Modes of Operation
Computerized Engine Control System:
The ECU contains memories for the basic discharge timing and duration of the fuel injectors at various engine speeds and manifold pressures. The basic discharge values are then modified by signals sent from various sensors to obtain the final injection values.
CONTROL FUNCTIONS:
1. Electronic Air Control:
The ECU controls the idle speed by varying the amount of air that is allowed to bypass the throttle plate. The bypass air is regulated with the electronic air control valve (EACV). This system is able to compensate for various loads on the engine like the A/C compressor, power steering pump load, transmission in gear (A/T only), etc., to keep the idle speed constant.
2. Ignition Timing Control:
The ECU is programmed with basic ignition timing values for various engine speeds, loads and coolant temperatures.
3. Starting Control:
When the engine is started, the ECU provides a rich mixture.
4. Fuel Pump Control:
When the ignition switch is first turned ON, the ECU supplies a ground for the main relay which supplies current to the fuel pump for 2 seconds to pressurize the injection system. The ECU maintains the ground to the main relay as long as the engine is running (as indicated by an ignition signal).
The main relay is used to interrupt power to the fuel pump when the engine is not running and the ignition is in the ON position.
5. Fuel Cut-Off Control:
During deceleration with the throttle valve closed and the engine speed is above 1400 rpm M/T, 1100 rpm A/T, current to the injectors is cut off to improve fuel economy. Current to the injectors is also cut off when the engine speed exceeds 6600 rpm to prevent over-revving.
6. A/C Compressor Clutch Relay:
When the ECU receives a demand for cooling from the air conditioning system (A/C compressor control unit), it delays the compressor from being energized and enriches the mixture to assure smooth transition to the A/C mode.
7. Purge Control Solenoid Valve:
When the engine coolant temperature is below 167°F (75°C), the ECU supplies a ground to the purge cut-off solenoid valve which cuts vacuum to the purge control valve.
8. Bypass Control Solenoid Valve (BPCSV):
When the engine speed is below 4800 rpm, the BPCSV is activated by a signal from the ECU. Intake air flows through the long intake path, resulting in high torque output. At engine speeds over 4900 rpm, both solenoid valves are deactivated by the ECU, the intake air flows through the short intake path to reduce airflow resistance.
9. EGR Control Solenoid Valve (EGR CSV):
The EGR system is controlled by the ECU. When EGR is required to reduce NOx emissions, (as determined by pre-programmed values in the ECU), the ECU supplies a ground to the EGR control solenoid valve which supplies regulated vacuum to the EGR valve allowing it to open.
10. Intake Control Solenoid Valve:
With engine speed below 4000 rpm, a ground is supplied by the ECU to the intake control solenoid valve. The valve opens allowing intake manifold vacuum to reach the intake control diaphragm.
11. Alternator Control:
Voltage produced by the alternator is controlled by the system, in accordance with the electric load and drive mode. It then reduces the engine load to improve fuel economy.
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. When the ignition is initially turned ON, the ECU supplies ground for the "CHECK" engine warning light for about 2 seconds.