Feedback Control System

Fig. 43 Feedback Control Wiring and Vacuum hose Diagram:

Air-Fuel mixture ratio feedback on this vehicle is performed using the oxygen sensor, the Electronic Air Control Valve (EACV) and the control unit. The oxygen sensor, sends the signal to the control unit in order to judge whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio. The control unit receives other signals from the speed sensor, TW sensor, vacuum switch, ignition coil, MAP sensor, and TA sensor and sends the electric current to the EACV.
The EACV opens the air passage from the air cleaner case to the intake manifold in proportion to the intensity of the electric current received from the control unit.
This feedback system has four functions:

1. air fuel ratio control
2. shot air control
3. deceleration air supply
4. hot engine start control

1. Air-Fuel Ratio Control:
The system is designed to achieve a stoichiometric air-fuel mixture ratio making the most of the three-way catalytic converter performance to give a simultaneous reduction of hydrocarbons, carbon monoxide, and oxides of nitrogen. The carburetor air-fuel mixture is basically calibrated on the richer side of the stoichiometric ratio, and the air supply through the EACV dilutes the mixture for controlling the mixture close to the stoichiometric.
The system performs feedback function in most of the driving conditions based on the output from the oxygen sensor. However, the system stops this feedback function when the engine needs richer or leaner mixture for the operating condition, such as when the vehicle is in a power mode, or when the engine is warming up.

2. Shot Air Control:

The system provides air into the intake manifold to improve emissions performance and prevent afterburning due to the over-rich mixture during short deceleration.
The control unit receives signals of vehicle speed, engine coolant temperature, intake manifold vacuum and engine speed. And shot air is induced from the EACV when the manifold vacuum increases suddenly except when the vehicle is moving at a very low speed with the engine coolant temperature is below the normal operating level.
The amount of air supplied into the intake manifold depends on the amount of the manifold vacuum increase.

3. Deceleration Air Supply:

This system is designed to improve emission performance by supplying air into the intake manifold during deceleration at relatively high engine speed.
The control unit receives signals from the MAP, TW sensor, speed sensor, vacuum switch, gear position switch, and ignition coil, and identifies driving conditions for deceleration air supply. The control unit transmits the electric current to the EACV which opens and supplies air into the intake manifold.

4. Hot Engine Start Control:

This system is designed to provide air into the intake manifold for engine starting when engine coolant temperature is very high.
The control unit receives the signal of engine coolant temperature. When it is higher than the normal temperature, the EACV is activated to supply air into the intake manifold before the vehicle speed exceeds the set speed.