Dodge Colt/Challenger/Conquest/Vista 1971-1989 Repair Guide

Feedback Carburetor (FBC) System

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DESCRIPTION



The feedback carburetor system provides the capability to perform closed loop fuel control (adjusting fuel/air mixture to meet all driving conditions). It also provides the capability to control the secondary air system, the deceleration spark control system and the throttle opener system.

Input signals from a variety of sensors are fed to a microprocessor based electronic control unit (ECU). The ECU the generates output signals for all of the control functions.

The feedback carburetor is a two barrel, downdraft carburetor designed for closed loop system. When used in the closed loop system of mixture control, the carburetor includes special design features for optimum air/fuel mixtures during all ranges of engine operation. Fuel metering is accomplished through the use of three solenoid operated on/off valves (jet mixture, enrichment and deceleration solenoids), adding or reducing fuel to the engine.

The activation of the on/off valve is controlled by the length of the time current is supplied to the solenoid. The solenoid operates at a fixed frequency. By varying the amount of time the solenoid is energized during each cycle (defined as duty cycle), the air/fuel mixture delivered to the engine can be precisely controlled. The duty cycle to the solenoid is controlled by the ECU in response to the signals from the exhaust oxygen sensor, throttle position sensor, coolant sensor, engine speed and other sensors.

Incorporated in the feedback carburetor are eight basic systems of operation: fuel inlet, primary metering, secondary metering, accelerating pump, choke, jet mixture, enrichment and fuel cutoff.

OPERATION



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 the sensors and converts them into digital signals. These signals and various discrete inputs are processed and used by the ECU in controlling fuel delivery, secondary air, deceleration spark and throttle opener managements.

Air/Fuel Control

The feedback carburetor air/fuel control ratio is controlled by the ECU. The ECU monitors the throttle position, engine speed, coolant temperature, intake air temperature, and exhaust oxygen concentration to calculate the fuel flow required to yield the desired air/fuel ratios for all operating conditions. Closed loop control is used to adjust the fuel flow to yield a near stoichiometeric air/fuel ratio (optimum of 15:1) when required. The fuel flow is modified to account for special operating conditions, such as hot starts, acceleration and deceleration.

Adaptive Memory Control

During closed loop operation, the ECU controls the duty cycle of the jet mixture control solenoid, based on the output voltage signal from the exhaust oxygen sensor. The mean values of the duty cycle are stored in a Random Access Memory (RAM) and the last values are stored, even when the ignition is turned OFF.

Secondary Air Control

A solenoid is used to control the air control valve signal vacuum. The solenoid is controlled by the ECU, based on the engine speed, idle position and coolant temperature. The valve sends air to the exhaust manifold.

Deceleration Spark Control

In order to decrease the hydrocarbon (HC) emissions during vehicle deceleration, ignition timing is advanced by the solenoid operated vacuum valve on the distributor. The valve changes the vacuum to intake manifold vacuum. The solenoid is controlled by the ECU, based on engine speed.

Exhaust Oxygen Sensor

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.

Coolant Temperature Sensor

The coolant temperature sensor is installed in the intake manifold. The sensor provides data to the ECU for use in controlling fuel delivery and secondary air management.

Engine Speed Sensor

The engine speed sensor signal comes from the ignition coil. Electric signals are sent to the ECU, where the time between pulses is used to calculate engine speed. Engine speed is used in controlling fuel delivery, secondary air management, deceleration spark and throttle opener management.

Throttle Position Sensor

The throttle position sensor is a carburetor mounted potentiometer. The TPS provides throttle angle information to the ECU to be used in controlling the fuel delivery and secondary air management.

Vacuum Sensor Switch

The switch is mounted on the floor board or the inside fender and is turned on when the throttle valve is in the closed (engine idling) position. Information from the switch is provided to the ECU for use in controlling fuel delivery and secondary air management.

Intake Air Temperature Sensor

The sensor is located in the air cleaner. The function of this sensor is to measure the air intake temperature. The temperature information is provided to the ECU for use in controlling fuel delivery.

Jet Mixture System

The jet mixture system supplies fuel to the engine through jet mixture passages and jet valves for optimum air/fuel mixtures. This system is calibrated by the jet mixture solenoid, which responses to an electrical impulse from the ECU. If the oxygen sensor detects a lean condition, the ECU energizes the solenoid at increasing duty cycles to enrich the mixture. If the oxygen sensor detects a rich condition, the solenoid receives a signal from the ECU to decrease the duty cycle to lean out the mixture. Thus, the solenoid is constantly responding to an electrical signal from the ECU to provide efficient control of the air/fuel mixture.

Enrichment System

The enrichment system consists of a metering jet and an enrichment solenoid operated on/off valve, which constantly provides additional fuel for the main metering system. The activation of the on/off valve is controlled by the length of time the current is supplied to the solenoid. When additional fuel is required, such as under heavy acceleration, heavy engine loads, cold start or warm up operation, the ECU energizes the solenoid at preset duty cycles.

 
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