Feedback Carburetor System Honda Prelude III

DESCRIPTION

The feedback carburetor system maintains proper air/fuel ratio during engine operation. Air control valves are used to bypass additional air into the intake manifold to adjust temporary rich mixture condition. The system consists of 2 feedback systems, "X" and "M".

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

The "M" system consists of 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 diaphragm chamber of valve. Frequency solenoid B is modulated by the control unit, and controls the amount of vacuum to air control valve B. On Accord and Civic models, a feedback control solenoid is provided to stop feedback operation under low engine speed and cold engine temperatures. On Prelude models, manifold vacuum applied at frequency solenoid valve B is constantly modulated by the constant vacuum valve.

Mixture is more precisely controlled by the "M" system, based on engine load. The diaphragm chamber of air control valve A receives the same carburetor vacuum as EGR control valves A and B, while air is supplied to the intake manifold in proportion to 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 preset (stochiometric) mixture.

Scheme 25

Scheme 26

Scheme 27

1. Ensure engine is at normal operating temperature. On Accord models, disconnect hose No. 14 from power valve control solenoid valve located within control box, and plug hose. (Scheme 25)and (Scheme 26).
2. On Civic 1500 models, disconnect vacuum hose from power valve at carburetor and plug hose. (Scheme 31) On Prelude models, disconnect hose No. 14 from control box No. 1 and plug hose. (Scheme 29)and (Scheme 30). (Scheme 25): Accord Control Box
3. On Civic 1500 models with manual transmission and without power steering, remove cover from control box No. 2. On all models, start engine and check that frequency solenoid valves A and B are operating while running engine at 2500 RPM. (Scheme 26): By-Passing Accord Vacuum Control Valve NOTE: On Civic 1500 models equipped with automatic transmission 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.
4. 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 27): Civic 1500 Control Boxes No. 1 & 2
5. 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 zero volts, replace solenoid valves and retest.
6. If voltmeter indicates 12 volts and engine operating condition is normal, check vacuum lines of system for blockage, leaks or disconnected hoses. If no problem exists, check both air control valves A and B.
7. If voltmeter indicates zero volts, proceed to «TROUBLE SHOOTING»(/honda/prelude/iii-1986-1989/remont/testing-diagnostics/#feedback-carburetor-system). If voltmeter indicates 12 volts and engine operating condition is abnormal, or engine stops, proceed to «TROUBLE SHOOTING»(/honda/prelude/iii-1986-1989/remont/testing-diagnostics/#feedback-carburetor-system).
8. Reconnect vacuum hoses. With A/C, radiator fan, heater blower motor, rear window defogger and headlights off, ensure that frequency solenoid valves are inoperable while engine is at idle speed.
9. If solenoid valves are inoperable, system is okay and testing is complete. If solenoid valves do operate, proceed to «TROUBLE SHOOTING»(/honda/prelude/iii-1986-1989/remont/testing-diagnostics/#feedback-carburetor-system).

Prelude Control Boxes No. 1 & 2. Scheme 28

Scheme 29

1. Disconnect vacuum hose No. 16 (Accord), 29 (Civic) or 15 (Prelude) from air control valve A. Connect vacuum pump and disconnect air hose between air control valve A and frequency solenoid valve A. Connect hand vacuum pump and pressure gauge to hose. (Scheme 30) (Scheme 29): View of Frequency Solenoid Valves Accord model is shown; other models are similar.
2. When vacuum is applied to vacuum hose side, air should flow without resistance and pressure gauge should not indicate any pressure. When vacuum is released, air should flow with resistance and pressure should build-up on gauge and release gradually. If air control valve fails either test, valve should be replaced.

Checking Air Control Valves Accord model is shown; other models are similar. Scheme 30

Accord & Prelude

Air control valve B is tested in same manner as control valve A with one exception; disconnect air hose between air control valve B and frequency solenoid valve B. See AIR CONTROL VALVE A .

Scheme 31

1. Disconnect air suction hose at control box No. 2. Force air into hose using a hand vacuum pump, "T" fitting and pressure gauge. (Scheme 32) (Scheme 31): Testing Air Control Valve B on Civic 1500 Models
2. Air should not flow and pressure gauge should indicate build-up. If air does not pass through, proceed to steps 4) and 5). If air passes through, proceed to step 3).
3. Remove control box cover. Pinch off air suction hose to frequency solenoid valve A. Force air into air suction hose. If air flows through, replace air control valve B and retest. If air does not flow through, replace frequency solenoid A and retest.
4. Disconnect vacuum hose No. 24 from surge tank B and connect vacuum pump to hose. Disconnect hose between air control valve B and frequency solenoid valve A. Connect vacuum pump (on pressure side) to hose.
5. Pinch off hose routed from feedback control solenoid valve. Apply vacuum to air control valve B (diaphragm side) and force air into 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.

