Powertrain Control Module Description PCM Function

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 injector, the idle speed control (ISC) motor, the fuel pump relay, etc. The PCM performs the OBD II diagnostic tests of the emission related systems. The PCM supplies a buffered 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 PCM is also responsible for a self-diagnosis function and a fail-safe function.

The PCM (1) is located behind the glove box and below the dash board.

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:

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 (in which the diagnostic runs) without the malfunction occuring. 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:

Control Module Learning Ability

The 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.

Input Components

The powertrain control module (PCM) supplies a buffered 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:

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 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:

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

Powertrain Control Module Description Primary Sys. Based Diagnostics

The powertrain control module (PCM) uses certain diagnostic strategies known as primary system based diagnostics that evaluate the various primary system operations. The primary system based diagnostics also evaluate the various primary system operations affect on vehicle emissions. Some of the primary system based diagnostics are listed below with a brief functional description of the diagnostics involved.

Catalyst Monitor Diagnostic Operation

The OBD II catalyst monitor diagnostic measures the oxygen storage capacity of the three-way catalytic converter (TWC). Heated oxygen sensors are installed before (pre-catalyst) and after (post-catalyst) the TWC. Voltage variations between the sensors allow the powertrain control module to determine the performance of the TWC catalyst. When the TWC catalyst becomes less effective in promoting chemical reactions, the catalyst's capacity to store and release oxygen is generally degraded. The OBD II catalyst monitor diagnostic is based on a correlation between the conversion efficiency of the TWC catalyst and the oxygen storage capacity of the catalyst. A good catalyst (e.g. 95 percent hydrocarbon conversion efficiency) will show a relatively flat output voltage on the post-catalyst Heated Oxygen Sensor (HO2S 2). A degraded catalyst (65 percent hydrocarbon conversion) will show greatly increased activity in the output voltage from the post catalyst HO2S.

The post-catalyst HO2S is used to measure the oxygen storage/release capacity of the catalyst in the TWC. A high oxygen storage capacity indicates a good catalyst. A low oxygen storage capacity indicates a failing catalyst. The TWC and the HO2S 2 must be at operating temperature in order to achieve the correct oxygen sensor voltages (like those shown in the post-catalyst HO2S Outputs graphic).

The catalyst monitor diagnostic is sensitive to the following conditions:

Exhaust system leaks may cause any of the following results:

The presence of HO2S contaminants may prevent the catalyst monitor diagnostic from functioning properly.

Three-Way Catalyst Oxygen Storage Capacity

The three-way catalytic converter (TWC) catalyst must be monitored for efficiency. In order to accomplish this, the control module monitors the pre-catalyst HO2S and post-catalyst HO2S oxygen sensors. When the TWC is operating properly, the post-catalyst oxygen sensor will have significantly less activity than the pre-catalyst oxygen sensor. The TWC stores and releases oxygen as needed during its normal reduction and oxidation process. The control module will calculate the oxygen storage capacity using the difference between the pre-catalyst and post catalyst oxygen sensors voltage levels. If the activity of the post-catalyst oxygen sensor approaches that of the pre-catalyst oxygen sensor, the catalyst's efficiency is degraded.

Stepped or staged testing levels allow the powertrain control module to statistically filter test information. This prevents falsely passing or falsely failing the catalyst monitor oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. Post-catalyst oxygen sensor activity should not be used to determine oxygen storage capacity unless directed by the service manual.

A two stage test is used to monitor the catalyst efficiency. Failure of the first stage of the test will indicate that the catalyst requires further testing in order to determine the catalyst efficiency. The second stage test looks at the inputs from the pre-catalyst and post-catalyst HO2S sensors more closely in order to determine if the catalyst is actually degraded. This two stage test further increases the accuracy of the oxygen storage capacity monitor. Failing the first stage test DOES NOT indicate a failed catalyst. The catalyst may be marginal or the fuel's sulfur content could be very high.

