Engine Control Module: Description and Operation

Fig. 16 PGM-FI electronic controls:

Fig. 17 PGM-FI electronic controls:

The ECU, located under the driver's seat on Accord, or under passenger side of dash beneath carpet on Prelude, contains memories for basic discharge durations at various manifold pressures and engine speeds. The ECU modifies the basic discharge duration by signals sent by various sensors (inputs) to operate various outputs, Figs. 16 and 17.
When the engine is started, the ECU provides a rich mixture.
When the ignition switch is first turned on, the ECU supplies ground to the main relay which supplies current to the fuel pump for 2 seconds to pressurize fuel system. When engine is running, the ECU supplies ground to main relay which supplies current to the fuel pump. When engine is not running and ignition switch is in On position, the ECU cuts ground to the main relay which cuts current to the fuel pump.
During deceleration with throttle valve closed, current to the injectors is cut off at speeds over 1200 RPM to improve fuel economy. Fuel cut off also occurs at engine speeds above 6600 RPM on Accord, or above 7000 RPM on Prelude, regardless of position of throttle valve to prevent engine from over running.
When engine is cold, A/C compressor is on, automatic transmission is in gear, power steering oil pressure is high or the alternator is charging, the ECU controls current to the EACV to maintain correct idle speed.
When EGR is required for control of NOx emissions, the ECU provides ground to the EGR control solenoid valve which supplies regulated vacuum to the EGR valve.
The ECU incorporates a fail-safe function which allows it to ignore an abnormal input signal and assume a pre-programmed value that permits the engine to continue to operate.
The system incorporates a backup function that, when an abnormality occurs in the ECU, controls the injections by a backup circuit independent of the system to allow minimal driving.
This system also has a self-diagnosis function. When an abnormality occurs in one of the inputs, the ECU lights the PGM-FI warning light and stores the failure code in erasable memory and indicates the code with a LED on the ECU anytime the ignition is on. When the ignition is first turned on, the ECU supplies ground for the PGM-FI warning light for approximately 2 seconds.
On Accord, when coolant temperature is below 212°F, the ignition control solenoid valve is operated by the ECU in response to signals from engine speed and manifold vacuum to provide maximum performance and fuel economy, while protecting engine by preventing detonation.
On Prelude, the ECU contains memories for basic ignition timing at various engine speeds and manifold pressures. Ignition timing is also adjusted for coolant temperature.
On Prelude, when the ECU receives a demand for cooling from the compressor control unit, it delays the compressor from being energized and enriches the mixture to assure smooth transition to the A/C mode.
On Prelude, when engine speed is below 5000 RPM and the bypass control solenoid valve is activated by a signal from the ECU, intake air flows through the long intake path, causing high torque to be delivered. At speeds above 5000 RPM, both solenoid valves are deactivated by the ECU, and intake air flows through the short intake path in order to reduce the resistance to air flow.