Computers and Control Systems: Description and Operation
PGM-FI SystemThe Programmed Fuel Injection (PGM-FI) system is a sequential multiport fuel injection system.
Air Conditioning (A/C) Compressor Clutch Relay
When the ECM/PCM receives a demand for cooling from the A/C system, it delays the compressor from being energized, and enriches the mixture to assure smooth transition to the A/C mode.
Alternator Control
The alternator signals the Engine Control Module (ECM) / Powertrain Control Module (PCM) during charging. The ECM/PCM then controls the voltage generated at the alternator according to the electrical load determined by the Electrical Load Detector (ELD) and driving mode. This reduces engine load to improve fuel economy.
Barometric Pressure (BARO) Sensor
The BARO sensor is inside the ECM/PCM. It converts atmospheric pressure into a voltage signal that modifies the basic duration of the fuel injection discharge.
Crankshaft Position (CKP) Sensor
The CKP sensor detects engine speed and determines ignition timing and timing for fuel injection of each cylinder.
Engine Coolant Temperature (ECT) Sensor
The ECT sensor is a temperature dependent resistor (thermistor). The resistance of the thermistor decreases as the engine coolant temperature increases.
Ignition Timing Control
The ECM/PCM contains the memory for basic ignition timing at various engine speeds and manifold absolute pressure. It also adjusts the timing according to engine coolant temperature.
Injector Timing and Duration
The ECM/PCM contains the memory for basic discharge duration 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.
By monitoring long term fuel trim, the ECM/PCM detect long term malfunctions in the fuel system and set a Diagnostic Trouble Code (DTC).
Intake Air Temperature (IAT) Sensor
The IAT sensor is a temperature dependent resistor (thermistor). The resistance of the thermistor decreases as the intake air temperature increases.
Knock Sensor
The knock control system adjusts the ignition timing to minimize knock.
Malfunction Indicator Lamp (MIL) Indication (In relation to Readiness Codes)
The vehicle has certain "readiness codes" that are part of the on-board diagnostics for the emissions systems. If the vehicle's battery has been disconnected or gone dead, if the DTCs have been cleared, or if the ECM/PCM has been reset, these codes are reset. In some states, part of the emissions testing is to make sure these codes are set to complete. If all of them are not set to complete, the vehicle may fail the test.
To check if the readiness codes are set, turn the ignition switch ON (II), but do not start the engine. The MIL will come on for 15 - 20 seconds. If it then goes off, the readiness codes are set. If it blinks several times, one or more readiness codes are not set to complete. To set each code, drive the vehicle or run the engine as described in the procedures to set them in this section.
Manifold Absolute Pressure (MAP) Sensor
The MAP sensor converts manifold absolute pressure into electrical signals to the ECM/PCM.
Primary Heated Oxygen Sensor (Primary HO(2)S)
The primary HO(2)S detects the oxygen content in the exhaust gas and sends signals to the ECM/PCM which varies the duration of fuel injection accordingly. To stabilize its output, the sensor has an internal heater. The primary HO(2)S is installed in the TWC (D17A1 engine) or in the exhaust manifold (D17A2 engine). By controlling the air fuel ratio with primary HO(2)S and secondary HO(2)S, the deterioration of the primary HO(2)S can be evaluated by its feedback period. When the feedback period exceeds a certain value during stable driving conditions, the sensor is considered deteriorated and the ECM/PCM sets a DTC.
Secondary Heated Oxygen Sensor (Secondary HO(2)S)
The secondary HO(2)S detects the oxygen content in the exhaust gas downstream of the three way catalytic converter (TWC) and sends signals to the ECM/PCM which varies the duration of fuel injection accordingly. To stabilize its output, the sensor has an internal heater. The secondary HO(2)S is installed in the TWC.
Starting Control
When the engine is started, the ECM/PCM provides a rich mixture by increasing injector duration.
Throttle Position (TP) Sensor
The TP sensor is a potentiometer connected to the throttle valve shaft. As the throttle position changes, the sensor varies the signal voltage to the ECM/PCM. The TP sensor is not replaceable apart from the throttle body.
Top Dead Center (TDC) Sensor
The TDC sensor detects the position of the No.1 cylinder as a reference for sequential fuel injection to each cylinder.
Vehicle Speed Sensor (VSS)
The VSS is driven by the differential. It generates a pulsed signal from an input of 5 volts. The number of pulses per minute increases/decreases with the speed of the vehicle.
Idle Control System
When the engine is cold, the A/C compressor is on, the transmission is in gear, the brake pedal is pressed, the power steering load is high, or the alternator is charging, the ECM/PCM controls current to the Idle Air Control (IAC) valve to maintain the correct idle speed. Refer to the System Diagram to see the functional layout of the system.
Brake Pedal Position Switch
The brake pedal position switch signals the ECM/PCM when the brake pedal is pressed.
Idle Air Control (IAC) Valve
To maintain the proper idle speed, the IAC valve changes the amount of air bypassing the throttle body in response to an electrical signal from the ECM/PCM.
Power Steering Pressure (PSP) Switch
The PSP switch signals the ECM/PCM when the power steering load is high.