Aftermarket HO2S characteristics may be significantly different from the original equipment manufacturer HO2S. An inferior HO2S may lead to a false pass or a false fail of the catalyst monitor diagnostic. An aftermarket catalytic converter that does not contain the same amount of cerium as the original catalytic converter can cause a false DTC to set. An incorrect amount of cerium in the catalyst can alter the correlation between the oxygen storage and the conversion efficiency of the TWC.

Catalyst Monitor (Good Catalyst)

A good TWC catalyst will show a very active output voltage on the pre-catalyst heated oxygen sensor (1). A good catalyst (95 percent hydrocarbon conversion) will show a relatively flat output voltage on the post-catalyst heated oxygen sensor (2).

Catalyst Monitor (Bad Catalyst)

A degraded (bad) TWC catalyst (65 percent hydrocarbon conversion) will show greatly increased activity in the output voltage from the post-catalyst heated oxygen sensor (2). The degraded catalyst post-catalyst HO2S output voltage will therefore appear similar to the typically active output voltage of the pre-catalyst heated oxygen sensor (1).

Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on crankshaft rotational velocity (reference period) variations. The powertrain control module determines the crankshaft rotational velocity using the crankshaft position sensor and the camshaft position sensor. When a cylinder misfires the crankshaft actually slows down momentarily. By monitoring the crankshaft and the camshaft position sensor signals, the control module can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic will be required to report a misfire that is present within 1000 to 3200 engine revolutions.

For a catalyst damaging misfire, the diagnostic will respond to a misfire that is within 200 engine revolutions.

Rough roads may cause a false misfire detection. A rough road will cause torque to be applied to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity and cause a false misfire detection.

On automatic transaxle equipped vehicles, the torque converter clutch (TCC) will be disabled whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the effect of the drive wheel inputs (torque) on the crankshaft rotation.

When the TCC has been disabled as a result of a misfire detection, the TCC will be re-enabled after approximately 3200 engine revolutions with no misfire is detected. The TCC will remain disabled whenever a misfire is detected. This allows the misfire diagnostic to evaluate the system.

Fuel Trim System Monitor Diagnostic Operation

The fuel system monitor diagnostic averages of short-term and long-term fuel trim values. If these fuel trim values stay at their limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares the averages of the short-term fuel trim values and the long-term fuel trim values to the rich and lean thresholds. If either value is within the thresholds, a pass is recorded. If both values are outside their thresholds, a rich or lean DTC will be recorded.

In order to meet OBD ll requirements, the control module uses weighted fuel trim cells in order to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if the fuel trim counts in the weighted fuel trim cells exceed the specifications. A vehicle that has a fuel trim problem that is causing a concern under certain conditions but operates fine under other conditions may not set a fuel trim DTC. For example an engine that is idling high due to a small vacuum leak or an engine that is running rough due to a large vacuum leak may set an idle speed DTC or an HO2S DTC but not a fuel trim DTC.

A fuel trim DTC may be triggered by many different vehicle faults. Use all diagnostic information available when diagnosing a fuel trim fault.

Powertrain Control Module Description Common OBD II Systems

Table 1:

Storing Freeze Frame and Failure Record Data

The Chevrolet Metro conforms to all OBD II mandated emission regulations. The powertrain control module (PCM) is responsible for monitoring and modifying engine controls in order to meet all OBD II requirements. Certain minimum criteria for how the PCM monitors and diagnoses the emisssion system is specified in the OBD II regulations. The following information describes the common operations and terms of the OBD II regulations that the Metro PCM is designed to adhere to.

Diagnostic

The word diagnostic, when used as a noun, refers to any on-board test run by the vehicle's powertrain control module (PCM). A diagnostic is simply a test run on a system or component in order to determine if the system or the component is operating according to specifications. The following list defines the major vehicle on-board diagnostics. Depending on the emission requirements in the area of vehicle sale, certain diagnostics listed below may not apply.

