Engine Control Module: Description and Operation

The electronic control unit compares the exhaust gas at the exhaust manifold to the correct air/fuel ratio. The air/fuel ratio is judged to be a rich mixture when the electromotive force of the oxygen sensor is higher than the specified level. At this time the ECU issues a signal to the solenoid valves to increase air flow into the carburetor. If the air/fuel ratio is judged to be lean, the electromotive force of the ECU is lower. The ECU will issue a signal to the solenoid valves which will correct the air/fuel ratio.
The solenoid valve repeats opening and closing at a short cycle according to ECU signal outputs. The average flow rate of air passing through the solenoid valve to carburetor air bleed depends upon the period of time during which the solenoid valve is opened, adjusting the air/fuel ratio.