Dodge Aspen/Volare 1976-1980 Repair Guide

Lean Burn System



Click image to see an enlarged view

Fig. Fig. 1 Schematic of the Lean Burn ignition system

The Chrysler Corporation Lean Burn System, introduced in 1976 on the Cordoba, has been made available on a number of Aspen/Volare- models for 1978. This system is based on the principle that lower NO x emissions would occur if the air/fuel ratio inside the cylinder area was raised from its current point (15.5:1) to a much leaner point (18:1).

In order to make the engine workable, a solution to the problems of carburetion and timing had to be found since a lean running engine is not the most efficient in terms of driveability. Chrysler adapted a conventional carburetor to handle the added air coming in, but the real advance of the system is the Spark Control Computer mounted on the air cleaner.

Since a lean burning engine demands precise ignition timing, additional spark control was needed for the distributor. The computer supplies this control by providing an infinitely variable advance curve. Input data is fed instantaneously to the computer by a series of seven sensors located in the engine compartment which monitor timing, water temperature, air temperature, throttle position, 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 a thousandth of a second to complete a circuit from sensor to distributor.

The components of the system are as follows: Modified carburetor; Spark Control Computer, consistent of two interacting modules: the Program Schedule Module which is responsible for translating input data, and the Ignition Control Module which transmits data to the distributor to advance or retard the timing; and the following sensors.

Start Pick-up Sensor, located inside the distributor, supplies a signal to the computer providing a fixed timing point which is only used for starting the car. It 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 engine will be able to run, but not very well.

The Run Pick-up Sensor, also located in the distributor, provides timing data to the computer once the engine is running. It also monitors engine speed and helps the computer decide when the piston is reaching the top of its compression stroke.

Coolant Temperature Sensor, located on the water pump housing, informs the computer when the coolant temperature is below 150°.

Air Temperature Sensor, inside the computer itself, monitors the temperature of the air coming into the air cleaner.

Throttle Position Transducer, located on the carburetor, monitors the position and rate of change of the throttle plates. When the throttle plates start to open and as they continue to open toward full throttle, more and more spark advance is called for by the computer. If the throttle plates are opened quickly even more spark advance is given for about one second. The amount of maximum advance is determined by the temperature of the air coming into the air cleaner. Less advance under acceleration will be given if the air entering the air cleaner is hot, while more advance will be given if the air is cold.

Carburetor Switch Sensor, located on the end of the idle stop solenoid, tells the computer if the engine is at idle or off-idle.

Vacuum Transducer, located on the computer, monitors the amount of intake manifold vacuum present; the more vacuum, the more spark advance to the distributor. In order to obtain this spark advance in the distributor, the carburetor switch sensor has to remain open for a specified amount of time during which the advance will slowly build up to the amount indicated as necessary by the vacuum transducer. If the carburetor switch should close during that time, the advance to the distributor will be cancelled. From here the computer will start with an advance count-down. If the carburetor switch is reopened within a certain amount of time, the advance will continue from a point where the computer decides it should. If the switch is reopened after the computer has counted down to "no advance," the vacuum advance process must start over again.


When you turn the ignition key on, the start pick-up sends its signal to the computer which relays back information for more spark advance during cranking. As soon as the engine starts, the run pick-up takes over and receives more advance for about one minute. This advance is slowly eliminated during the one minute warm-up period. While the engine is cold (coolant temperature below 150° as monitored by the coolant temperature sensor), no more advance will be given to the distributor until the engine reaches normal operating temperature, at which time normal operation of the system will begin.

In normal operation, the basic timing information is relayed by the run pick-up to the computer along with input signals from all the other sensors. From this data the computer determines the maximum allowable advance or retard to be sent to the distributor for any situation.

If either the run pick-up or the computer should fail, the back-up system of the start pick-up takes over. This supplies a fixed timing signal to the distributor which allows the car to be driven until it can be repaired. In this mode, very poor fuel economy and performance will be experienced. If the start pick-up or the ignition control module section of the computer should fail, the engine will not start or run.


Click image to see an enlarged view

Fig. Fig. 2 An "under cap" view of the Lean Burn ignition distributor. Note the additional pick-up and wire assembly, and the lack of a vacuum canister

See the Non-Lean Burn Inspection section above.


Refer to , Emissions, for test procedures on Lean Burn equipped vehicles.

AutoZone Logo