Chrysler Full-Size Vans 1967-1988 Repair Guide

Description and Operation


The computer provides the engine with Ignition Spark Control during starting and during 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 and then directs the ignition control module to advance or retard the timing as necessary. This whole process is going 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 will only function during engine cranking and starting. The run mode only functions after the engine start and during engine operation. The two will 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 pick-up coils or the start mode of the computer occur, the engine will not start.

The pick-up 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. Advance from the vacuum will be given by the computer when the carburetor switch is open. The amount is programmed into the computer and is proportional to the amount of vacuum and engine rpm.
  3. Advance from speed is given by the computer when the carburetor switch is open and is programmed to engine rpm.


The computer consists of one electronic printed circuit board which simultaneously receives signals from all the sensors and within milliseconds, analyzes them to determine how the engine is operating and then advances or retards the ignition timing by signaling the ignition coil to produce the electrical impulses to fire the spark plugs at the exact instant when ignition is required.


The microprocessor is an electronic module located within the computer that processes the signals from the engine sensor for accurate engine spark timing. Its digital electronic circuitry offers more operating precision and programming flexibility than the voltage dependent analog system used previously.


The start and the run pick-up sensors are located inside the distributor, suppling a signal to the computer to provide a fixed timing point that is used for starting (start pick-up) and the second for normal engine operation (run pick-up). The start pick-up also has a back-up function of taking over engine timing in case the run pick-up fails. Since the timing in this pick-up is fixed at one point, the car will be able to run, but not very well. The run pick-up sensor also monitors engine speed and helps the computer decide when the piston is reaching the top of its compression stroke.

The two systems will not operate at the same time.


The coolant temperature sensor, located in the intake manifold, informs the computer when the coolant temperature reaches a predetermined operating level. This information is required when the engine is equipped with a feedback carburetor, to prevent changing of the air/fuel ratio with the engine in a non-operating temperature mode. Its signals to the computer also help to control the amount of spark advance with a cold engine.


The carburetor switch sensor is located on the end of the idle stop solenoid and tells the computer when the engine is at idle or off-idle. With the carburetor switch grounding out at idle, the computer cancels the spark advance and the idle control of the air/fuel ratio at the carburetor.


The vacuum transducer, located on the computer, monitors the amount of intake manifold vacuum present in the engine. The engine vacuum is one of the factors that will determine how the computer will advance/retard the ignition timing and with a feedback carburetor, change the air/fuel ratio.


The detonation sensor is mounted in the number two branch of the intake manifold and is tuned to the frequency characteristic of engine knocking. When detonation (knocking) occurs, the sensor sends a low voltage signal to the computer, which retards ignition timing in proportion to the strength and frequency of the signal. The maximum amount of retard is 11° for 1984 models and 20° for 1985 and later models. When the detonation has ceased, the computer advances timing to the original value.


The oxygen sensor is used when the engine is equipped with a feedback carburetor. The sensor is located in the exhaust manifold and through the use of a self-produced electrical current, signals the computer as to the oxygen content within the exhaust gases flowing past it. Since the electrical output of the oxygen sensor reflects the amount of oxygen in the exhaust, the results are proportional to the rich and lean mixture of the air/fuel ratio. The computer then adjusts the air/fuel ratio to a level that maintains the operating efficiency of the three-way catalytic converter and the engine.


The charge temperature switch is located in the No. 8 runner of the intake manifold. When the intake air temperature is below approximately 60°F., the CTS will be closed, allowing no EGR timer function or valve operation. The air injection air is switched to the exhaust manifold (upstream). The CTS opens when the intake air temperature is above approximately 60°F., thus allowing the EGR timer to time out, the EGR valve to operate and switches the air injection air to the catalytic converter (downstream).