Enable Criteria

The term enable criteria is engineering language for the conditions necessary for a given diagnostic test to run. Each diagnostic has a specific list of conditions which must be met before the diagnostic will run. Enable criteria is another way of saying conditions required.

The enable criteria for each diagnostic is listed on the first page of the DTC description under the heading Conditions for Setting the DTC. The enable criteria varies with each diagnostic, and typically includes, but is not limited to the following items:

Trip

Technically, a trip is a key on-run-key off cycle in which all the enable criteria for a given diagnostic are met, allowing the diagnostic to run. Unfortunately, this concept is not quite that simple. A trip is official when all the enable criteria for a given diagnostic are met. Because the enable criteria vary from one diagnostic to another, the definition of a trip varies as well. Some diagnostics are run when the vehicle is at operating temperature. Some diagnostics are run when the vehicle first starts up. Some diagnostics require that the vehicle is cruising at a steady highway speed. Some diagnostics run only when the vehicle is at idle. Some diagnostics function with the torque converter clutch disabled. Some diagnostics run only immediately following a cold engine start-up.

A trip then, is defined as a key on-run-key off cycle in which the vehicle was operated in such a way as to satisfy the enable criteria for a given diagnostic. This diagnostic will consider this cycle to be one trip. However, another diagnostic with a different set of enable criteria (which were not met) during this driving event, would not consider it a trip. No trip will occur for that particular diagnostic until the vehicle is driven in such a way as to meet all the enable criteria.

Passive and Active Diagnostic Tests

A passive test is a diagnostic test which simply monitors a vehicle system or a vehicle component. An active test actually takes some sort of action when the performing diagnostic functions. An active test is often in response to a failed passive test. For example, the exhaust gas recirculation (EGR) diagnostic active test may force the EGR valve open during a closed throttle deceleration maneuver. Or the EGR diagnostic active test may force the EGR valve closed during a period of steady speed driving. Either action should result in a change in the manifold pressure.

Warm-Up Cycle

A warm-up cycle means that the engine temperature must reach a minimum of 70°C (160°F) and rise at least 22°C (40°F) over the course of a trip.

Freeze Frame and Failure Records

Freeze Frame is an element of the Diagnostic Management System that stores various vehicle information in the PCM memory at the moment an emissions-related fault occurs that commands the MIL On. Freeze Frame data is useful in identifying the cause of an emission-related fault. Freeze Frame data is accessed by the scan tool and can be saved with the Capture Data feature. Refer to Storing And Erasing the Freeze Frame Data in Powertrain Control Module Diagnosis for more detailed information.

The PCM can store one Freeze Frame and save the recorded data of three additional Freeze Frames as Failure Records. Therefore the PCM can store up to 4 frames of Freeze Frame and Failure Record data. The 1st frame stores data of the fault that was detected first. The data from the 1st detected fault is also stored in the 2nd frame as a Failure Record. The Failure Record data stored in the 2nd frame is permanent, and will not change when a new fault is detected. The 1st frame of Freeze Frame data will remain unchanged unless a fault of a higher priority occurs. A misfire fault (DTC P0300-P0303) or a fuel trim fault (DTC P0171 and P0172) will replace the data in the 1st frame of Freeze Frame data because these DTCs have a higher priority under OBD 2 rules. The 2nd through 4th frames of Failure Records will store fault data in the order that the faults were detected regardless of the fault's priority. Utilizing the 4 frames of Freeze Frame and Failure Record data can provide information on the order in which the faults were first detected. The following table indicates how Freeze Frame and Failure Record data is stored when two or more faults are detected.

