S15/T15 Jimmy 2WD L4-119 1.9L VIN A 2-BBL (1983)
Engine Control Module: Description and Operation
Closed Loop Fuel Control System
The ECM generates a control signal to the vacuum control solenoid which controls the air/fuel ratio through vacuum signals. This signal continuously
cycles the solenoid between on and off time as a function of the input voltages from the sensors. The ECM control signal is selected from one of the
following operational modes:
1.
Inhibit Mode. No signal to vacuum control solenoid.
2.
Enrichment Mode. A fixed pre-programmed duty cycle signal to vacuum control solenoid.
3.
Open Loop Mode. A fixed pre-programmed duty cycle signal to vacuum control solenoid.
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 the voltage output of the oxygen sensor. When the exhaust is lean (low voltage), the ECM
commands the carburetor to enrich the mixture. When the exhaust is rich (high voltage), the ECM commands a leaner mixture.
The ECM can also store in memory the current duty cycle being used for either idle or off idle conditions. When the vacuum switch signals the ECM
that a specific condition exists, the ECM steps to the duty cycle last recorded for optimum operation.
The fuel cut solenoid valve, located in the carburetor, is controlled by electrical signals from the ECM. The ECM senses coasting conditions by means
of signals from and transmission gear position, clutch position and idle position switches, then opens circuit to mixture control solenoid whenever engine
speed is above a predetermined value. This causes fuel flow to carburetor to be discontinued. The circuit to the slow cut solenoid is opened only when
the following four conditions exist simultaneously:
1.
Transmission in gear.
2.
On manual transmission models, clutch engaged.
3.
With carburetor throttle valve at idle position.
4.
Engine speed exceeds specified limit.
