Engine Control Module: Description and Operation

The ECM generates a controlling signal to the mixture control solenoid which controls carburetor air/fuel ratio. This ECM controlling signal is continuously cycling the solenoid between ON and OFF time (duty cycle) as a function of the inputs form the various sensors. By changing the duty cycle, the ECM controls the air/fuel ratio. The ECM controlling signal is selected from four operational models as follows.
1. Inhibit Mode-Fixed control signal to the mixture control solenoid.
2. Pre-wide open throttle (WOT) and WOT Mode-A pre-selected duty cycle is generated based on engine RPM.
3. Open Loop Mode-A pre-selected duty cycle is generated based on various operating parameters.
4. Closed Loop Mode-A calculated duty cycle is generated based on oxygen sensor and other sensor outputs.
During closed loop operation, the ECM monitors oxygen sensor voltage output. As sensor voltage output increases and passes through a predetermined ECM set point, the proportional gain immediately changes the duty cycle of the output signal. The duty cycle is further changed by the integral gain until sensor input voltage decreases and passes through the predetermined ECM set point. The selection of integral and proportional gain rates by the ECM is based upon engine operating conditions (engine RPM and oxygen sensor transition condition).
The ECM also stores, in an adaptive memory, the current duty cycle being used for either idle or off-idle conditions. For example, when the ECM senses a transition from idle condition to an off-idle condition, it immediately stops at the last recorded condition for stoichiometric (ideal air/fuel operation.)
The ECM outputs the signal to control the slow cut solenoid valve incorporated within the carburetor assembly.