Computers and Control Systems: Description and Operation

System Description - PGM-FI System
The PGM-FI system on this model is a sequential multiport fuel injection system.

Fuel Injector Timing and Duration
The PCM 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.

Idle Air Control
Idle Air Control Valve (IAC Valve)
When the engine is cold, the A/C compressor is on, the transmission is gear, the brake pedal is depressed, the P/S load is high, or the alternator is charging, the PCM controls current to the (IAC Valve to maintain the correct idle speed.

Ignition Timing Control
- The PCM contains memories for basic ignition timing at various engine speeds and manifold air flow rates. Ignition timing is also adjusted for engine coolant temperature.
- A knock control system is also used. When detonation is detected by a knock sensor (KS), the ignition timing is retarded.

Other Control Functions
1. Starting Control
When the engine is started, the PCM provides a rich mixture by increasing fuel injector duration.

2. Fuel Pump Control
- When the ignition switch is initially turned on, the PCM supplies ground to the PGM-FI main relay that supplies current to the fuel pump for two seconds to pressurize the fuel system.
- When the engine is running, the PCM supplies ground to the PGM-FI main relay that supplies current to the fuel pump.
- When the engine is not running and the ignition is on, the PCM cuts ground to the PGM-FI main relay which cuts current to the fuel pump.

3. Fuel Cut-off Control
- During deceleration with the throttle valve closed, current to the fuel injectors is cut off to improve fuel economy at speeds over 1,000 rpm.
- Fuel cut-off action also takes place when engine speed exceeds 5,900 rpm, regardless of the position of the throttle valve, to protect the engine from over-revving.

4. A/C Compressor Clutch Relay
When the PCM receives a demand for cooling from the air conditioning system, it delays the compressor from being energized, and enriches the mixture to assure a smooth transition to the A/C mode.

5. Evaporative Emission (EVAP) Purge Control Solenoid Valve
When the engine coolant temperature is below 99°F (37°C)*1, 140°F (60°C): *2, the PCM controls the EVAP purge control solenoid valve which cuts vacuum to the EVAP purge control canister diaphragm.
:*1 '96-99 model
:*2 '00 model

6. Intake Air Bypass (IAB) Low Control Solenoid Valve, Intake Air Bypass (IAB) High Control Solenoid Valve
When engine speed is below 3,200 rpm, the IAB High Control Solenoid Valve and IAB Low Control Solenoid Valve are activated by a signal from the PCM. Intake air flows through a long chamber path, increasing torque at low RPM. When engine speed is 3,200 - 3,800 rpm, the IAB Low Control Solenoid Valve is deactivated by the PCM. Intake air flows through a short chamber path, increasing mid-range torque.
When the engine rpm is above 3,800 rpm, the IAB Low Control Solenoid Valve and IAB High Control Solenoid Valve are deactivated by the PCM. This creates a very short intake path and increases high-speed torque.

7. Exhaust Gas Recirculation (EGR) Control Solenoid Valve
When the EGR is required for control of oxides of nitrogen (NOx) emissions, the PCM controls the EGR control solenoid valve which supplies regulated vacuum to the EGR valve.

ECM Fail-safe/Back-up Functions
1. Fail-safe Function
When an abnormality occurs in a signal from a sensor, the PCM ignores that signal and assumes a pre-programmed value for that sensor that allows the engine to continue to run.

2. Back-up Function
When an abnormality occurs in the PCM itself, the fuel injectors are controlled by a back-up circuit independent of the system in order to permit minimal driving.

3. Self-diagnosis Function [Malfunction Indicator Lamp (MIL)]
When an abnormality occurs in a signal from a sensor, the PCM supplies ground for the MIL and stores the DTC in erasable memory. When the ignition is initially turned on, the PCM supplies ground for the MIL for two seconds to check the MIL bulb condition.

4. Two Driving Cycle Detection Method
To prevent false indications, the "two driving cycle detection method" is used for the HO2S, fuel metering-related, idle control system, ECT sensor, EGR system, TWC and EVAP control system and other self-diagnostic functions. When an abnormality occurs, the PCM stores it in its memory. When the same abnormality recurs after switch is turned OFF and ON (II) again, the PCM informs the driver by turning on the MIL.