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 ECM/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 in gear (A/T only) the brake pedal is depressed, the P/S load is high, or the alternator is charging, the ECM/PCM controls current to the IAC Valve to maintain correct idle speed.
Ignition Timing Control
- The ECM/PCM contains memories for basic ignition timing at various engine speeds and manifold pressures. Ignition timing is also adjusted for engine coolant temperature.
- A Knock Control System is adopted which sets the ideal ignition timing for the octane number of gasoline used.
Other Control Functions
1. Starting Control
When the engine is started, the ECM/PCM provides a rich mixture by increasing fuel injector duration.
2. Fuel Pump Control
- When the ignition switch is initially turned on, the ECM/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 ECM/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 ECM/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 following rpm:
- B18B1 engine: 970 rpm (Canada model: 1,050 rpm)
- B18C1 engine: 970 rpm (Canada model: 1,050 rpm)
- B18C5 engine: 1,300 rpm (Canada model: 1,400 rpm)
- Fuel cut-off action also takes place when engine speed exceeds 7,000 rpm (B18B1 engine), 8,100 rpm (B18C1 engine) or 8,600 rpm (B18C5 engine), regardless of the position of the throttle valve, to protect the engine from over-revving. With A/T model, the PCM cuts the fuel at engine speeds over 5,000 rpm when the vehicle is not moving.
4. A/C Compressor Clutch Relay
When the ECM/PCM receives a demand for cooling from the air conditioning system, it delays the compressor from being energized, and enriches the mixture to assure smooth transition to the A/C mode.
5. Evaporative Emission (EVAP) Purge Control Solenoid Valve
When the engine coolant temperature is above 154 °F (68 °C) and the air conditioner on, the ECM/PCM controls the EVAP purge control solenoid valve which controls vacuum to the EVAP purge control canister.
6. Intake Air Bypass (IAB) Control Solenoid Valve
When the engine rpm is below 5,750 rpm, the IAB control solenoid valve is activated by a signal from the ECM/PCM, intake air flows through the long intake path, then high torque is delivered. At speeds higher than 5,750 rpm, the solenoid valve is deactivated by the ECM/PCM, and intake air flows through the short intake path in order to reduce the resistance in airflow.
ECM Fail-safe/Back-up Functions
1. Fail-safe Function
When an abnormality occurs in a signal from a sensor, the ECM/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 ECM/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 ECM/PCM supplies ground for the MIL and stores the code in erasable memory. When the ignition is initially turned on, the ECM/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 Secondary H02S, fuel metering related, idle control system, ECT sensor, TWC and EVAP control system and other self-diagnostic functions. When an abnormality occurs, the ECM/PCM stores it in its memory. When the same abnormality recurs after the ignition switch is turned OFF and ON (II) again, the ECM/PCM informs the driver by lighting the MIL. However, to ease troubleshooting, this function is cancelled when you short the service check connector. The MIL will then blink immediately when an abnormality occurs.