Feedback Carburetor System Honda Civic III

DESCRIPTION

All Honda models use a feedback carburetor system (except Civic 1300 and 1500HF). The feedback carburetor system maintains the proper air/fuel ratio during engine operation. Air control valves are used to by-pass additional air into the intake manifold to adjust a temporarily rich mixture condition. The system consists of 2 feedback systems, the "X" and "M" systems.

The "X" system consists of an air control valve B, frequency solenoid B, check valves A, B, C and E, surge tanks A and B, and the control unit. Surge tank A is used as a reservoir. Surge tank B is used to dampen vacuum pulses to the air control valve B.

The "M" system consists of a check valve A (Civic 1500 and Prelude), air control valve A, frequency solenoid valve A and the control unit.

OPERATION

The "X" system is the main feedback system. It controls air/fuel mixture by controlling the opening of air control valve B according to the amount of vacuum at the diaphragm chamber of the valve. The frequency solenoid B is modulated by the control unit, and controls the amount of vacuum to air control valve B. On Civic models, a feedback control solenoid is provided to stop feedback operation under low engine speed and cold engine temperature.

Mixture is more finely controlled by the "M" system, based on engine load. The diaphragm chamber of air control valve A receives the same carburetor vacuum as the EGR control valves A and B, and air is supplied to the intake manifold in proportion to the carburetor intake air volume. Frequency solenoid valve A receives signals from the oxygen sensor through the control unit and opens the valve when air/fuel mixture is richer than pre-set (stochiometric) mixture.

Scheme 32

Scheme 33

1. Ensure engine is at normal operating temperature. On Civic 1500 models, disconnect vacuum hose from the power valve at the carburetor and plug the hose. (Scheme 32)and (Scheme 35).
2. On 1500 models, disconnect vacuum hose from the power steering, remove cover from control box No. 2. On all models, start engine and check that frequency valves A and B are operating while running engine at 2500 RPM. NOTE: Civic 1500 models equipped with auto. trans. and power steering, frequency solenoid valves are located outside of control box No. 2. Hose No. 6 leads to valve A and hose No. 21 leads to valve B. (Scheme 32): View of Civic 1500 Power Valve Vacuum Hose
3. Frequency valves should emit a clicking sound. It may be easier to place hand on each individual valve to feel movement. If either solenoid A or B is inoperable, replace valve and retest. If both solenoid valves operate properly, proceed to step 6). (Scheme 33): Civic 1500 Control Boxes Nos. 1 & 2
4. If either solenoid A or B are inoperable, check for voltage at terminal lead wires while engine is running at 2500 RPM. If voltmeter fluctuates between 12 volts and 0 volts, replace solenoid valves and retest.
5. If voltmeter indicates 0 volts, proceed to TROUBLE SHOOTING. If voltmeter indicates 12 volts and engine operating condition is abnormal, or the engine stops, proceed to TROUBLE SHOOTING.
6. Reconnect vacuum hoses. Then, with A/C, radiator fan, heater blower motor, rear window defogger and headlights off, ensure that frequency solenoid valves are inoperable while at engine is at idle speed.
7. If solenoid valves are inoperable, the system is okay and testing is complete. If solenoid valves do operate, proceed to TROUBLE SHOOTING.

Scheme 34

1. Disconnect vacuum hose No. 29 (Civic) from air control valve A and connect a vacuum pump. Disconnect the air hose between air control valve A and frequency solenoid valve A. Connect a hand vacuum pump and pressure gauge to the hose. (Scheme 34) (Scheme 34): View of Frequency Solenoid Valves Other models similar.
2. When vacuum is applied to vacuum hose side, air should flow without resistance and the pressure gauge should not register any build up. When vacuum is released, air should flow with resistance and pressure should register on gauge and be released gradually. If air control valve fails either test, replace it.

Checking Air Control Valves Other models similar. Scheme 35

Scheme 36

1. Disconnect air suction hose at vacuum hose manifold of control box No. 2. Blow air into the hose using a hand vacuum pump, T-fitting and pressure gauge. (Scheme 36) (Scheme 36): Testing Air Control Valve B on Civic 1500 Models
2. Air should flow and pressure gauge should register a build up. If no air will pass through, proceed to steps 4) and 5). If air passes through, proceed to step 3).
3. Remove control box cover. Pinch off the air suction hose to frequency solenoid valve A. Blow air into air suction hose. If air flows through, replace the air control valve B and retest. If air does not flow, replace frequency solenoid A and retest.
4. Disconnect vacuum hose No. 24 from surge tank B and connect a vacuum pump to hose. Disconnect hose between air control valve B and frequency solenoid valve A and connect a vacuum pump (on pressure side) to the hose.
5. Pinch off hose routed from the feedback control solenoid valve. Apply vacuum to air control valve B (diaphragm side) and blow into air control valve B (valve side). If air passes through, valve is okay. If air does not pass through, replace air control valve B and retest.

Power Steering Models

1. Disconnect vacuum hose No. 24 from air control valve B and connect a hand vacuum pump to the valve. Disconnect air hose No. 6 from air control valve B and connect a pressure gauge and hand vacuum pump. Blow air into air control valve B.
2. When vacuum is applied to the diaphragm of air control valve B, air should flow and pressure gauge should not register any build up. When vacuum is released, air should not flow and pressure gauge should register build up. If the valve fails either test, replace it and retest.

