Air/Fuel Ratio Control System

There are two ways to control the air-fuel ratio. These are open loop control and closed loop control, which are switched back and forth by the signal of water temperature, engine rpm, battery voltage, exhaust gas sensor signal and manifold vacuum.





OPEN-LOOP CONTROL
For improved driveability, air-fuel ratio solenoid is controlled by open-loop control under the following conditions:
- When starting engine
- When engine is cold
- When hot-restarting
- During deceleration
- When driving at high speeds
- At full throttle
With open-loop control, the air-fuel ratio is determined by the E.C.C. control unit, corresponding to the condition of engine rpm, engine warm-up, battery voltage, exhaust gas sensor signal and manifold vacuum.

CLOSED-LOOP CONTROL
(Air-fuel ratio feedback control)
This system is designed to control the air-fuel ratio precisely to the stoichiometric point so that the three-way catalyst can minimize CO, HC and NOx emissions simultaneously. The system uses the exhaust gas sensor located in the exhaust manifold to give an indication of whether the air- fuel ratio is richer or leaner than the stoichiometric point. The sensor transmits a nonlinear voltage to the E.C.C. control unit. The control unit adjusts the feedback pulse width according to the sensor voltage so the mixture ratio will be within the narrow window of the three-way catalyst. During engine warm-up period, however, this system remains open until the sensor reaches the operating temperature.








AIR/FUEL RATIO CONTROL SOLENOID VALVE
This solenoid valve is opened or closed repeatedly every 65 msec.
Under closed loop control, the percentage of opening time and closing time is feedback controlled in E.C.C. control unit by the signal from exhaust gas sensor.
Under the open loop control, the percentage of opening time and closing time is kept at the specified level according to condition of engine.

E.C.C. CONTROL UNIT
The E.C.C. control unit consists of a micro-computer, connectors for signal input and output and power supply, and an exhaust gas sensor monitor lamp. The control unit controls the feedback pulse width and fuel shut-off operation, etc.
For Non-California model, a barometric pressure sensor is installed in order to correct the mixture ratio according to the elevation.





WARNING:
- When installing CB ham radio equipment or a vehicle phone, be sure to observe the following as it may adversely affect electronic control systems depending on its installation location.
1) Keep the antenna as far as possible away from the electronic control units.
2) Also keep the antenna feeder line more than 20 cm (7.9 in) away from the harness of electronic controls. Do not let them run parallel for a long distance.
3) Adjust the antenna and feeder line so that the standing-wave ratio can be kept smaller.





EXHAUST GAS SENSOR
The exhaust gas sensor (02 -sensor), which is built into the exhaust manifold, monitors the density of oxygen in the exhaust gas.
It consists of ceramic titania which electric resistance drastically changes at the stoichiometric air-fuel ratio.