The feedback carburetor system provides positive air/fuel ratio control for maximum reduction of emissions. The electronic control unit receives signals from various sensors and then modulates 2 solenoid valves installed on the carburetor to control the air/fuel ratio.
The ECU also controls the ignition timing, electric choke and the idle up solenoid by switching the solenoid valves ON and OFF.
See Figures 1, 2 and 3
The air/fuel ratio control is maintained by the ECU in 1 of 2 operating modes.Closed Loop Control (Feedback Control)
Closed loop control is used after engine warm-up. The air/fuel mixture is determined by the feedback control, based on the oxygen sensor signal.
Oxygen sensor output voltage changes sharply at the stoichiometric ratio (14.7:1). The control unit senses this signal and uses the feedback solenoid valve to regulate the ratio. By providing a stoichiometric ratio, the best purification rate of the 3-catalyst converter may be kept. In this state, the slow cut solenoid valve is kept wide open (100 percent duty).Open Loop Control
During engine start, warm-up, high load operation and deceleration, the air/fuel ratio is in open loop. Control is based on preset values contained in the ECU. These values are set by the factory and cannot be changed.
During deceleration, the slow cut solenoid valve limits fuel flow for better fuel economy and for prevention of overheating of the catalysts.
Electronic Control Unit (ECU)
The electronic control unit is mounted in the passenger compartment and consists of a printed circuit board mounted in a protective metal box. It receives analog inputs from sensors and converts them into digital signals. These signals and various inputs are processed and used by the ECU in controlling the fuel delivery, secondary air, deceleration spark and throttle opener management.Coolant Temperature Sensor
The coolant temperature sensor is installed in the intake manifold. This sensor provides data to the ECU for use in controlling fuel delivery and secondary air management.Throttle Position Sensor
The Throttle Position Sensor (TPS) is a potentiometer mounted to the carburetor. The TPS provides throttle angle information to the ECU to be used in controlling the fuel delivery and secondary management.Engine Speed Sensor
The engine speed sensor signal comes from the negative terminal voltage of the ignition coil. Electric signals are sent to the ECU, where the time between these pulses is used to calculate engine speed, which is used in controlling fuel delivery, secondary air management, deceleration spark and throttle opener management.Oxygen Sensor (Federal)
The oxygen sensor is mounted in the exhaust manifold. The output signal from the sensor, which varies with the oxygen content of the exhaust gas stream, is provided to the ECU for use in controlling closed loop compensation of fuel delivery.Oxygen Sensor (California)
The oxygen sensor is mounted in the exhaust manifold. The oxygen sensor used in California models contains a heater element to obtain closed loop in a shorter amount of time. The output signal from the sensor, which varies with the oxygen content of the exhaust gas stream, is provided to the ECU for use in controlling closed loop compensation of fuel delivery.Vacuum Switch
The switch is installed on the floor board or the fender and is electronically turned ON (switch closed) when the throttle valve is in the closed (idling) position. Information to this switch is provided from the ECU for use in controlling fuel delivery and secondary air management.Feedback Solenoid Valve
The feedback solenoid valve is installed in the carburetor float chamber cover. The ECU controls the air to fuel ratio by controlling the duty cycle of the valve. As the duty ratio increases, the mixture becomes leaner.Slow Cut Solenoid Valve
The slow cut solenoid valve is located in the carburetor float bowl chamber cover. The ECU controls the carburetor slow system fuel flow by controlling the duty cycle of the valve.Idle Up Solenoid
When the tail switch or the blower motor switch is ON, the engine idle speed is increased by changing the throttle valve opening. In order to decrease HC emissions emitted during vehicle deceleration and to improve engine performance under a load, the ECU de-energizes the solenoid valve which supplies the manifold vacuum to the throttle opener at or above the preset engine speed and then the throttle opener operation is suspended.Mixture Control Valve (MCV)
When the throttle is closed suddenly during deceleration or shifting, the fuel remaining in the intake manifold causes an over rich mixture temporarily. In order to prevent this, air is supplied from another passage to correct the air/fuel ratio and reduce HC emission. The increased manifold vacuum acts so air is supplied to the intake manifold. The vacuum is also supplied to chamber B but with some delay due to an orifice. When the vacuum is supplied to both chambers B and A , the spring causes the valve to close, stopping the supply of air. The check valve located at the diaphragm prevents high vacuum from remaining in chamber B during acceleration or deceleration. If a high vacuum remains in chamber B , the valve may fail to operate when vacuum acts on chamber A .