DESCRIPTION
The Hitachi Computerized Ignition system controls ignition timing by adjusting to engine operating conditions. The optimum ignition timing for each driving condition is pre-programmed in the system. Ignition timing is determined by an electric signal contained in the unit.
CRANK ANGLE SENSOR
The crank angle sensor is a basic component of the entire computerized ignition system and is built into the distributor. It monitors engine RPM, and piston position and sends this information to the Electronic Control Unit (ECU).
The crank angle sensor has a rotor plate and a wave forming circuit. The rotor plate has 360 slits in one degree increments for engine speed signal and 4 or 6 slits for piston position signal. Light Emitting Diodes (LEDs) and photo diodes are built in the wave forming circuit. When the signal rotor passes between the LED and photo diode, the slits in the rotor plate alternately cuts the light. This is converted into an on-off pulse by the wave forming circuit, which is sent to the ECU. (Scheme 1)
Hitachi Computerized Ignition System. Scheme 1
IGNITION COIL
The ignition coil has a built-in power transistor which amplifies the signal from the control unit. The amplified signal turns the ignition coil primary circuit on and off, inducing the proper high voltage in the secondary circuit.
Needle Type Speedometer Models
The needle type speed sensor consists of a reed switch, which is installed in the speedometer unit and transforms vehicle speed into a pulse signal to the ECU.
Digital Type Speedometer Models
The digital type speed sensor consists of an LED, photo diode, shutter and wave forming circuit. Its principle is the same as that of the crank angle sensor.
VEHICLE SPEED SENSOR (VSS)
Raise drive wheels. Remove ECU 16-pin connector. Turn ignition switch to "ON" position. Rotate drive wheel by hand. Using an ohmmeter, check resistance between harness connector terminal No. 29 and ground. It should alternate between zero and infinity ohms on needle type speed sensor. On digital type speed sensor, voltage reading should vary between zero volt to approximately 5 volts.
The ignition coil primary resistance should be 0.84-1.02 ohms and the secondary coil resistance should be 8000-12,000 ohms. Replace ignition coil if not within specifications.
POWER TRANSISTOR
Note. On models equipped with dual coils, DO NOT disconnect "T" type harness connector for ignition coil.
- Turn engine off. Before proceeding with test, check harness for continuity between power transistor and ECU. Check fusible link "G" (located in fusible link holder), ignition switch and continuity of ignition coil. All testing should be made from terminal side of harness connector.
- Disconnect ignition coil and power transistor harness connector. Connect ohmmeter positive lead to terminal No. 1 (coil positive) and ohmmeter negative lead to terminal No. 2 (ECU), reading should be approximately 15 ohms.
- Connect ohmmeter positive lead to terminal No. 4 (coil negative) and ohmmeter negative lead to terminal No. 3 (ground). Reading should be approximately 15 ohms.
- Connect ohmmeter between terminal No. 2 (ECU connector) and ground. Reading should be approximately one ohm.
- Connect ohmmeter between terminal No. 1 (coil positive) and terminal No. 4 (coil negative). Reading should be zero ohm.
- If readings are not as specified, replace power transistor.