Storing Freeze Frame and Failure Record Data

Order	Fault	Frame 1	Frame 2	Frame 3	Frame 4
--	No Fault detected	No Freeze Frame data	No Failure Record data	No Failure Record data	No Failure Record data
1	DTC P0118	P0118 data stored	P0118 data stored	--	--
2	DTC P0171	P0171 data replaces P0118 data	P0118 data still stored	P0171 data stored	--
3	DTC P0300	P0171 data still stored	P0118 data still stored	P0171 data still stored	P0300 data stored
4	DTC P0301	P0171 data still stored	P0118 data still stored	P0171 data still stored	P0300 data still stored

Diagnostic Information

The diagnostic tables and the functional checks are designed to locate a faulty circuit or component through a process of logical decisions. The diagnostic tables are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are not multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complimented by the diagnostic procedures contained in this service information. The language of communicating the source of the malfunction is a system of diagnostic trouble codes (DTCs). When a malfunction is detected by the powertrain control module, a DTC is set and the malfunction indicator lamp (MIL) lamp is illuminated.

Malfunction Indicator Lamp (MIL)

The malfunction indicator lamp (MIL) looks the same as the MIL you may be already familiar with (Service Engine Soon or Check Engine). OBD II regulations require that the MIL illuminate according to a strict set of guidelines. Under OBD II the MIL is turned on when the PCM detects a malfunction that will impact the vehicle emissions.

The MIL is controlled by the PCM. The MIL will be illuminated if an emissions-related diagnostic test indicates a malfunction has occurred. The MIL will stay illuminated until the system or the component passes the same test for three consecutive trips.

A vehicle that is experiencing a misfire malfunction that may cause damage to the three-way catalytic converter (TWC) will flash the MIL once per second. The MIL will continue to flash once per second until the vehicle is outside of the speed and the load conditions that may cause damage to the TWC catalyst. The MIL will stop flashing and remain on steady once the vehicle is outside of the speed and the load conditions that may cause damage to the TWC catalyst.

Extinguishing the MIL

The powertrain control module (PCM) will turn Off the malfunction indicator lamp (MIL) after three consecutive trips that a test passed has been reported for the diagnostic test that originally caused the MIL to illuminate.

The diagnostic trouble code (DTC) will remain in the PCM memory and the Freeze Frame record until forty (40) warm-up cycles have been completed and no faults exist.

An MIL that was illuminated by either a fuel-trim DTC or a misfire-related DTC has additional requirements that must be met in order to turn Off the MIL. The additional requirements are as follows:

Meeting these requirements ensures that the fault which turned on the MIL has been corrected.

DTC Types

DTCs are categorized by type. The type indicates the action the PCM will take in recording the DTC failure and illuminating the MIL. The following table indicates what action the PCM takes for the 3 different DTC types when a DTC failure occurs.

DTC Type	MIL Illumination	Freeze Frame Stored	Failure Records Stored*
A	Yes	Yes	Yes
B	Yes (with two fails)	Yes (on second failure)	Yes (on second failure)
C	No	No	No
* The PCM in this vehicle stores up to three Failure Records

In order for a type B DTC to request MIL illumination the DTC must fail in two consecutive drive trips in which the DTC tests.

Refer to the DTC List table in Specifications for the type of DTC the particular PCM supports.

There are two type C DTCs. The C1 type DTC will illuminate an information lamp or display a message. The type C0 DTC DOES NOT illuminate an information lamp or display a message. Type C0 DTCs were formerly referred to as Type D.

Always refer to the diagnostic support information within each DTC to obtain the Action Taken when the DTC sets and the Conditions for Clearing the DTC. These will indicate any variations from the general type A, B and C actions.

Special Cases Of Type B Diagnostic Tests

The powertrain control module (PCM) has the capability of alerting the vehicle operator to potentially damaging levels of misfire. A misfire condition that could potentially damage the catalytic converter as a result of high misfire levels will command the PCM to flash the malfunction indicator lamp (MIL) at a rate of once per second. This once per second flashing of the MIL will occur during the entire time that the catalyst damaging misfire condition is present. This flashing of the MIL is unique to the misfire diagnostic.

Maintenance Schedule

Refer to the General Motors Maintenance Schedule of the appropriate service category for the maintenance that the owner or the technician should perform in order to retain emission control performance.