Powertrain

The powertrain used in this vehicle consists of a 3.4L pushrod 60° V-6 engine mated to a 4T65-E electronically controlled transaxle. Refer to Engine Component Description in Engine Mechanical for more information on the powertrain. The powertrain has electronic controls to reduce exhaust emissions while maintaining excellent driveability and fuel economy. The powertrain control module (PCM) is the essence of this control system.

The PCM is designed to maintain exhaust emission levels to Federal/California/other standards while providing excellent driveability and fuel efficiency. Review the components and wiring diagrams in order to determine which systems are controlled by the PCM. The PCM monitors numerous engine 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, the PCM stores a diagnostic trouble code (DTC). The problem area is identified by the particular DTC that is set. The control module supplies a buffered voltage to various sensors and switches. The input and output devices in the PCM include analog-to-digital converters, signal buffers, counters, and output drivers. The output drivers are electronic switches that complete a ground or voltage circuit when turned on. Most PCM controlled components are operated via output drivers. The PCM monitors these driver circuits for proper operation and, in most cases, can set a DTC corresponding to the controlled device if a problem is detected.

Torque Management

Torque management is a function of the PCM that reduces the engine power under certain conditions. Torque management is performed for 3 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 the engine output torque. The PCM then monitors the torque converter status, the transaxle gear ratio, and the brake switch inputs and determines if any torque reduction is required. If torque reduction is required, the PCM retards the spark in order to reduce the engine torque output. In the case of abusive maneuvers, the PCM may also shut off fuel to certain cylinders in order to reduce the engine power.

There are 4 instances when an engine power reduction is likely to be experienced:

In the first 2 instances, the driver is unlikely to even notice the torque management actions. In the other situations, the engine power output will be moderate at full throttle.

When the PCM determines that an engine power reduction is required, the PCM calculates the amount of spark retard necessary to reduce the 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 the fuel injectors in order to obtain the necessary amount of torque reduction.

Traction Control

Traction control is a function of the PCM and the electronic brake control module (EBCM) that reduces front wheel slip during acceleration by applying the front brakes and reducing the engine power. Refer to ABS Operation in Antilock Brake System for an explanation of the role of the EBCM in traction control. The PCM continuously sends out a signal that indicates the torque output of the powertrain. This signal is delivered through the serial data link, and is used by the EBCM to determine what action is required when the EBCM senses the front wheels slipping. The EBCM may apply the front brakes only or apply the front brakes and request a reduced torque output from the powertrain. The PCM then reduces the engine torque by adjusting the ignition timing.

The PCM responds by shutting off fuel to one or more cylinders. The PCM does not shut off any fuel injectors if any of the following conditions are present:

The disabled fuel injectors will be enabled one by one as the need for traction control decreases.

Several DTCs disable the traction control when set. The DTCs also trigger a Traction Off light or message. The PCM traction control override also disables the traction control and triggers the message. To diagnose a Traction Off light or message, diagnose any DTCs set first. Then inspect the traction control override and, if active, deactivate the override. If the Traction Off light or message is still present, refer to Diagnostic Starting Point in Antilock Brake System for further diagnosis.

Class II Serial Data

The Class II serial data circuit allows the control modules to communicate with each other. The modules send a series of digital signals pulsed from approximately 7 volts to 0 volts. These signals are sent in variable pulse widths of one or 2 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. The speed, or baud rate, at which the control modules communicate depends on the message content. A large message content lowers the baud rate, while a small message content increases the baud rate. The average baud rate is approximately 10.4 Kbps (10,400 bits per second). When the key is ON, each module sends a state of health (SOH) message to the other control modules using the Class II serial data line. This ensures that the modules are working properly. When the module stops communicating, a loss of the SOH message occurs. The control modules that should receive the message detect the loss and set a loss of communication DTC.

Data Link Connector (DLC)

The provision for communicating with the control module is the data link connector (DLC). The connector is usually located under the instrument panel. The DLC is used to connect to a scan tool. Some common uses of the scan tool are listed below:

Service Engine Soon/Malfunction Indicator Lamp (MIL)

The Service Engine Soon or 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 condition that affects vehicle emissions, may cause powertrain damage, or severely impacts driveability.

