Powertrain Control Module Description. Geo Prizm 1997

For 1990-2009 cars

The powertrain control module (PCM) is a precision unit consisting of a one chip microprocessor, an A/D (Analog/Digital) converter, and an I/O (Input/Output) unit. The PCM is an essential part of the electronic control system. The PCM is responsible for such major functions as control of the fuel injectors, the idle air control (IAC) valve, the fuel pump relay, etc. The PCM performs the OBD II diagnostic tests of the emission related systems. The PCM supplies a buffered voltage (called reference voltage) to the various information sensors and switches. The PCM controls most components with an electronic switch that completes a ground circuit when turned ON. The electronic switch is commonly referred to as an output driver. The PCM is also responsible for a self-diagnosis function and a fail-safe function.

The PCM is located in the center console below the entertainment center (radio/cassette/CD player).

### Self-Diagnosis Function

The powertrain control module (PCM) diagnoses any troubles which may occur in the engine control system when the ignition switch is in the ON position with the engine running. The PCM indicates a malfunction by illuminating the malfunction indicator lamp (MIL) when a fault occurs in any of the following systems:

• | Oxygen Sensor 1 (O2S 1)

• | Heated Oxygen Sensor 2 (HO2S 2)

• | Engine Coolant Temperature (ECT) sensor

• | Throttle Position (TP) sensor, including the CTP switch

• | Vehicle Speed Sensor (VSS)

• | Intake Air Temperature (IAT) sensor

• | Manifold Absolute Pressure (MAP) sensor

• | Camshaft Position (CMP) sensor

• | Crankshaft Position (CKP) sensor

• | Exhaust Gas Recirculation (EGR) control system

• | Idle Air Control (IAC) system

• | Misfire Detection

• | Fuel-trim

• | Catalyst Monitor

• | Central Processing Unit (CPU) of the PCM

When the PCM detects a malfunction in one of the above areas, the PCM will illuminate or flash the MIL in order to notify the driver of the occurrence of a fault. The PCM will store a diagnostic trouble code (DTC) when the PCM illuminates the MIL.

The PCM will turn Off the MIL after 3 consecutive ignition cycles without the malfunction occurring. The DTC will remain stored in the PCM memory after the MIL is Off.

### Fail-Safe Function

When a malfunction occurs within the engine control system, the powertrain control module (PCM) maintains control over the fuel injection system, the idle speed control system, etc. The PCM controls these systems by using calculated values and/or backup programs stored within the PCM.

This function is called the fail-safe function. With the fail-safe function, a certain level of engine performance is available even when a malfunction occurs. The fail-safe function prevents a complete loss of engine performance.

The systems covered by the fail-safe function are as follows:

• | The Engine Coolant Temperature (ECT) sensor

• | The Throttle Position (TP) sensor

• | The Manifold Absolute Pressure (MAP) sensor

• | The Intake Air Temperature (IAT) sensor

• | The Central Processing Unit (CPU) in the PCM

• | Fuel Cutoff

### Control Module Learning Ability

The powertrain control module has a learning ability which enables the control module to make corrections for minor variations in the fuel system. This learning ability can improve driveability. Disconnecting the battery resets the learning process. A change in the vehicles performance may be noticed when a reset occurs. The vehicle operator can teach the control module in order to regain some of the lost vehicle performance.

In order to teach the control module, ensure that the engine is at operating temperature and drive the vehicle at part throttle with moderate acceleration. The vehicle may also be operated at idle conditions until normal performance returns.

### Check Mode

The powertrain control module (PCM) has the ability to operate in the normal mode or in the check mode. The two modes of operation are identical except that when the PCM is operating in the check mode, the PCM has an increased ability to detect malfunctions. In order to request that the PCM operate in the check mode, scan tool communication with the PCM is necessary.

### Input Components

The powertrain control module (PCM) supplies a buffered (reference) voltage to the various information sensors and switches. The PCM monitors the input components for circuit continuity and out-of-range values. The PCM also provides performance checking. Performance checking refers to the PCM indicating a fault when the signal from an input does not seem reasonable (i.e. a throttle position (TP) sensor that indicates high throttle position at low engine loads or low manifold absolute pressure sensor voltage). The input components may include, but are not limited to the following sensors and switches:

• | Crankshaft Position (CKP) sensor

• | Throttle Position (TP) sensor

• | Engine Coolant Temperature (ECT) sensor

• | Camshaft Position (CMP) sensor

• | Manifold Absolute Pressure (MAP) sensor

• | Oxygen Sensors (O2S)

• | Vehicle Speed Sensor (VSS)

• | The A/C Compressor Control Module (A/C relay)

For more information on some of the PCM input components refer to Information Sensors/Switches Description.

### Output Components

The powertrain control module (PCM) is responsible for the control and operation of many output components. The PCM controls many components with an electronic switch (called an output driver) that completes a ground circuit when turned ON. The PCM monitors the output components for the proper response to the PCM commands. Components where functional monitoring is not feasible will be monitored for circuit continuity and out-of-range values if applicable.

Output components to be monitored include, but are not limited to the following circuits:

• | The Idle Air Control (IAC) valve

• | The Circuit Opening Relay

• | The Malfunction Indicator Lamp (MIL) control

• | The A/C Compressor Control Module (A/C relay)

For more information on some of the PCM output components refer to PCM Outputs Description.