Anti-Afterburn System Honda Prelude II

DESCRIPTION

The anti-afterburn system supplies fresh air to intake manifold when manifold vacuum suddenly increases, as during sudden deceleration. This leans the air/fuel mixture and helps maintain proper mixture for complete combustion.

OPERATION

The anti-afterburn valve is sensitive only to sudden increases in vacuum. The length of time it stays open is controlled by a diaphragm that senses change in manifold vacuum.

When manifold vacuum suddenly increases, as during sudden deceleration, diaphragm/valve unit is pulled downward. Air flow from air chamber to sensing chamber is restricted by an orifice, creating a pressure differential on diaphragm, which holds valve open.

This unbalanced condition lasts for a few seconds until pressure in both chambers is equalized by air entering through orifice. When both chambers are equalized, spring pressure pushes diaphragm up, closing valve.

Cold Engine

Note. Engine coolant temperature must be below 158°F (70°C).

1. Disconnect air suction lower hose at anti-afterburn valve. Start engine and quickly raise engine speed to 2500 RPM. Close throttle suddenly. If there is no vacuum, proceed to step 5).
2. If there is vacuum, check for voltage at anti-afterburn control solenoid valve (Blue/Yellow wire). If there is no voltage, check for voltage at control unit connector (Blue wire) with ignition switch on. There should be voltage. If no voltage, check Blue wire between ignition coil and control unit.
3. Check speed sensor and thermosensor. See «SPEED SENSOR»(/honda/prelude/ii-1983-1987/remont/testing-diagnostics/#anti-afterburn-system__speed-sensor) and «THERMOSENSOR»(/honda/prelude/ii-1983-1987/remont/testing-diagnostics/#anti-afterburn-system__thermosensor) . Check for voltage at control unit connector (Black/Yellow wire) with ignition switch on. There should be voltage. If no voltage, check wiring. Check Brown/Black wire for continuity between control unit and ground. There should be continuity.
4. If there is voltage, disconnect vacuum hose No. 21 from anti-afterburn valve. Check for vacuum at hose. If there is no vacuum, check for leaks or disconnected hose. If there are no defects, replace anti-afterburn control solenoid valve and retest. If there is vacuum, replace anti-afterburn valve and retest.
5. Raise front of vehicle and support with safety stands. Block rear wheels and set parking brake. Place shift or selector lever in second or "2" position and accelerate. Release throttle suddenly and check for vacuum during deceleration above 3000 RPM (man. trans.) or above 15 MPH (auto. trans.).
6. If there is vacuum, proceed to HOT ENGINE test. If there is no vacuum, check for voltage at anti-afterburn control solenoid valve above 3000 RPM (man. trans.) or above 15 MPH (auto. trans.).
7. If there is voltage, check for voltage at control unit connector (Blue wire) with ignition switch on. There should be voltage. If no voltage, check Blue wire between ignition coil and control unit. Check speed sensor and thermosensor. See «SPEED SENSOR»(/honda/prelude/ii-1983-1987/remont/testing-diagnostics/#anti-afterburn-system__speed-sensor) and «THERMOSENSOR»(/honda/prelude/ii-1983-1987/remont/testing-diagnostics/#anti-afterburn-system__thermosensor) .
8. Check for voltage at control unit connector (Black/Yellow wire) with ignition switch on. There should be voltage. If no voltage, check wiring. Check Brown/Black wire for continuity between control unit and ground. There should be continuity.
9. If there is no voltage, disconnect vacuum hose No. 21 from anti-afterburn valve and check for vacuum above 3000 RPM (man. trans.) or above 15 MPH (auto. trans.). If there is vacuum, replace anti-afterburn control valve and retest.
10. If there is no vacuum, disconnect vacuum hose No. 20 from anti-afterburn valve. Check for vacuum. If there is no vacuum, check for disconnected hoses or leaks and retest. If there is vacuum, replace anti-afterburn valve and retest.

Hot Engine

1. Disconnect air suction lower hose at anti-afterburn valve. Start engine and quickly raise engine speed to 3500 RPM. Close throttle suddenly. There should be vacuum. If there is vacuum, system is okay.
2. If there is no vacuum, check for voltage at anti-afterburn control solenoid valve (Blue/Yellow wire). If voltage is available, check for voltage at control unit connector (Blue wire) with ignition switch on. There should be voltage. If no voltage, check Blue wire between ignition coil and control unit.
3. Check speed sensor and thermosensor. See «SPEED SENSOR»(/honda/prelude/ii-1983-1987/remont/testing-diagnostics/#anti-afterburn-system__speed-sensor) and «THERMOSENSOR»(/honda/prelude/ii-1983-1987/remont/testing-diagnostics/#anti-afterburn-system__thermosensor) . . Check for voltage at control unit connector (Black/Yellow wire) with ignition switch on. There should be voltage. If no voltage, check wiring.
4. Check Brown/Black wire for continuity between control unit and ground. There should be continuity. If there is no voltage, replace anti-afterburn control solenoid valve and retest.

CONTROL UNIT CONNECTOR

1. Check for voltage at control unit connector Blue wire with ignition on. There should be voltage. If there is no voltage, check wiring between control unit and ignition coil.
2. Check for voltage at Black/Yellow wire with ignition on. There should be voltage. If there is no voltage, check wiring. Check Brown/Black wire for continuity between control unit and ground. There should be continuity.

SPEED SENSOR

1. Raise and support front of vehicle. Push voltmeter or test light positive probe into Yellow/Black wire (Yellow wire on Accord) terminal at control unit connector.
2. Connect negative probe to ground. Start engine. Place vehicle in 2nd position (gear selector in "2" position) and accelerate slowly. Battery voltage should be as shown in SPEED SENSOR VOLTAGE CHECK table. SPEED SENSOR VOLTAGE CHECK Speed Voltage Below 11 MPH Yes Above 16 MPH No
3. If voltmeter readings do not correspond to appropriate MPH ranges, install a new speed sensor and retest. If there was no voltage during speed sensor test, check for bad electrical connection, fuse or faulty speed sensor. Replace or repair as necessary and retest.

THERMOSENSOR

1. Ensure engine coolant temperature is less than 104°F (40°C). Disconnect vacuum hose from charcoal canister. If there is no vacuum at idle, thermovalve is open and test may be performed.
2. Disconnect connector at control unit. Check for resistance between Blue/Yellow and Brown/Black terminals. Resistance should be above 1000 ohms. Run engine until cooling fan comes on and check resistance again. Resistance should be about 200 ohms.
3. If resistance is outside ranges, replace thermosensor and retest. If ohmmeter shows no continuity, check wires. If there are no defects, replace thermosensor and retest.

See also:
• SPEED SENSOR
• THERMOSENSOR