Modes of Operation

Fig. 21 Electronic Control Unit Functions:

The PGM-FI ECU is a complete engine management system, it uses various sensors and output devices to control engine operation. Listed below are some of the ECU functions:

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 memory, is further modified by signals sent from various sensors to obtain the final discharge duration.

Electronic Air Control

When the engine is cold, the A/C compressor is on, the transmission is in gear (A/T only), power steering oil pressure is high, or the alternator is charging, the ECU 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 and manifold pressures. Ignition timing is also adjusted for coolant temperature.

Other Control Functions

1. Starting Control
When the engine is started, the ECU provides a rich mixture.

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 for about 2 seconds to pressurize the fuel system.
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 off the 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 1,300 rpm.
b. Fuel cut-off action also takes place when engine speed exceeds 6,800 rpm regardless of the position of the throttle valve to protect the engine from over-running.

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 80°C (176°F), 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)
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 control 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) (California A/T)
When the EGR is required to 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 warning light, LED indicator)
When an abnormality occurs in the signal from a sensor, the ECU lights the CHECK ENGINE warning light, stores the failure code in eraseable 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 warning light for 2 seconds.