Computers and Control Systems: Description and Operation

Fig. 46 Air bleed (carburetor feedback) system schematic.:

Fig. 47 Air bleed (carburetor feedback) system.:

3A-C, 3E, 4A-C & 4A-LC ENGINES

This system Figs. 46 and 47, is used on Corolla and Tercel. An electronic control unit receives a signal from the oxygen sensor and uses this information to control carburetor primary side main air bleed and slow air bleed volume to maintain optimum air/fuel mixture in accordance with existing driving conditions. This reduces the emission of hydrocarbons, carbon monoxide, and nitrous oxides. This system also improves driveability and fuel economy.
On 3A-C, 4A-C and 4A-LC engines, when coolant temperature is below 45°F, the thermal vacuum switching valve passage between ``J'' and ``L'' is open, vacuum switch B and the computer are off, the electric air bleed control valve is off, and the air bleed system is off.
On 3A-C, 4A-C and 4A-LC engines, when coolant temperature is above 63°F, the thermal vacuum switching valve passage between ``K'' and ``L'' is open. At idle with engine RPM below 1300, the computer is off, the electric air bleed control valve is closed, and the air bleed system is off.
On 3E engines, with a coolant temperature of below 41°F, BVSV passages ``K'' and ``L'' is open, throttle position switch is on, vacuum switch is off, computer is off, EBCV is closed and air bleed is off. With engine idling and coolant temperature above 66°F, BVSV passages ``K'' and ``J'' is open, throttle position switch is off, computer is off, EBCV is closed and the air bleed is off.
Under normal driving conditions, when engine RPM is above 1500, vacuum switches A and B are on. When there is a rich air/fuel mixture in the exhaust manifold, the computer is on, the electric air bleed valve is open, and there is feedback air bleed. When there is a lean air/fuel mixture in the exhaust manifold, the computer is off, the electric air bleed valve is closed, and there is feedback air bleed.
Under heavy loads, when intake manifold vacuum is below 3.35 inches Hg, vacuum switch ``A'' is on, vacuum switch ``B'' is off, the computer is off, the electric air bleed valve is closed, and the air bleed system is off.
During deceleration above 1500 RPM, vacuum switch ``A'' is off, vacuum switch ``B'' is on, the computer is on, the electric air bleed valve is open, and the air bleed system is open.