Civic 1500 (Power Steering Models)

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

1. Disconnect wire harness from frequency solenoid valve A. Disconnect air hose from air control valve A. Connect pressure gauge and vacuum pump. (Scheme 33)
2. Apply vacuum to solenoid valve A. Valve should hold vacuum. If vacuum holds, apply battery voltage to frequency solenoid valve and apply vacuum.
3. Solenoid valve should not hold vacuum. If valve does not hold vacuum, solenoid valve is working properly. If vacuum is held, replace frequency solenoid valve A.

Checking Frequency Solenoid Valve A Accord model is shown; Prelude models are similar. Scheme 32

Civic 1500

1. Disconnect air hose between air control valve A and frequency solenoid valve A at control valve and force air from air hose. Air should not flow.
2. If air flows, apply battery voltage to frequency solenoid valve A (control box No. 2) Green/White wire and force air from 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.

Scheme 33

1. Disconnect hose No. 30 from frequency solenoid valve B and plug inlet pipe of valve. Disconnect hose No. 31 from frequency solenoid valve B and connect vacuum pump. (Scheme 34) (Scheme 33): Checking Frequency Solenoid Valve B Accord model is shown; Prelude models are similar.
2. Apply vacuum to frequency solenoid valve B. If valve does not hold vacuum, replace frequency solenoid valve B and retest. If valve holds vacuum, apply 12 volts to frequency solenoid valve B and apply vacuum.
3. If vacuum holds, valve is okay. Reconnect hoses. If frequency solenoid valve B does not hold vacuum, replace valve and retest.

1. Disconnect and plug hose No. 10 from surge tank A (rear tank). Disconnect vacuum hose No. 23 from surge tank B (front tank) and connect vacuum pump. Apply battery voltage to frequency solenoid valve B and draw vacuum from hose No. 23.
2. If vacuum holds, proceed to step 3). If vacuum does not hold, replace frequency solenoid valve B and retest.
3. Remove 12 volts from frequency solenoid valve B. Vacuum should bleed off. If vacuum bleeds off, reconnect vacuum hoses No. 10 and 23 and proceed to «FEEDBACK CONTROL SOLENOID VALVE»(/honda/prelude/iii-1986-1989/remont/testing-diagnostics/#feedback-carburetor-system) test. If vacuum holds, replace frequency valve B and retest.

Scheme 34

1. Disconnect hose No. 30 from frequency solenoid valve B and connect vacuum gauge to hose. (Scheme 35) Start engine and allow to run at idle speed. (Scheme 34): Checking Constant Vacuum Valve
2. Vacuum reading after engine stabilizes, should be 4-8 in. Hg vacuum. If vacuum is within range, constant vacuum valve is okay. Reconnect hose to frequency valve B. If vacuum stabilizes above or below specified range, replace constant vacuum valve.

CONSTANT VACUUM GENERATOR

1. Disconnect hose No. 33 from frequency solenoid valve B and connect vacuum gauge. Start engine and allow to run at idle. After vacuum stabilizes, reading should be 6-11 in. Hg vacuum.
2. If vacuum stabilizes within range, constant vacuum generator is okay. Reconnect hose to frequency valve B. If vacuum stabilizes above or below specified range, replace constant vacuum generator.

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

1. Disconnect vacuum hose No. 24 from surge tank B and connect vacuum pump to hose. Apply battery voltage to feedback control solenoid valve (control box No. 1, Blue/Red wires) and draw vacuum.
2. If vacuum does not hold, replace feedback control solenoid valve and retest. If vacuum holds, remove 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, turn ignition on and check for voltage at control unit connector Blue wire. Voltage should be present. If voltage is not present, check Blue wire between ignition coil and control unit.
2. On all models, turn ignition on and check for voltage at control unit connector Black/Yellow wire. Voltage should be present. If voltage is not present, check wiring and ignition fuse.
3. Check Brown/Black (Civic and Prelude models) or Black (Accord models) wire for continuity between control unit and body ground. Continuity should be present.

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). If sensor does not operate as described, replace sensor.

Civic, Civic CRX & Prelude

1. Insert voltmeter positive lead into White wire of terminal from control unit connector. Connect negative lead to ground.
2. Start engine and warm to operating temperature. Raise engine RPM several times while observing voltmeter. Voltmeter should indicate .5 volt or above during this test.
3. If voltmeter does not indicate voltage, check fuse and wiring. If both are okay, replace oxygen sensor and retest.

