Modes of Operation

Computerized Engine Control System:

The PGM-FI Control (ECU) Unit in this vehicle is a dedicated microcomputer which controls virtually all engine functions. Based on information received from the inputs, the ECU determines the current demands being placed on the engine. Once this is known, the ECU, based on the program contained in permanent memory, uses the ECU outputs to tailor engine performance to more efficiently meet those demands. A generalized list of the different control functions and the pertinent inputs and the outputs that control them are as follows:

INJECTOR TIMING AND DURATION
The ECU contains memories for the basic discharge durations at various engine speeds and manifold pressures. The basic discharge duration, after being read out from the memory, is further modified by signals sent from various sensors to obtain the final discharge duration. The most vital of these sensors are: Coolant Temperature (TW) sensor, Intake Air Temperature (TA) sensor, Oxygen sensor, and the Atmospheric Pressure (PA) sensor.

ELECTRONIC AIR CONTROL:
When the engine is cold, the A/C compressor is on, the transmission is in gear (A/T only) or the alternator is charging, the controls current to the Electronic Air Control Valve (EACV) to maintain correct idle speed.

IGNITION TIMING CONTROL:
The ECU contains memories for basic ignition timing at various engine speeds, manifold pressures, and coolant temperatures. The ignition timing is adjusted by the ECU through the igniter unit.

OTHER CONTROL FUNCTIONS:
1. Starting Control
When the starter signal is received the ECU will, based on the coolant temperature, enrich the air-fuel mixture to improve starting performance.
2. Fuel Pump Control
a. When the ignition switch is initially turned on, the ECU supplies ground to the main relay which supplies current to the fuel pump.
b. When the engine is running, the ECU supplies ground to the main relay which supplies current to the fuel pump.
c. When the engine is not running and the ignition is on, the ECU cuts ground to the main relay which cuts current to the fuel pump.
3. Fuel Cut-off Control
a. During deceleration with the throttle valve closed, current to the injectors is cut off to improve fuel economy at speeds over 1300 rpm.
b. Fuel cut-off action also takes place when engine speed exceeds 6800 rpm regardless of the position of the throttle valve to protect the engine from over-revving.
4. A/C Compressor Clutch Relay
When the ECU receives a demand for cooling from the air conditioning system (A/C switch), it delays the compressor from being energized to assure smooth transition to the A/C mode.
5. Purge Cut-off Solenoid Valve
When the coolant temperature is below 176°F (80°C), the ECU supplies ground to the Purge Cut-off Solenoid Valve which cuts vacuum to the purge control valve.
6. Tandem Valve Control Solenoid Valve
Depending on coolant temperature and engine speed, the ECU supplies ground to the Tandem Valve Control Solenoid Valve which opens the tandem valve. The opening and closing of the tandem valve maintains good atomization of the fuel injected by the main fuel injector.
7. 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.
8. Alternator Control
The system controls the voltage generated by the alternator in accordance with the electric load and drive mode, and reduces the engine load to improve the fuel economy.
9. EGR Control Solenoid Valve (EGR CSV) (CAL A/T only)
When the EGR is required for control of oxides of nitrogen (NOx) emissions, the ECU supplies ground to the EGR CSV which supplies regulated vacuum to the EGR valve.

ECU BACK-UP FUNCTIONS:
1. Fail-safe Function
When an abnormality occurs in a 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 minimal driving.
3. Self-diagnosis Function (Check Engine Light, LED Indicator)
When an abnormality occurs in a signal from a sensor, the ECU lights the Check Engine Light, stores the failure code in erasable memory and indicates the code with the LED on the ECU anytime the ignition is on. When the ignition is initially turned on, the ECU supplies ground for the Check Engine Light for two seconds.