The powertrain has electronic controls to reduce exhaust emissions while maintaining excellent driveability and fuel economy. The powertrain control module (PCM) manages the operation of the engine control system.

Review the components and wiring diagrams in order to determine which systems are controlled by the PCM. The PCM monitors numerous engine, transmission, and vehicle functions. The following are some of the functions that the PCM controls:

Powertrain Control Module Function

The PCM constantly looks at the information from various sensors and other inputs, and controls systems that affect vehicle performance and emissions. The PCM also performs diagnostic tests on various parts of the system. The PCM can recognize operational problems and alert the driver via the malfunction indicator lamp (MIL). When the PCM detects a malfunction, it stores a diagnostic trouble code (DTC). The PCM supplies a buffered voltage ( 5V or 12V) to various sensors and switches. The input and output devices in the control module include analog to digital converters, signal buffers, counters, multiple function drivers, and output drivers. The multiple function drivers (i.e., EGR control, fuel pump relay) are electronic switches that supply ignition voltage to energize the circuit. The output drivers are electronic switches that, when energized, complete the ground path. Most PCM controlled components are operated via output drivers. The PCM monitors these driver circuits for proper operation. In most cases, if a condition is detected, a DTC corresponding to the controlled device sets.

Torque Management

Torque Management is a function of the PCM that reduces engine power under certain conditions. Torque management is performed for the following reasons:

The PCM uses manifold vacuum, intake air temperature, spark retard, engine speed, engine coolant temperature, A/C clutch status, and EGR valve position to calculate engine output torque. The PCM also monitors the torque converter clutch status, transaxle gear ratio, and brake switch inputs and determines if any torque reduction is required. If torque reduction is required, the PCM retards spark as appropriate to reduce engine torque output. In the case of abusive maneuvers, the PCM may also shut off fuel to certain cylinders to reduce engine power.

There are five instances when engine power reduction is likely to be experienced:

In the first two instances, the driver is unlikely to even notice the torque management actions. In the other cases, engine power output will be moderate at full throttle.

When the PCM determines that engine power reduction is required, it calculates the amount of spark retard necessary to reduce power by the desired amount. This spark retard is then subtracted from the current spark advance. In the case of abusive maneuvers, the PCM momentarily disables fuel injectors to obtain the necessary amount of torque reduction.

Traction Control

Traction Control is a function of the PCM and electronic brake and traction control module (EBTCM) to reduce wheel slip during acceleration. When wheel slip is detected, the EBTCM applies the front brakes, and the PCM reduces engine power. Refer to Brakes for an explanation of the EBTCMs role in traction control. The PCM continuously sends the EBTCM a PWM signal to indicate the torque output of the powertrain. This signal, referred to as the delivered torque signal, is used by the EBTCM to determine the required action when it sees the front wheels slipping. The EBTCM may decide to apply the front brakes only, or apply the front brakes and signal the PCM to reduce the torque output of the powertrain. The EBTCM requests reduced torque using another PWM signal. This signal, referred to as the desired torque signal, is used by the PCM to determine the amount of torque reduction requested by the EBTCM. The PCM adjusts the ignition timing in response to the desired torque signal.

The desired torque signal varies within a range of 95 percent and 5 percent duty cycle. A duty cycle at 95 percent indicates no torque reduction. A duty cycle at 10 percent indicates full torque reduction. The desired torque signal to the PCM will normally be at a 90 percent duty cycle. The EBTCM will decrease the duty cycle of the signal by the amount of torque reduction desired. The PCM responds to the signal by adjusting the ignition timing. The PCM may shut off the fuel to one or more cylinders unless the following conditions are present:

The PCM re-enables the fuel injectors as the need for traction control ends.

Traction control is disable when certain DTCs set. The PCM will request a traction off light/message via class 2 serial data when the DTCs set.

Class 2 Serial Data

The Class 2 serial data circuit allows the control modules to communicate with each other. The modules send a series of digital signals pulsed from high to low voltage (approximately 7 volts to 0 volts). These signals are sent in variable pulse widths of one or two bits. A string of these bits creates a message that is sent in a prioritized data packet. This allows more than one module to send messages at the same time without overloading the serial data line.

When the key is ON, each module sends a State of Health message to the other control modules using the class 2 serial data line. This ensures that the modules are working properly. When the module stops communicating, a loss of the state of health message occurs. The control modules that expect to receive the message detect the loss, and will set a loss of the state of health diagnostic trouble code (DTC).

Data Link Connector (DLC)

The Data Link Connector (DLC) provides a class 2 data circuit that allows bi-directional communication between the scan tool, the PCM, and other system control modules. Usually located under the instrument panel, the DLC provides power and ground for the scan tool. Some common uses of the scan tool are listed below:

Service Engine Soon/Malfunction Indicator Lamp (MIL)

The Service Engine Soon/Malfunction Indicator Lamp (MIL) is located in the instrument panel cluster (IPC). The MIL is controlled by the PCM and is used to indicate that the PCM has detected a problem that affects vehicle emissions, may cause powertrain damage, or severely impacts driveability.

Trip

A trip is an interval of time during which the diagnostic test runs. A trip may consist of only a key cycle to power up the PCM, allow the diagnostic to run, then cycle the key off to power down the PCM. A trip may also involve a PCM power up, meeting specific conditions to run the diagnostic test, then powering down the PCM. The definition of a trip depends on the diagnostic. Some diagnostic tests run only once per trip (i.e., catalyst monitor) while other tests run continuously during each trip (i.e., misfire, fuel system monitors).

