computer-controlled carburetor uses a solenoid-operated valve to respond to commands from
the microcomputer (electronic control unit). The system uses various sensors to send
information to the computer that calculates how rich or lean to set the carburetor
air-fuel mixture. The system is also known as a computer controlled emission system which
consists of the following: oxygen sensor, temperature sensor, pressure sensor,
electromechanical carburetor, mixture control solenoid, computer, and idle speed actuator.
The function of each is as follows:
oxygen sensor, or exhaust gas sensor, monitors the oxygen content in the engine
exhaust. The amount of oxygen in the exhaust indicates the richness (low oxygen content)
or leanness (high oxygen content) of the air- fuel mixture. The sensor voltage output
changes with any change in oxygen content in the exhaust gases.
temperature sensor detects the operating temperature of the engine. Its resistance
changes with the temperature of the engine. The change in resistance allows the computer
to enrich the fuel mixture during cold engine operations.
manifold pressure sensor (MAP) measures intake manifold vacuum and engine load.
engine load or power output causes intake manifold vacuum to drop. The pressure sensor
then signals the computer with a change in resistance and current flow. As manifold
pressure drops, the computer increases the air-fuel mixture for added power. As the
manifold pressure increases, the computer makes the carburetor setting leaner for improved
electromechanical carburetor has both electrical and mechanical control devices. It
is commonly used with a computer control system.
mixture control solenoid alters the air-fuel mixture in the electromechanical
carburetor. Electrical signals from the computer activate the solenoid to open and close
air and fuel passages in the carburetor..
computer, also called the electronic control unit (ECU), uses sensor information to
operate the mixture control solenoid of the carburetor.
idle speed actuator is a tiny electric motor and gear mechanism that allows the
computer to change engine idle speed by holding the throttle lever in the desired
Many of the
components and sensors are also used in gasoline fuel injection systems, which we will
discuss later in this lesson.
computer-controlled carburetor, the air-fuel ratio is maintained by cycling the mixture
solenoid ON and OFF several times a second. Control signals from the computer are used to
meter different amounts of fuel out of the carburetor. When the computer sends a rich
command to the solenoid, the signal voltage to the mixture solenoid is in the OFF position
more than it is ON, causing the solenoid to stay open more. During a lean signal the
mixture solenoid has more ON time, causing less fuel to pass through the solenoid valve
and the mixture becomes leaner.
carburetor systems vary. For exact detail on a particular system, refer to the
manufacturers service manual, which will explain how the specific system functions.