Computers and Control Systems: Description and Operation

Fig.1 Electronically controlled Hitachi carburetor. Schematic.:

Fig.2 Electronically controlled Carter-Weber carburetor. Schematic.:

This system, Figs. 1 and 2, is used on 1982-1984 carburetted models and maintains the air/fuel ratio at nearly ideal conditions at all times, thereby controlling emissions while providing satisfactory driveability. The main components of this system are an oxygen sensor, electronic control module, duty solenoid valve, thermosensor, engine speed sensing circuit and two vacuum switches.
The vacuum switch (l), thermosensor, and engine speed sensing circuit assure better driveability by preventing application of feedback or closed loop operation during full throttle operation or engine operation when the coolant is below the specified value. The vacuum switch (ll), controls operation of duty solenoid valves.
The electronic control unit compares the exhaust gas at the exhaust manifold to the correct air/fuel ratio stored in it's memory. The air/fuel ratio is judged to be rich when the voltage of the oxygen sensor is higher than the specified level. At this time the ECU issues a signal to the duty solenoid valve(s) to increase air flow into the carburetor. If the air/fuel ratio is judged to be lean, the ECU will issue a signal to the duty solenoid valve(s) to reduce air flow to the carburetor, thus richening the mixture.
The solenoid valve repeats opening and closing at short intervals according to ECU signal outputs. The average flow rate of air passing through the valve to the carburetor air bleed depends upon the period of time during which the solenoid valve is opened, thus adjusting the air/fuel ratio. There are two duty solenoid valves used in the Hitachi carburetor system and both the valves are located on the intake manifold. The Carter-Weber system uses only one duty solenoid valve and it is positioned onto the carburetor.