General System Description

Fig. 151 Electronic Control Unit Inputs and Outputs (PGM-FI Engine):

The ECU contains memories for the basic discharge durations of the fuel injectors at various engine speeds and manifold pressures. The basic discharge duration of the fuel injector is modified by signals from various sensors to obtain the final discharge duration.

ELECTRONIC IDLE CONTROL
When the engine is cold, the A/C compressor is ON, the transmission is in gear (automatic transmission ), and/or the alternator is charging, the ECU controls current to the electronic air control valve (EACV) to maintain correct idle speed.

OTHER CONTROL FUNCTIONS
1. Starting control
When the engine is started, the ECU provides a rich mixture.

2. Fuel Pump Control
When the ignition is initially switched ON, the ECU supplies ground to the main relay which supplies current to the fuel pump for 2 seconds to pressurize the system.
When the engine is running, the ECU supplies ground to the main relay which supplies current to the fuel pump.
When the engine is not running and the ignition is turned ON, the ECU cuts the ground to the main relay which cuts the current to the fuel pump.

3. Fuel Cut-Off Control
During deceleration with the throttle valve closed, current to the injectors is cut-off at speeds over 1200 rpm, to improve fuel economy.
Fuel cut-off action also takes place when engine speed exceeds 6600 rpm regardless of throttle position to protect the engine from over-running.

4. Ignition Control Solenoid Valve (ICSV)
The ICSV is operated by the ECU which recieves signals from the engine speed, coolant temperature, and manifold pressure sensors to provide maximum performance and fuel economy, while protecting the engine by preventing detonation from occurring.

5. EGR Control Solenoid Valve
When the EGR is required for control of oxides of nitrogen (NOx) emissions, the ECU supplies ground to the EGR control solenoid valve which supplies regulated vacuum to the EGR valve.

6. Purge Cut-Off Control Solenoid Valve
When the engine coolant temperature is above the set temperature of the TW sensor, (55°C/131°F), the purge cut-off solenoid valve is activated by the control unit receiving signals from each sensor.

ECU BACK-UP FUNCTIONS

1. Fail Safe Function
When an abnormality occurs in the signal from a sensor, the ECU ignores that signal and assumes a pre-programmed value that allows the engine to continue to run.

2. Back-Up Function
When an abnormality occurs in the ECU itself, the injectors are controlled by a back-up circuit independent of the system in order to permit normal driving.

3. Self-Diagnosis Function
When an abnormality occurs in a signal from a sensor, the ECU lights the "CHECK ENGINE" warning light, stores the failure code in erasable memory and indicates the code with an LED on the ECU anytime the ignition is ON. When the ignition is initially turned on, the ECU supplies ground for the "CHECK ENGINE" warning light for about 2 seconds.