Spark Control System Honda Prelude III

DESCRIPTION & OPERATION

All Honda engines have ignition control systems to lower emission levels, maximize fuel economy and improve engine performance. The system is designed to advance and retard ignition timing as necessary.

All models use a distributor advance with 2 separate vacuum diaphragms. Diaphragm A operates on intake manifold vacuum. Diaphragm B operates on intake manifold vacuum via an inline thermovalve, vacuum control solenoid valve or cold advance solenoid valve. During cold engine operation, thermovalve B allows vacuum to enter diaphragm B to improve engine performance.

CONTROL UNIT CONNECTOR

1. On A/T models, check for voltage at control unit connector (Blue wire) with ignition on. There should be voltage. If there is no voltage, check Blue wire between ignition coil and control unit.
2. Check for voltage at the 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.

INTAKE AIR TEMPERATURE SENSOR

Unplug air intake temperature sensor connector. Check for continuity between sensor terminals when air temperature is below 40°F (4.5°C). Check for no continuity when air temperature is above 73°F (23°C). Replace sensor if not within specifications.

Auto. Trans.

1. Raise and support front of vehicle. Connect voltmeter or test light positive probe into Yellow/Black wire terminal at the control unit connector.
2. Connect negative probe to ground. Start engine. Place vehicle in second position (gear selector in "2" position) and accelerate slowly. Battery voltage should be as shown in SPEED SENSOR VOLTAGE CHECK table.
3. If no voltage was present during test, check for bad electrical connection, fuse or faulty speed sensor. Replace or repair as necessary and retest.

SPEED SENSOR VOLTAGE CHECK

THERMOSENSOR

1. Ensure engine coolant temperature is below 104°F (40°C). Disconnect connector at control unit. Check for resistance between Blue/Yellow and Brown/Black terminals. Resistance should be above 1000 ohms.
2. Run engine until cooling fan comes on and check resistance again. Resistance should be about 200 ohms. 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.

1. Disconnect vacuum hose No. 15 to vacuum switch C and connect vacuum pump to hose. Apply vacuum to switch. Check for continuity between Black/Yellow and Blue/Black wire terminals of control box 6P connector. Continuity should be present when vacuum is released, replace vacuum switch C and retest.

COLD ENGINE

Note. Engine coolant temperature must be below 140°F (60°C).

Ignition Timing Controls. Scheme 13

1. Disconnect vacuum hose No. 8 from vacuum advance diaphragm. Connect vacuum gauge to hose. Start engine and run at idle. There should be vacuum. If there is vacuum, proceed to «HOT ENGINE»(/honda/prelude/iii-1986-1989/remont/testing-diagnostics/#spark-control-system__hot-engine) test.
2. If there is no vacuum, check for voltage at vacuum control solenoid valve in emission control box No. 1 (4P connector Blue/Black wire). If there is voltage, replace vacuum control solenoid and retest. If there is no voltage, check the following.
3. On man. trans. models, check thermosensor and intake air temperature sensor. Check for voltage at control box connector (Black/Yellow wire) with ignition on. There should be voltage. If there is no voltage, check wiring. Ensure Brown/Black wire between control unit and ground has continuity.
4. On auto. trans. models, check for voltage at control unit connector (Blue wire) with ignition on. There should be voltage. If there is no voltage, check Blue wire between control unit and ignition coil. Check speed sensor, thermosensor, intake air temperature sensor and vacuum switch C.
5. Check for voltage at control box connector (Black/Yellow wire) with ignition on. There should be voltage. If there is no voltage, check wiring. Ensure Brown/Black wire between control unit and ground has continuity.

HOT ENGINE

1. Ensure engine is at normal operating temperature. Disconnect vacuum hose No. 8 at vacuum advance diaphragm B. Connect a vacuum gauge to hose.
2. Start engine and check for vacuum at idle. If there is no vacuum present, proceed to step 6). If vacuum is present, check for voltage at vacuum control solenoid valve in emission control box 4P connector Blue/Black wire.
3. On man. trans. models, if there is voltage, check thermosensor and intake air temperature sensor. Check for voltage at control box connector (Black/Yellow wire) with ignition on. There should be voltage. If there is no voltage, check wiring. Ensure Brown/Black wire between control unit and ground has continuity.
4. On auto. trans. models, if there is voltage check for voltage at control unit connector (Blue wire) with ignition on. There should be voltage. If there is no voltage, check Blue wire between control unit and ignition coil. Check speed sensor, thermosensor, intake air temperature sensor and vacuum switch C.
5. Check for voltage at control box connector (Black/Yellow wire) with ignition on. There should be voltage. If there is no voltage, check wiring. Ensure Brown/Black wire between control unit and ground has continuity. If there is no voltage, replace vacuum control solenoid valve and retest.
6. Disconnect both vacuum hoses from the distributor advance unit. Plug both disconnected hoses. Using a "T" fitting and vacuum hose, connect a vacuum pump to both diaphragms.
7. Pinch vacuum hose to diaphragm B. Apply 16 in. Hg vacuum to diaphragm A. If ignition timing advances and holds steady, proceed to step 8). If timing will not advance or does not hold vacuum, check distributor breaker plate for binding condition. If there is no trouble found, replace vacuum advance unit.
8. Release pinched vacuum hose and apply 16 in. Hg vacuum to both diaphragms. If ignition timing advances more than step 7) and holds steady, system is okay. If ignition timing does not advance as described, replace vacuum advance unit and retest.

See also:
• HOT ENGINE