Accord

1. Start engine and warm to operating temperature (cooling fans on). Measure voltage between White (+) terminal and Black (-) terminal. Voltmeter should indicate as shown in «OXYGEN SENSOR VOLTAGE SPECIFICATION»(/honda/prelude/iii-1986-1989/remont/testing-diagnostics/#feedback-carburetor-system) table. OXYGEN SENSOR VOLTAGE SPECIFICATION Throttle Position Voltage Wide Open Throttle Above 0.6 Idle Below 0.4
2. If voltmeter indicates as shown in table, O2 sensor is okay. If voltmeter does not indicate as shown in table, disconnect main wire harness from O2 sensor.
3. Measure voltage between connector terminal and body ground. Voltmeter should indicate as shown in «OXYGEN SENSOR VOLTAGE SPECIFICATION»(/honda/prelude/iii-1986-1989/remont/testing-diagnostics/#feedback-carburetor-system) table.
4. If voltmeter does not indicate as shown in table, replace O2 sensor. If voltmeter indicates as shown in table, repair open or short in White wire between O2 sensor and control unit.

SPEED SENSOR

1. Raise and support front of vehicle. Insert voltmeter or test light positive probe into Yellow and Black (Accord) or Yellow/Black (Civic and Prelude) wire terminal at control unit connector.
2. Connect negative probe to ground. Start engine. Place vehicle in 2nd gear and accelerate slowly. Check for battery voltage as shown in table. SPEED SENSOR VOLTAGE CHECK Speed Voltage Accord Below 10 MPH Yes Above 20 MPH No Civic Below 6 MPH Yes Above 12 MPH No Prelude Below 11 MPH Yes Above 16 MPH No
3. If voltage does not correspond to above MPH ranges, install a new speed sensor and retest. If voltage was not present during step 2), check for bad electrical connection, fuse or faulty speed sensor. Replace or repair as necessary and retest.

Remove terminal leads from thermosensor. With cylinder head temperature above 158°F (70°C), check for continuity. Continuity should be present. With cylinder head temperature below 158°F (70°C), continuity should not be present. If thermosensor does not operate as described, replace thermosensor.

Civic 1500 (Thermosensor A)

Remove terminal leads from thermosensor. With engine block temperature below 167°F (75°C), check for continuity. Continuity should be present. With engine block temperature above 167°F (75°C), continuity should not be present. If thermosensor 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. Continuity should be present. With cylinder head above 149°F (65°C), continuity should not be present. If thermosensor does not operate as described, replace thermosensor.

Prelude

Remove terminal leads from thermosensor. With cylinder head temperature below 40°F (4.5°C), check for continuity. Continuity should be present. With cylinder head temperature above 73°F (23°C), continuity should not be present. If thermosensor does not operate as described, replace thermosensor.

1. Disconnect vacuum hose No. 4 (Accord) or No. 2 (Prelude) to control box at vacuum hose manifold. Connect hand vacuum pump to hose. Check for continuity between White/Blue wire terminal and body ground (Accord) or Black/Yellow and Brown/White wire terminals (Prelude).
2. Apply at least 1.2 in. Hg vacuum (Accord) or over 3.9 in. Hg vacuum (Prelude) to vacuum switch. Continuity should be present. Release vacuum. continuity should not be present. If vacuum switch does not operate as described, replace vacuum switch.

1. Disconnect vacuum hose to vacuum switch and connect vacuum pump. Apply vacuum and check for voltage between 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 with vacuum above 1.2 in., replace vacuum switch.

1. Disconnect hose No. 7 to control box at vacuum hose manifold. Connect hand vacuum pump to hose. Apply vacuum to switch.
2. On Accord models, check for continuity between Black/Yellow and Blue/White (Accord) or Black/Yellow and Gray (Prelude) wires at control box connector. Continuity should be present when vacuum above 1.2 in. Hg is applied. No continuity should be present when vacuum is released.

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

1. Disconnect vacuum hose No. 14 at control box (vacuum hose manifold). Connect vacuum pump to hose. Remove control box from firewall and remove cover.
2. Check for continuity between vacuum switch terminal leads. Continuity should be present when vacuum above 3.9 in. Hg is applied. Continuity should not be present when vacuum is released.

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

1. Disconnect vacuum hose No. 15 to control box at vacuum manifold. Connect vacuum pump to hose. Apply vacuum to switch. Check for continuity between Black/Yellow and Blue/Black wire terminals of control box 6-pin connector.
2. Continuity should be present when vacuum above 12 in. Hg is applied. If continuity is not present, replace vacuum switch C and retest.

Accord Wiring Diagram (Carburetor). Scheme 35

Civic CRX HF Federal & High Altitude Wiring Diagram (Carb). Scheme 36

Civic CRX HF California Wiring Diagram (Carburetor). Scheme 37

Civic 1500 & CRX Wiring Diagram (Carburetor). Scheme 38

Civic 1300 Wiring Diagram (Carburetor). Scheme 39

Prelude Wiring Diagram (Carburetor). Scheme 40

See also:
• TROUBLE SHOOTING