DESCRIPTION
This feedback carburetor system uses closed loop feedback circuitry. The system has 3 basic components, the carburetor, feedback computer and an oxygen sensor in the exhaust manifold.
Note. Some 1987 vehicles have been retrofitted with 1988 carburetors. See NEW CARB SYSTEM - FUEL 002, JULY 1987 article in TSB/RECALLS/TECH INFO section.
The system operates the engine on leanest possible fuel mixture. During normal engine operation air/fuel mixture is controlled through fuel metering. Fuel metering is accomplished by the use of 3 solenoid operated on/off valves. The 3 valves (jet mixture, enrichment and deceleration) add or reduce the amount of fuel supply to the engine. Activation of on/off valve is controlled by length of time electrical current is supplied to solenoid. (Scheme 10)
Each solenoid operates at a fixed frequency. By varying the amount of time the solenoid is energized during the duty cycle, the air/fuel mixture to the engine can be precisely controlled. The duty cycle for each solenoid is controlled by the Electronic Unit (ECU) in response to signals from the vacuum switch, exhaust oxygen sensor, throttle position sensor, coolant temperature sensor, engine speed sensor and intake air temperature sensor.
OXYGEN SENSOR
The oxygen sensor is mounted in exhaust manifold ahead of catalytic converter. Output signal from the oxygen sensor varies with percent of oxygen in the exhaust stream. This signal is sent to ECU for use in controlling closed loop compensation of fuel delivery.
COOLANT TEMPERATURE SENSOR
The coolant temperature sensor is mounted in intake manifold. Sensor sends a signal to ECU which is used to determine control of fuel delivery and secondary air management.
ENGINE SPEED SENSOR
The engine speed signal is a pulse from ignition coil. Time between these pulses is used to calculate engine speed.
THROTTLE POSITION SENSOR
This is a potentiometer mounted to carburetor. The TPS sends throttle angle signal to ECU. This signal is used in controlling fuel delivery and secondary air management.
VACUUM SWITCH
The vacuum switch is mounted on instrument panel and is activated when throttle valve is at closed (idling) position. Vacuum switch signal is sent to ECU for use in controlling fuel delivery and secondary air management.
INTAKE AIR TEMPERATURE SENSOR
The intake air temperature sensor is mounted in air cleaner. Sensor measures temperature of intake air in air cleaner. This sensor signal is sent to ECU for use in controlling fuel delivery.
ELECTRONIC CONTROL UNIT (ECU)
The ECU is mounted in passenger compartment. ECU receives analog inputs from sensors and converts them into digital signals. These digital signals are processed by the ECU to compute fuel delivery, secondary air, deceleration spark and throttle opener managements.
FEEDBACK CARBURETOR AIR/FUEL CONTROL
The feedback carburetor air/fuel mixture is controlled by the ECU. The ECU monitors throttle position, engine speed, coolant temperature, intake air temperature and exhaust oxygen concentration. The ECU calculates fuel flow required to yield desired air/fuel ratios. Closed loop control is used to adjust fuel flow. Fuel flow is modified to account for special operating conditions such as hot or cold starts, acceleration and deceleration.
ADAPTIVE MEMORY CONTROL
During closed loop operation, ECU controls the duty cycle of jet mixture control solenoid valve. Duty cycle values of jet mixture control solenoid valve are from output signal of oxygen sensor.
SECONDARY AIR CONTROL
Air control valve vacuum is controlled by signal from secondary air control solenoid. The solenoid is controlled by ECU based on engine speed, idle position and coolant temperature. This valve sends air to exhaust manifold.
DECELERATION SPARK CONTROL
To decrease hydrocarbons emitted during vehicle deceleration, ignition timing is advanced by solenoid operated vacuum on distributor, changing vacuum supplied to valve from carburetor ported vacuum to intake manifold vacuum. Solenoid valve is controlled by ECU based on engine speed, and a dashpot delays throttle valve closure to its normal idling position.
IDLE-UP CONTROL
When tail switch or blower motor switch is on, engine idle speed is increased by changing throttle valve opening. To decrease hydrocarbons emitted during vehicle deceleration and to improve engine brake performance, ECU de-energizes solenoid valve which supplies manifold vacuum to throttle opener at or above preset engine speed. Throttle opener operation is suspended.
JET MIXTURE SYSTEM
The jet mixture system supplies fuel to engine through jet mixture passages and jet valves for best air/fuel mixture. This system is calibrated by jet mixture solenoid which responds to electrical impulse from ECU.