Warm-up Cycle

The PCM uses warm-up cycles to run some diagnostics, and to clear any diagnostic trouble codes (DTCs). A warm-up cycle occurs when the engine coolant temperature raises 22°C (40°F) from start-up. The engine coolant must also achieve a minimum temperature of 71°C (160°F). The PCM counts the number of warm-up cycles in order to clear the malfunction indicator lamp (MIL). The PCM will clear the DTC(s) when 40 consecutive warm-up cycles occur without a malfunction.

Diagnostic Trouble Code Display

DTCs can only be displayed with the use of a scan tool.

DTC Status

When the scan tool displays a DTC, the status of the DTC is also displayed. The following DTC statuses will be indicated only when they apply to the DTC that is set.

Fail This Ign. (Fail This Ignition): Indicates that this DTC failed during the present ignition cycle.

Last Test Fail: Indicates that this DTC failed the last time the test ran. The last test may have run during a previous ignition cycle if an A or B type DTC is displayed. For type C DTCs, the last failure must have occurred during the current ignition cycle to appear as Last Test Fail.

MIL Request: Indicates that this DTC is currently requesting the MIL. This selection will report type B DTCs only when they have requested the MIL. (failed twice).

Test Fail SCC (Test Failed Since Code Clear): Indicates that this DTC that has reported a failure since the last time DTCs were cleared.

History: Indicates that the DTC is stored in the PCMs History memory. Type B DTCs will not appear in History until they have requested the MIL (failed twice). History will be displayed for all type A DTCs and type B DTCs (which have requested the MIL) that have failed within the last 40 warm-up cycles. Type C DTCs that have failed within the last 40 warm-up cycles will also appear in History.

Not Run SCC (Not Run Since Code Clear): DTCs will be listed in this category if the diagnostic has not run since DTCs were last cleared. This status is not included with the DTC display since the DTC can not be set if the diagnostic has not run. This information is displayed when DTC Info is requested using the scan tool.

Clearing Diagnostic Trouble Codes

Use a scan tool to clear DTCs from the PCM memory. Disconnecting the vehicle battery to clear the PCM memory is not recommended. This may or may not clear the PCM memory and other vehicle system memories will be cleared. Do not disconnect the PCM connectors solely for clearing DTCs. This unnecessarily disturbs the connections and may introduce a new problem. Before clearing DTCs the scan tool has the capability to save any data stored with the DTCs and then display that data at a later time. Capture DTC info before beginning diagnosis (refer to Capturing DTC Info). Do not clear DTCs until you are instructed to do so within a diagnostic procedure.

Many PCM DTCs have complex test and failure conditions. Therefore, simply clearing DTCs and watching to see if the DTC sets again may not indicate whether a problem has been corrected. To verify a repair after it is complete, you must look up the test conditions and duplicate those conditions. If the DTC runs and passes, chances are good that the problem is fixed.

Freeze Frame Data

Government regulations require that engine operating conditions be captured whenever the MIL is illuminated. The data captured is called Freeze Frame data. The Freeze Frame data is very similar to a single record of operating conditions. Whenever the MIL is illuminated, the corresponding record of operating conditions is recorded as Freeze Frame data. A subsequent failure will not update the recorded information.

The Freeze Frame data parameters stored with a DTC failure include the following:

Freeze frame data can only be overwritten with data associated with a misfire or fuel trim malfunction. Data from these faults take precedence over data associated with any other fault. The Freeze Frame data will not be erased unless the associated history DTC is cleared.

Failure Records Data

When a diagnostic test fails, the PCM stores the information as Failure Records data. Unlike Freeze Frame data, Failure Records are stored for multiple DTCs or non-emission related DTCs (DTCs that do not illuminate the MIL). The Failure Records information is updated the first time the test fails during each ignition cycle.

Freeze Frame, Failure Records data may be retrieved through the scan tool. If more than one DTC is set review the odometer or engine run time data located in the Freeze Frame, Failure Records info to determine the most current failure. The stored information will look like normal PCM data.

Important: Always capture the Freeze Frame and Failure Records information with the scan tool BEFORE proceeding with diagnosis. Loss of this data may prevent accurate diagnosis of an intermittent condition.

Capturing DTC Info (Capture Info)

Selecting this option on the scan tool allows the technician to record the Freeze Frame and Failure Records that may be stored in the PCMs memory. Capture DTC info before beginning diagnosis. This is a step in the OBD System Check. At the end of the diagnostic tables, you are instructed to clear DTCs, verify that the repair was successful and then to review captured information. The captured information will contain any additional DTCs and related data that was stored originally (if multiple DTCs were stored).

Storing And Erasing Freeze Frame Data

Government regulations require that engine operating conditions be captured whenever the MIL is illuminated. The data captured is called freeze frame data. The freeze frame data is very similar to a single record of operating conditions. Whenever the MIL is illuminated, the corresponding record of operating conditions is recorded to the freeze frame buffer.

Each time a diagnostic test reports a failure, the current engine operating conditions are recorded in the freeze frame buffer. A subsequent failure will update the recorded operating conditions. The following operating conditions for the diagnostic test which failed typically include the following parameters:

Freeze frame data can only be overwritten with data associated with a misfire or fuel trim malfunction. Data from these faults take precedence over data associated with any other fault. The freeze frame data will not be erased unless the associated history DTC is cleared.

PCM Snapshot Using A Scan Tool

The scan tool can be set up to take a snapshot of the parameters available via serial data. The Snapshot function records live data over a period of time. The recorded data can be played back and analyzed. The scan tool can also graph parameters singly or in combinations of parameters for comparison. The snapshot can be triggered manually at the time a symptom is noticed, or set up in advance to trigger when a DTC sets. An abnormal value captured in the recorded data may point to a system or component that needs to be investigated further. The snapshot will remain in the scan tool memory even after the tool is disconnected from it's power source. Refer to the scan tool user instructions for more information on the Snapshot function.