Checking Frequency Solenoid Valve A Accord model shown; Prelude model similar. Scheme 37

Civic 1500

1. Disconnect air hose between air control valve A and frequency solenoid valve A at the control valve and blow air from the air hose. Air should not flow.
2. If air flows, apply battery voltage to frequency solenoid valve A (control box number 2) Green/White wire and blow air from the air hose. If air flows, valve is okay. If air does not flow, replace frequency solenoid valve A and retest.

Note. For power steering models, frequency solenoids are located outside of control box No. 2.

Checking Frequency Solenoid Valve B Accord model shown; Prelude model similar. Scheme 38

1. Disconnect hose No. 10 from surge tank A (rear tank) and plug the hose. Disconnect vacuum hose No. 23 from surge tank B (front tank) and connect a vacuum pump. Apply battery voltage to frequency solenoid valve B and draw vacuum from hose No. 23.
2. Vacuum should hold. If vacuum holds, proceed to step 3). If vacuum does not hold, replace frequency solenoid valve B and retest.
3. Disconnect 12 volts from frequency solenoid valve B. Vacuum should bleed off. If vacuum bleeds off, reconnect vacuum hoses Nos. 10 and 23 and proceed to FEEDBACK CONTROL SOLENOID VALVE test. If vacuum holds, replace frequency valve B and retest.

Checking Constant Vacuum Valve. Scheme 39

FEEDBACK CONTROL SOLENOID VALVE

Note. Air control valve B must be functional before checking the feedback control solenoid valve.

1. Disconnect vacuum hose No. 24 from surge tank B and connect a vacuum pump to the hose. Apply battery voltage to the feedback control solenoid valve (Control Box No. 1, Blue/Red wires) and draw vacuum.
2. Vacuum should hold. If vacuum does not hold, replace the feedback control solenoid valve and retest. If vacuum holds, disconnect battery voltage from solenoid valve. Vacuum should bleed off.
3. If vacuum bleeds off, feedback control solenoid valve is okay. Reconnect vacuum hose No. 24. If vacuum does not bleed off, replace feedback control solenoid and retest.

CONTROL UNIT CONNECTOR

1. On all models, check for voltage at control unit connector Blue wire with ignition switch "ON". There should be voltage. If there is no voltage, check Blue wire between ignition coil and control unit.
2. On all models, check for voltage at control unit connector Black/Yellow wire with ignition switch "ON". There should be voltage. If there is no voltage, check wiring and ignition fuse.
3. Check Brown/Black (Civic Models) wire for continuity between control unit and body ground. There should be continuity.

INTAKE AIR TEMPERATURE SENSOR

Check for continuity when air temperature is below 40°F (4.5°C). Check for no continuity when air temperature is above 73°F (40°C). Replace sensor if sensor does not operate as described.

OXYGEN SENSOR

1. Push a voltmeter positive test lead into the control unit connector White wire (Civic). Connect the negative test lead to body ground.
2. Warm engine to normal operating temperature. Raise engine speed several times. Voltmeter should read 0.5 volts or higher. If not, check wiring and fuse. If no problem is found, replace oxygen sensor and retest.

SPEED SENSOR

1. Jack up and support the front of vehicle. Push the voltmeter or test light positive probe into the Yellow/Black (Civic) wire terminal at the control unit connector.
2. Connect the negative probe to ground. Start engine. Place vehicle in 2nd gear and accelerate slowly. Battery voltage should be as shown in table. SPEED SENSOR VOLTAGE CHECK Speed Voltage Below 6 MPH Yes Above 12 MPH No
3. If voltage does not correspond to above MPH ranges, install a new speed sensor and retest. If there was no voltage during step 2), check for bad electrical connection, fuse or faulty speed sensor. Replace or repair as necessary and retest.

Civic 1500 (Thermosensor A)

Remove terminal leads from thermosensor. With engine block temperature below 167°F (75°C), check for continuity. Continuity should exist. With engine block temperature above 167°F (75°C), no continuity should exist. If sensor does not operate as described, replace thermosensor.

Civic 1500 (Thermosensor B)

Remove terminal leads from thermosensor. With cylinder head temperature below 149°F (65°C), check for continuity. It should exist. With cylinder head above 149°F (65°C), no continuity should exist. If sensor does not operate as described, replace thermosensor.

1. Disconnect vacuum hose to the vacuum switch and connect a vacuum pump to the hose. Apply vacuum and check for voltage between the vacuum switch Blue/Black (for hatchback and sedan models with man. trans.) or Black/Orange (for all other models) terminal lead and body ground.
2. When less than 1.2 in. Hg vacuum is applied voltage should be indicated on voltmeter. If voltage is indicated above 1.2 in. Hg vacuum, replace vacuum switch.

1. Disconnect the vacuum hose to the vacuum switch and connect a vacuum pump to the hose. Apply vacuum and check for voltage at the vacuum switch.
2. Check for voltage between the Brown/White wire at the control box No. 1 connector and ground. There should be voltage when a vacuum above 9.8 in. Hg is applied. If not, replace vacuum switch B.

1. Disconnect the vacuum hose to the vacuum switch and connect a vacuum pump to the hose. Apply vacuum and check for voltage at the vacuum switch.
2. Check for voltage between the Black/Red wire at the control box No. 1 connector and ground. There should be voltage when a vacuum is below 3.0 in. Hg (Calif. man. trans.) or 6.0 in. Hg vacuum (all other models) is applied. If not, replace vacuum switch B.