Computers and Control Systems: Description and Operation

A/C Compressor Clutch Relay
When the 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.


A/C Switch
The A/C (air conditioning) switch signals the PCM whenever there is a demand for cooling.


Alternator Control
The alternator signals the PCM during charging. The 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 PCM. It converts atmospheric pressure into a voltage signal that modifies the basic duration of the fuel injection discharge.





Cylinder Position (CKP) Sensor
The CKP sensor determines timing for fuel injection of each cylinder and also detects engine speed.





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 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 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 PCM detects long term malfunctions in the fuel system, and sets 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 for the octane rating of the gasoline used.

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 to complete, 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.






Manifold Absolute Pressure (MAP) Sensor
The MAP sensor converts manifold absolute pressure into electrical signals to the PCM.






Primary Heated Oxygen Sensors (Primary H02S)
The primary HO(2)S detects the oxygen content in the exhaust gas and sends signals to the 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 exhaust pipe.

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 PCM sets a DTC.






Secondary Heated Oxygen sensors (Secondary HO2S)
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 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 PCM provides a rich mixture by increasing fuel 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 PCM. The TP sensor is not replaceable apart from the throttle body.






[1][2]Top Dead Center (TDC) Sensors
The TDC1/TDC2 sensor determines ignition timing at start up (cranking) and when crank angle is abnormal.