Feedback Carburetor System Honda Prelude II

DESCRIPTION

All Honda models use a feedback carburetor system. 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, feedback control solenoid valve, frequency solenoid B, constant vacuum valve, 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, 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. Manifold vacuum applied at frequency solenoid valve B is constantly modulated by the constant vacuum valve.

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.

Hot Engine

1. Ensure engine is at normal operating temperature. Disconnect hose No. 14 from control box No. 1 and install a plug in control box No. 1. (Scheme 18)and (Scheme 19).
2. Start engine and check that frequency valves A and B are operating while running engine at 2500 RPM.
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).
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»(/honda/prelude/ii-1983-1987/remont/testing-diagnostics/#feedback-carburetor-system). If voltmeter indicates 12 volts and engine operating condition is abnormal, or the engine stops, proceed to «TROUBLE SHOOTING»(/honda/prelude/ii-1983-1987/remont/testing-diagnostics/#feedback-carburetor-system).
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»(/honda/prelude/ii-1983-1987/remont/testing-diagnostics/#feedback-carburetor-system).

Prelude Control Boxes Nos. 1 & 2. Scheme 18

Scheme 19

1. Disconnect vacuum hose No. 15 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 19) (Scheme 19): View of Frequency Solenoid Valves Accord model shown; 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 Accord model shown; other models similar. Scheme 20

AIR CONTROL VALVE B

The air control valve B is tested in the 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 .

FREQUENCY SOLENOID VALVE A

1. Disconnect the air hose from air control valve A. Connect a pressure gauge and vacuum pump. (Scheme 21) Blow air into the hose. If air flows and pressure gauge does not register a build up, replace the frequency solenoid valve A and retest.
2. If air does not flow through, apply battery voltage to frequency solenoid valve A and blow into the valve. Air should flow and the pressure gauge should not register any build up. If air flows through, frequency solenoid valve A is okay. If not, replace frequency solenoid valve A and retest.

Checking Frequency Solenoid Valve A (Typical). Scheme 21

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

Scheme 22

1. Disconnect hose No. 30 from frequency solenoid valve B and plug inlet pipe of the valve. Disconnect hose number 31 from frequency solenoid valve B and connect a vacuum pump. (Scheme 22) (Scheme 22): Checking Frequency Solenoid Valve B (Typical)
2. Apply vacuum to frequency solenoid valve B. If valve holds vacuum, replace frequency solenoid valve B and retest. If valve does not hold vacuum, apply 12 volts to frequency solenoid valve B and draw vacuum.
3. Vacuum should hold. If vacuum holds, valve is okay. Reconnect hoses. If frequency solenoid valve B does not hold vacuum, replace and retest.

Scheme 23

1. Disconnect hose No. 30 from the frequency solenoid valve B and connect a vacuum gauge to hose. (Scheme 23) (Scheme 23): Checking Constant Vacuum Valve
2. Start engine and allow run at idle speed. After vacuum stabilizes, vacuum should be 4-8 in. Hg. 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.

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 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 or White wire. 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 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 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 there was no voltage during step 2), check for bad electrical connection, fuse or faulty speed sensor. Replace or repair as necessary and retest.

THERMOSENSOR

Remove terminal leads from thermosensor. With cylinder head temperature below 40°F (4.5°C), check for continuity. It should exist. With cylinder head temperature above 73°F (23°C), no continuity should exist. If thermosensor does not operate as described, replace thermosensor.

VACUUM SWITCH A

1. Disconnect vacuum hose No. 2 to control box at vacuum hose manifold. Connect a hand vacuum pump to hose. Check for continuity between Black/Yellow and Brown/White wire terminals.
2. Apply over 3.9 in. Hg to vacuum switch. Continuity should be present. Release vacuum. There should be no continuity. If switch does not operate as described, replace vacuum switch.

VACUUM SWITCH B

1. Disconnect hose number 7 to control box at the vacuum hose manifold. Connect a hand vacuum pump to the hose. Apply vacuum to switch.
2. Check for continuity between Black/Yellow and Gray wires at control box connector. Continuity should be present when a vacuum above 1.2 in. Hg is applied. No continuity should be present when vacuum is released.

VACUUM SWITCH C

1. Disconnect vacuum hose No. 15 to the control box at the vacuum manifold and connect a vacuum pump to the hose. Apply vacuum to switch. Check for continuity between Black/Yellow and Blue/Black wire terminals of control box 6-pole connector.
2. Continuity should be present when a vacuum above 12 in. Hg is applied. If there is no voltage, replace the vacuum switch C and retest.

See also:
• TROUBLE SHOOTING