ENRICHMENT SYSTEM
The enrichment system consists of a metering jet and a solenoid-operated on/off enrichment valve which provides additional fuel for main metering system. The activation of on/off valve is controlled by length of time current is supplied to solenoid. Under heavy acceleration, heavy engine loads, cold start or warm-up, additional fuel is required. ECU energizes solenoid at pre-set duty cycles as additional fuel is needed.
FUEL CUT OFF SYSTEM
With ignition off, deceleration solenoid valve cuts off fuel flow to prevent engine run-on (dieseling). During deceleration, deceleration solenoid valve reduces fuel flow to decrease hydrocarbons and improve fuel economy.
DIAGNOSTIC CIRCUIT CHECK
Note. If battery terminals are disconnected while engine is running or when ignition switch is in "ON" position, malfunction of computer or damage to semi-conductors could result.
- Turn ignition switch to "OFF" position. Remove harness connectors "A" and "B" from Carburetor Control Unit (CCU). (Scheme 10)
- Set check switch of Feedback Carburetor Checker (09341-21000 ) to "OFF" position. (Scheme 11)
- Set select switch of checker to "A" position. (Scheme 12) Connect FBC wiring harness connector to connectors of FBC checker.
- Connect FBC connector to CCU connectors. Perform checks according to FBC Systems Check Chart.
- If checker shows variation from specifications, check defective sensor and electrical wiring. Repair or replace defective components.
- After repair or replacement of defective components, recheck system with checker. Set check switch of checker to "OFF" position.
- Set ignition switch to "OFF" position. Disconnect all harness connectors and reinstall in proper harness connectors.
Feedback Carburetor (FBC) System. Scheme 10
FBC Systems Check Procedure Chart. Scheme 11
Carburetor Control Unit Connectors. Scheme 12
FBC Checker & Harness Connectors. Scheme 13
FBC Checker. Scheme 14
Oxygen Sensor Checker. Scheme 15
Jet Mixture Solenoid Checker. Scheme 16
OXYGEN SENSOR TEST
After engine is at normal operating temperature, idle engine at 1300 RPM. Check voltage of oxygen sensor. It should be 0-1 volt and 2.7 volts pulsating. If voltage of oxygen sensor is not to specification, replace oxygen sensor.
JET MIXTURE SOLENOID (JMS)
Voltage of JMS checker should be 11 - 13 volts when ignition is on. After warming up engine, voltage should be 5 - 12 volts at idle. If the valve is not to specification, replace JMS valve. (Scheme 16)
THROTTLE OPENER
- Check that vacuum hoses and throttle opener control solenoid harness are connected. Disconnect vacuum hose at throttle opener nipple.
- Connect a hand vacuum pump to nipple of throttle opener. Install a tachometer to engine. Run engine at curb idle speed of 700-750 RPM.
- Apply 12.0 in. Hg vacuum to throttle opener nipple. Increase engine speed. If engine speed fails to increase, replace throttle opener/dash pot. (Scheme 17)
Throttle Opener. Scheme 17
THROTTLE OPENER CONTROL SOLENOID VALVE
Turn ignition switch to "OFF" position. Disconnect connector from throttle opener control solenoid valve. Check solenoid valve coil for continuity using an ohmmeter. Solenoid valve coil resistance should be 40 ohms. If open or short circuit is found in coil, replace solenoid coil.
SOLENOID VALVE
Disconnect solenoid valve connector. Check solenoid valve coil for continuity using an ohmmeter between terminals. (Scheme 18) If an open or short circuit is found, replace solenoid valve.
Solenoid Valve Connector. Scheme 18
TPS Connector. Scheme 19
| Valve | Ohms |
|---|---|
| Deceleration Solenoid (DSV) | 50.0 |
| Enrichment Solenoid (ESV) | 50.0 |
| Jet Mixture Solenoid (JSV) | 50.0 |
| Bowl Vent Solenoid (BVV) | 80.0 |
SOLENOID VALVE CONNECTOR TERMINALS
THROTTLE POSITION SENSOR (TPS)
Disconnect TPS connector at carburetor. Check resistance using an ohmmeter between terminals No. 2 and 3. (Scheme 16) Ensure that when throttle valve is slowly operated from closed to wide open throttle position, resistance changes smoothly. If resistance is not smooth, replace TPS.
| Throttle Position | Ohms |
|---|---|
| Closed | 1,200 |
| Wide Open | 4,900 |
THROTTLE POSITION SENSOR