through the starter solenoid circuit, which allows the start pickup to adjust the timing for starting purposes only. As soon as the starter solenoid is de-energized, the start/run relay switches the sensing function back to the run pickup.

The pickup circuit is used to sense the proper timing for the control unit switching transistor. The reluctor rotating with the distributor shaft produces a voltage pulse in the magnetic pickup each time a spark plug should be fired. This pulse is transmitted through the pickup coil to the power switching transistor in the control unit, and causes the transistor to interrupt the current flow through the primary circuit. This break in the primary circuit induces a high voltage in the secondary coil circuit and fires the appropriate spark plug.

The length of time the switching transistor allows the current to flow in the primary circuit is determined by the electronic circuitry in the control unit. This determines the dwell.

Dwell is not adjustable and there is no means to change it, since changes are not required.


The computer provides the engine with Ignition Spark Control during starting and engine operation, providing an infinitely variable spark advance curve. Input data is fed instantaneously to the computer by a series of sensors located in the engine compartment which monitor timing, water temperature, air temperature, idle/off-idle operation and intake manifold vacuum. The program schedule module of the Spark Control Computer receives the information from the sensors, processes it, then directs the ignition control module to advance or retard the timing as necessary. This whole process goes on continuously as the engine is running, taking only milliseconds to complete a circuit from sensor to distributor. The main components of the system are a modified carburetor and Spark Control Computer, which is responsible for translating input data and which transmits data to the distributor to advance or retard the timing.

There are two functional modes of the computer: start and run. The start mode only functions during engine cranking and starting. The run mode only functions after the engine starts and during engine operation. The two modes never operate together.

Should a failure of the run mode of the computer occur, the system will go into a "limp-in'' mode. This will enable the operator to continue to drive the vehicle until it can be repaired. However, while in this mode, very poor engine operation will result. Should failure of the pickup coils or the start mode of the computer occur, the engine will not start.

The pickup coil signal is a reference signal. When the signal is received by the computer, the maximum amount of timing advance is made available. Based on the data from all the sensors, the computer determines how much of this maximum advance is needed at that instant.

The amount of spark advance is determined by two factors: engine speed and engine vacuum. However, when it happens depends on the following conditions:

  1. Spark advance based on engine vacuum is determined by the computer when the carburetor switch is open. The amount is programmed into the computer, and is proportional to the amount of engine vacuum and rpm.
  3. Spark advance based on engine speed is determined by the computer when the carburetor switch is open, and is programmed based on engine rpm.