MIL Operation

The malfunction indicator lamp (MIL) is located on the instrument panel and is displayed as CHECK ENGINE lamp.

MIL Function

MIL Illumination

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 upon 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).

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 increases 22°C (40°F) from the start-up temperature. 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 DTCs 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.

Diagnostic Trouble Codes (DTCs)

The PCM is programmed with test routines that test the operation of the various systems the PCM controls. Some tests monitor internal PCM functions. Many tests are run continuously. Other tests run only under specific conditions, referred to as Conditions for Running the DTC. When the vehicle is operating within the conditions for running a particular test, the PCM monitors certain parameters and determines if the values are within an expected range. The parameters and values considered outside the range of normal operation are listed as Conditions for Setting the DTC. When the Conditions for Setting the DTC occur, the PCM executes the Action Taken When the DTC Sets. Some DTCs alert the driver via the MIL or a message. Other DTCs do not trigger a driver warning, but are stored in memory. Refer to the Diagnostic Trouble Code (DTC) List/Type for a complete list of PCM DTCs and the driver alerts the DTCs trigger. The PCM also saves data and input parameters when most DTCs are set. This data is stored in the Freeze Frame and/or Failure Records.

The DTCs are categorized by type. The DTC type is determined by the MIL operation and the manner in which the fault data is stored when a particular DTC fails. In some cases there may be exceptions to this structure. Therefore, when diagnosing the system it is important to read the Action Taken When the DTC Sets and the Conditions for Clearing the DTC in the supporting text .

There are different types of DTCs and different actions taken when the DTCs set. Refer to Diagnostic Trouble Code (DTC) Type Definitions for a description of the general characteristics of each DTC type.

DTC Status

When the scan tool displays a DTC, the status of the DTC is also displayed. The following DTC statuses are 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 PCM 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 to clear 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 any DTC info before beginning diagnosis. Refer to Capturing DTC Info. Do not clear the DTCs until you are instructed to do so within a diagnostic procedure.

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

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

In addition to Freeze Frame data the PCM may also store Failure Records data when a DTC reports a failure. Unlike the Freeze Frame data, the Failure Records data can be stored by DTCs that DO NOT illuminate the MIL.

The Freeze Frame and Failure Records data may be retrieved through the Diagnostic Trouble Code menu on the scan tool. If more than one DTC is set, review the odometer or engine run time data located in the Freeze Frame and Failure Records info to determine the most current failure.

Keep in mind that once the Freeze Frame or Failure Record is selected, the parameter and input data displayed will look just like the normal PCM data except the parameters will not vary because stored data is being displayed.

Important: Always capture the Freeze Frame and Failure Records information with the scan tool BEFORE proceeding with diagnosis. Clearing DTCs, disconnecting the battery, disconnecting the PCM or body connectors, or procedures performed during diagnosis may erase or overwrite the stored Freeze Frame and Failure Records data. Loss of this data may prevent accurate diagnosis of an intermittent or difficult to set DTC.

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 PCM 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. This 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:

The Freeze Frame data can only be overwritten with data associated with a misfire or a 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.

Storing and Erasing Failure Records Data

When a PCM DTC sets, the PCM does several things. Among these is saving useful data and input parameter information for service diagnosis. This information is referred to as Freeze Frame and Failure Records. You will see references to these in many PCM DTC diagnostic tables because this information can be useful in pinpointing a problem even if the problem is not present when the vehicle is in the service bay.

The Failure Records data may be retrieved through the DTC menu on a scan tool. If more than one DTC is set review the odometer or engine run time data located in the Freeze Frame and Failure Records to determine the most current failure.

Keep in mind that once Freeze Frame or Failure Record is selected, the parameter and input data displayed will look just like the normal PCM data except the parameters will not vary because stored data is being displayed.

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 PCM memory. This can be useful if the PCM or the battery must be disconnected and later review of the stored information may be desired.

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.