Feedback Carburetor System — Dodge Ram I (1981-1993)

DESCRIPTION

The Feedback Carburetor System (FBC) is a computerized air/fuel ratio control system. System consists of an Electronic Control Unit (ECU), a specially designed 2-barrel downdraft carburetor, various sensors, and other components. The ECU utilizes electronic signals from various sensors and switches to precisely control air/fuel mixture ratio in the carburetor. The system is designed to lower exhaust emissions while maintaining good economy and driveability.

OPERATION

During engine start-up, warm-up operation, high load operation and deceleration, the air/fuel ratio is open loop controlled by values established previously for engine speed, throttle valve angle and coolant temperature. After engine warm-up, air/fuel ratio control is made by closed loop control, based on oxygen sensor signal. The ECU controls air/fuel mixture by controlling the Feedback Solenoid Valve (FBSV) and the Slow-Cut Solenoid Valve (SCSV) mounted on the carburetor.

COOLANT TEMPERATURE SENSOR

The coolant temperature sensor is a thermistor which converts the temperature of engine coolant to an electrical signal for use by ECU. The ECU uses coolant temperature information to provide optimum fuel enrichment when engine is cold. Coolant temperature information is also used for electric choke relay control and secondary air control.

ENGINE SPEED SENSOR

The engine speed signal is received from ignition coil. Electrical signals from ignition coil are sent to ECU where time between signals is used to calculate engine speed. This information is used by ECU to control air/fuel ratio, throttle opener control, electric choke relay control and secondary air control.

OXYGEN SENSOR

The oxygen sensor is located in the exhaust manifold. Output voltage of oxygen sensor varies with oxygen content in exhaust gas stream. When oxygen sensor signal is being interpreted by ECU, information is used to control air/fuel ratio.

THROTTLE POSITION SENSOR

The sensor, a rotary potentiometer, signals ECU of changes in throttle valve angle. This information is used for controlling air/fuel mixture.

VACUUM SWITCH

The vacuum switch is a contact type that is operated by intake manifold vacuum. When throttle valve closes, intake manifold vacuum closes switch. At this point, voltage on ECU side is grounded and the ECU senses throttle valve opening is near idle position. This information is used for controlling air/fuel mixture at idle with secondary air control.

FBC SYSTEM

Turn ignition switch to "LOCK" position. Remove large and small harness connector from ECU. Set check switch of ECI checker to "OFF" position. Set select switch of ECI checker to "A" position. Connect FBC harness connector to ECI checker and then connect to ECU harness connectors. (Scheme 27)
Perform checks as outlined in «OUTPUT SIGNAL CHARTS»(/dodge/ram/i-1981-1993/remont/testing-diagnostics/#feedback-carburetor-system). If checker varies from specifications, check corresponding sensor and related electrical wiring, and repair as necessary. See «COMPONENT TESTING & INSPECTION»(/dodge/ram/i-1981-1993/remont/testing-diagnostics/#feedback-carburetor-system__component-testing-inspection). Recheck with ECI checker to confirm repair has corrected problem.

ECI Checker & FBC Harness. Scheme 27

Scheme 27: ECI Checker & FBC Harness

COMPONENT TESTING & INSPECTION

CAUTION

Ensure ignition switch is in "OFF" position. Before removing or installing a part, disconnect battery terminal.

Remove sensor from intake manifold. Immerse sensor portion in water. Hold sensor housing .12" (3 mm) away from surface of water. Gradually heat water and read resistance. See COOLANT TEMPERATURE SENSOR RESISTANCE table. If resistance deviates greatly from specification, replace coolant temperature sensor.

Temperature °F (°C)	Resistance (Ohms)
32 (0)	5900
68 (20)	2500
104 (40)	1100
176 (80)	300

COOLANT TEMPERATURE SENSOR RESISTANCE

Check for continuity between ignition coil terminal and appropriate ECU terminal. If continuity exists, sensor is functioning.

Warm engine until coolant temperature is 185-205°F (85-95°C). Remove the oxygen sensor connector and the connect voltmeter. With engine above 1300 RPM, measure output voltage. Voltage should be approximately one volt. If not, replace oxygen sensor.

Disconnect carburetor connector. Measure resistance between terminals No. 1 (power) and No. 8 (ground) on Raider and Ram-50. Measure resistance between terminals No. 1 and No. 3 on Colt. (Scheme 28) Standard resistance is 3500-6500 ohms.
Connect ohmmeter between terminals No. 2 (output) and No. 8 on Raider and Ram-50. Connect ohmmeter between terminals No. 3 and No. 2 on Colt. Operate throttle valve slowly from idle position to full open position. Check that resistance changes smoothly in proportion to throttle valve opening. Resistance should change from 500 ohms to 3500-6500 ohms.
If resistance is out of specification or does not change smoothly, replace the throttle position sensor.

Front View of Throttle Position Sensor Connector. Scheme 28

Scheme 28: Front View of Throttle Position Sensor Connector

Remove Green-striped vacuum hose from device box. Connect vacuum pump to device box nipple. Disconnect vacuum switch connector. Apply vacuum and check for continuity between switch terminals No. 1 and No. 4 on Raider and Ram-50. (Scheme 29) On Colt, check for continuity between switch terminals. At 7.7 In. Hg of vacuum, there should be no continuity. At 11.9 In. Hg of vacuum, there should be continuity.

View of Vacuum Switch Connector. Scheme 29

Scheme 29: View of Vacuum Switch Connector

Feedback Solenoid Valve (FBSV) & Slow-Cut Solenoid Valve (SCSV)

Disconnect connector at carburetor. Apply battery voltage between solenoid valve terminals (No. 4 and No. 5 or Yellow and Red for FBSV, No. 3 and No. 6 or Blue and Red for SCSV). (Scheme 30) Solenoid valve should click. If no click is heard, replace solenoid valve.