Some computer-controlled carburetors have an O2 feed-back solenoid that allows air into the idle system or main system to control the air/fuel ratio. The computer uses a duty-cycle principle to control this solenoid. When the O2 sensor signal indicates a rich air/fuel ratio, the computer energizes the solenoid longer, which allows more air into the idle or main system to make the air/fuel ratio leaner. Conversely, if the O2 sensor signal is low indicating a lean air/fuel ratio, the computer reduces the solenoid on time and shuts off some of the airflow into the idle or main system to provide a richer air/fuel ratio. Other computer-controlled carburetors have a stepper motor that controls the airflow past a tapered valve on the motor stem into the main system. This stepper motor is popular on some variable venturi carburetors. A lean O2 sensor signal causes the computer to move the stepper motor stem and valve inward to close off some airflow to provide a richer air/fuel ratio. If the O2 sensor indicates a rich air/fuel ratio, the computer moves the stepper motor stem outward and allows more air flow into the main system to make the air/fuel ratio leaner. Since a certain amount of fuel is moving through the main system with the engine idling, the stepper motor controls airflow only into the main system.
ELECTRONIC IDLE-SPEED CONTROL
In order to maintain federally mandated emission levels, it is necessary to control the idle speed. Most feedback systems operate in open loop when the engine is idling. To reduce emissions during idle, most feedback carburetors idle faster and leaner than non feedback carburetors. To adjust idle speed, many feedback carburetors have an idle speed control (ISC) motor that is controlled by an electronic control module. The ISC motor is a small, reversible, electric motor. It is part of an assembly that includes the motor gear drive, and plunger. The direction of motor rotation dictates whether the plunger moves in or out. The ISC motor is mounted so that the plunger can contact the throttle level. The PCM controls the ISC motor and can change the polarity applied to the motor's armature in order to control the direction in which it turns. When the idle tracking switch is open (throttle closed), the PCM commands the ISC motor to control idle speed. The ISC provides the correct throttle opening for cold or warm engine idle.
Based on the input signals from the system's sensors, the PCM increases the curb idle speed if the coolant is below a specific temperature, if a load (such is air conditioning, transmission, or power steering) is placed on the engine, or when the vehicle is operated above a specific altitude. During closed choke idle, the fast-idle cam holds the throttle blade open enough to lift the throttle linkage off the ISC plunger. This allows the ISC switch to open so that the PCM does not monitor idle speed. As the choke spring allows the fast-idle cam to fall away and the throttle returns to the warm idle position, the PCM notes the still-low coolant temperature and commands a slightly higher idle speed. As the engine warms up, the plunger is retracted by the PCM. If the A/C compressor is turned on, the PCM extends the plunger a certain distance to increase engine idle speed to compensate for the added load. When the throttle is opened and the lever is no longer in contact with the plunger, an idle tracking switch (ITS) at the end of the plunger signals the PCM. The PCM then fully extends the plunger. During deceleration, the plunger makes contact with the lever and the unit acts like a dashpot. The throttle lever slowly closes. When the engine is shut down, the plunger retracts, preventing the engine from dieseling. It then extends for the next engine startup. In some systems, if the engine starts to overheat, the PCM commands a higher idle speed to increase coolant flow. Also, if system voltage falls below a predetermined value, the PCM commands a higher idle speed in order to increase AC generator speed and output.
Throttle Kicker and Idle Stop Solenoid
Some computer-controlled carburetors have a vacuum- operated throttle kicker and an idle stop solenoid. The throttle kicker maintains engine idle speed when the engine accessory load is increased, such as during A/C compressor clutch operation. This kicker also maintains idle speed during warm-up, after the fast idle cam has dropped away from the fast idle screw. Vacuum to the throttle kicker is controlled by an electric solenoid, which in turn is controlled by the power train control module (PCM). One end of the idle stop solenoid winding is connected to a terminal on the solenoid housing, and the other end of this winding is connected to ground on the solenoid housing. The idle stop solenoid terminal is connected to the ignition switch. When the ignition switch is turned on, current flows through the solenoid winding to ground. The magnetic field from this current flow pulls the solenoid plunger outward. Since the solenoid plunger contacts the throttle linkage, this plunger action moves the throttle linkage to provide the specified idle rpm. When the ignition switch is turned off, the idle stop solenoid plunger retracts and allows the throttle to move toward the closed position until the linkage contacts the throttle stop screw. This action prevents the engine from dieseling. On some computer-controlled carburetors, the throttle kicker and idle stop solenoid are combined in one unit, but the operation of both components is basically the same.
Vacuum-operated Throttle kicker
When a vacuum-operated throttle kicker (TK) and solenoid are used to control idle speed, the TK stem pushes against the throttle linkage. Under these operating conditions, the PCM energizes the solenoid and manifold vacuum is supplied to the TK diaphragm:
(1) During engine warm-up,
(2) When the air conditioning (A/C) is turned on, and
(3) When the engine begins overheating.
During other engine operating conditions, the TK solenoid is de-energized and vacuum is shut off to the TK diaphragm. Under this condition, the TK diaphragm stem is retracted and the idle speed is controlled by the idle speed adjustment screw.
Temperature Compensated Pump
The temperature compensated pump (TCP) solenoid is operated by the PCM. When the engine coolant temperature is cold, the TCP solenoid is de-energized, and this solenoid shuts off vacuum to the TCP diaphragm. When this action is taken, the TCP diaphragm seats the ball check and full accelerator pump output is available for improved cold acceleration. Once the engine coolant temperature is warmed up, the PCM energizes the TCP solenoid, which supplies vacuum to the TCP diaphragm
Variable Voltage Choke Relay
The variable voltage choke (WC) relay is operated by the PCM and connected to the electric choke cover terminal. If the engine coolant temperature and intake air temperatures are cold, the PCM energizes the choke relay and supplies voltage to the choke heater in the choke cover
every 2.5 seconds. This action reduces current to the choke heater and slows the choke opening. As the engine coolant and intake air temperature increase, the PCM increases the choke relay duty on time, which increases current to the choke heater and speeds up the choke opening. When the intake air temperature reaches 80°F (26.6°C), the PCM energizes the choke relay at l0% on time, and current is supplied to the choke heater continually to ensure that the choke continues to open.
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