Modes of Operation
Computerized Engine Control System:
The ECU contains memories for the basic discharge durations of the fuel injectors (determining the air/fuel mixture) 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. The oxygen sensor is a main input component for the air/fuel mixture control.
CONTROL FUNCTIONS:
1. Electronic Air 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.
2. Ignition Timing Control:
The ECU contains memories for basic ignition timing at various engine speeds, manifold pressures and the coolant temperature.
3. Starting Control:
When the engine is started, the ECU provides a rich mixture.
4. 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.
5. Fuel Cut-Off Control:
During deceleration with the throttle valve closed, current to the injectors is cut-off at speeds over 950 rpm to improve fuel economy.
Fuel cut-off action also takes place when engine speed exceeds 7000 rpm regardless of throttle position to protect the engine from over-running.
6. A/C Compressor Clutch Relay:
When the air conditioner (A/C switch) is turned ON, it delays the compressor from being energized to assure smooth transition to the A/C mode.
7. Purge Cut-Off Control Solenoid Valve:
When the engine coolant temperature is below 57°C (135°F), the ECU supplies voltage to the purge cut-off solenoid valve which cuts vacuum to the purge control valve.
8. Fast Idle Control Solenoid Valve:
When the coolant temperature is extremely low, the ECU supplies ground to the fast idle control solenoid valve which supplies additional bypassed air.
9. Lock-Up Control Solenoid Valve (A/T only):
The speed and throttle angle sensor inputs to the ECU are used to send an on/off voltage signal to the lock-up control solenoid valve for precise timing of the torque converter lock-up system.
10. Alternator Control:
The system controls the voltage generated by the alternator in accordance with the electric load and drive mode. This reduces the engine load to improve the fuel economy.
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.