Modes of Operation

System Description:

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 above the following rpm.

M/T: 930 rpm
A/T: 920 rpm

Fuel cut-off action also takes place when engine speed exceeds the following rpm, regardless of throttle position to protect the engine from over-running.

7400 rpm

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 158°F (70°C), the ECU supplies voltage to the purge cut-off solenoid valve which cuts vacuum to the purge control valve.

8. 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 the ignition is initially turned on, the ECU supplies ground for the "CHECK ENGINE" warning light for about 2 seconds. When an abnormality occurs in a signal from a sensor, the ECU lights the "CHECK ENGINE" warning light, and stores the failure code in the erasable memory.