Computers and Control Systems: Description and Operation

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

Fuel Injector Timing and Duration
The ECM 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) or the alternator is charging, the ECM controls current to the IAC Valve to maintain correct idle speed.

Ignition Timing Control
The ECM contains memories for basic ignition timing at various engine speeds and manifold pressures. Ignition timing is also adjusted for engine coolant temperature.

Other Control Functions
1. Starting Control
When the engine is started, the ECM provides a rich mixture by increasing injector duration.
2. Fuel Pump Control
^ When the ignition switch is initially turned on, the ECM supplies ground to the main relay that supplies current to the fuel pump for two seconds to pressurize the fuel system.
^ When the engine is running, the ECM supplies ground to the main relay that supplies current to the fuel pump.
^ When the engine is not running and the ignition is on, the ECM 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 the following rpm in no-load conditions in which the headlights, blower fan, rear defogger, radiator fan and air conditioner are not operating.
- D15B8 engine 870 rpm
- D15B7 engine M/T: 920 rpm
A/T: 900 rpm
- D15Z1 engine 850 rpm
- D16Z6 engine M/T: 930 rpm
A/T: 920 rpm
^ Fuel cut-off action also takes place when engine speed exceeds 5,800 rpm (D1 5B8 engine), 6,650 rpm (D15B7 engine), 6,300 rpm (D15Z1 engine), 7,400 rpm (D16Z6 engine), regardless of the position of the throttle valve, to protect the engine from over-revving.
4. A/C Compressor Clutch Relay
When the ECM 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 below 1 67°F (75°C), the ECM supplies a ground to the EVAP purge control solenoid valve which cuts vacuum to the EVAP purge control diaphragm valve.
6. Exhaust Gas Recirculation (EGR) Control Solenoid Valve
When the EGR is required for control of oxides of nitrogen (NOx) emissions, the ECM supplies ground to the EGR control solenoid valve which supplies regulated vacuum to EGR valve.
7. Alternator Control
The system controls the voltage generated at the alternator in accordance with the electrical load and drive mode, which reduces the engine load to improve the fuel economy.
8. Lock-up Control Solenoid Valve (A/T)
The vehicle speed and throttle position sensor inputs to the ECM 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.

ECM Fail-safe/Back-up Functions
1. Fail-safe Function
When an abnormality occurs in a signal from a sensor, the ECM 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 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 supplies ground for the MIL and stores the code in erasable memory. When the ignition is initially turned on, the ECM supplies ground for the MIL for two seconds to check the MIL bulb condition.
4. Two Trip Detection Method
To prevent false indications, the Two Trip Detection Method is used for the HO2S, fuel metering and EGR-related self-diagnostic functions. When an abnormality occurs, the ECM stores it in its memory. When the same abnormality recurs after the ignition switch is turned OFF and ON again, the ECM 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.