Ignition System: Description and Operation

Electronic Spark Advance System:

The electronic spark advance (ESA) system directly controls the ignition timing advance by way of the engine control unit (ECU) rather than relying on a combination of vacuum and mechanical advance. This system consists of input devices, the ECU, and the distributor.
The input devices that are computerized engine control related are the EGR position sensor, neutral switch (M/T only), inhibitor switch (A/T only), clutch switch (M/T only), brake light switch, throttle sensor, ignition coil, airflow meter, and the water thermosensor.
The EGR position sensor detects the EGR valve opening stroke and sends a signal to the ECU. The EGR position sensor is installed on the EGR control valve. The neutral switch detects in gear condition and sends a signal to the engine control unit. The switch is on when the transmission is in gear. The inhibitor switch detects in-gear conditions and sends a signal to the ECU. The switch is on when the transmission selector is in the "neutral" or "park" position. The clutch switch detects in-gear condition and sends a signal to the control unit. The switch closes when the clutch pedal is released. The brake light switch detects braking operation (deceleration) and sends a signal to the engine control unit. The throttle sensor detects throttle valve opening angle and sends a signal to the engine control unit. The throttle sensor is installed on the throttle body. Incorporated into the throttle sensor is an idle switch. The idle switch detects when the throttle valve is fully closed and sends a signal to the engine control unit. The negative terminal of the ignition coil is used as an engine speed input by the engine control unit. The water thermosensor detects coolant temperature and sends a signal to the engine control unit. The airflow meter detects the amount of intake air and sends a signal to the engine control unit. The engine control unit uses these signals to decide the amount of fuel to be injected with each engine cycle.
The ECU receives the input signals from the above sensors and determines the proper amount of timing advance necessary for peak engine performance and reduced exhaust emissions.