Powertrain

The powertrain used in this vehicle consists of a twin cam V6 engine mated to a 4T65-E electronically controlled transaxle. The powertrain has electronic controls to reduce exhaust emissions while maintaining excellent driveability and fuel economy. The powertrain control module (PCM) is the heart of this control system.

The PCM is designed to maintain exhaust emission levels to Federal or California standards while providing excellent driveability and fuel efficiency. Review the components and the 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 the 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 module stores a diagnostic trouble code (DTC). The problem area is identified by the particular DTC that is set. The module supplies a buffered voltage to various sensors and switches. The input and output devices in the control module include analog to digital converters, signal buffers, counters, and output drivers. The output drivers are electronic switches which complete a ground 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

The torque management is a function of the PCM that reduces engine power under certain conditions. Torque management is performed for 3 reasons:

The PCM uses the following data to calculate engine output torque:

The PCM then looks at the torque converter status, the transaxle gear ratio, and the brake switch inputs and determines if any torque reduction is required. If a torque reduction is required, the PCM retards the spark as appropriate to reduce the engine torque output. In the case of abusive maneuvers, the PCM may also shut OFF the fuel to certain cylinders to reduce the engine power.

The following are 4 instances when 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 cases, the engine power output will be moderate at full throttle.

When the PCM determines that an engine power reduction is required, thew PCM 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 will momentarily disable the fuel injectors to obtain the necessary amount of torque reduction.

Traction Control

Traction control is a function of the PCM and the electronic brake traction control module (EBTCM) that reduces any front wheel slip during acceleration by applying the front brakes and reducing the engine power. Refer to ABS Description and Operation in Antilock Brake System. The PCM continuously sends out a pulse width modulated (PWM) signal that indicates the torque output of the powertrain. This signal, referred to as the delivered torque signal, is used by the EBTCM to determine what action is required when the front wheels slip. The EBTCM may decide to apply the front brakes only or to apply the front brakes and request a reduced torque output from the powertrain. The EBTCM requests a reduced torque using another PWM signal. This signal, referred to as the Desired Torque signal, is used by the PCM to determine if the EBTCM is requesting a reduced torque output from the powertrain. The PCM reduces the engine torque by adjusting the ignition timing.

The Desired Torque will normally be a 90 percent duty cycle signal to the PCM. When the EBTCM decides to request a reduced engine power, the duty cycle of the Desired Torque signal is decreased by the amount of torque reduction required. A 90 percent duty cycle means no torque reduction, while a 10 percent duty cycle means 100 percent torque reduction. The PCM responds by shutting OFF the fuel to one or more cylinders depending on the percent torque reduction requested. The PCM will 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 the traction control ends.

Several DTCs disable the traction control when set. The DTCs will 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 - Antilock Brake System in Antilock Brake System.

UART Serial Data

Two methods of data transmission are used. One method involves a universally asynchronous receiving and transmitting (UART) protocol. The UART is an interfacing device that allows the on-board computer to send and receive serial data. Serial data refers to information that is transferred in a linear fashion-over a single line, one bit at a time. A data bus is the electronic pathway through which serial data travels. The UART receives data in a serial format, converts the data to parallel format, and places the data on the data bus which is recognizable to the on-board computer. This method had been the common strategy for establishing a communication link between the on-board control module and the off-board monitor or scanner since 1981. The UART is sometimes used to communicate between modules within the vehicle.

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 approximately 7-0 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. 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 2 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 expect to receive the message detect the loss, and will 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 DLC 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 problem that affects the vehicle emissions, that may cause damage to the powertrain, or that 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 DTCs. A warm-up cycle occurs when the engine coolant temperature (ECT) rises 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 MIL. The PCM will clear the DTCs when 40 consecutive warm-up cycles occur without a malfunction.

Diagnostic Trouble Code Display

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

Diagnostic Trouble Codes (DTCs)

The PCM is programmed with test routines that inspect the operation of the various systems the PCM controls. Some tests verify the internal PCM functions. Many of the tests are run continuously. Other tests run only under specific conditions, referred to as the Conditions for Running the DTC. When the vehicle is operating within the conditions for running a particular test, the PCM checks certain parameters and determines whether or not the values are within an expected range. The parameters and the values that are considered outside the range of normal operation are listed as the Conditions for Setting the DTC. When the Conditions for Setting the DTC are present, 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 DTC Type Table for a complete list of the PCM DTCs and the driver alerts that the DTCs trigger. The PCM also saves the data and input parameters when most DTCs are set. This data is stored in the Freeze Frame/Failure Records.

The DTCs are categorized by type. The DTC type relates to how the MIL operates and how the fault data is stored when a particular DTC sets. In some cases there may be exceptions to this structure. Therefore, you need to read the Action Taken When the DTC Sets and the Conditions for Clearing the MIL/DTC in the supporting text when diagnosing the system.

The following list describes 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 will be indicated only when the statuses 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 has reported a failure since the last time the 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: A DTC 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 the 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 the DTCs. This unnecessarily disturbs the connections and may introduce a new problem. Before clearing the 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 the DTC info before beginning a 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 service 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 the Freeze Frame data. The Freeze Frame data is very similar to a single record of the operating conditions. Whenever the MIL is illuminated, the corresponding record of operating conditions is recorded as the 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:

The Freeze Frame data can only be overwritten with data associated with a misfire or a fuel trim malfunction. The data from these faults takes precedence over any 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 the Freeze Frame data the PCM may also store the 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/Failure Records data may be retrieved through the DTC menu on the scan tool. If more than one DTC is set review the odometer or the engine run time data located in the Freeze Frame/Failure Records to determine the most current failure.

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

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

Capturing DTC Info-Capture Info

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

Storing and Erasing Freeze Frame Data

Government regulations require that the 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 the operating conditions. Whenever the MIL is illuminated, the corresponding record of the 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 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. The data from these faults take precedence over any 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 including saving useful data and input parameter information for a service diagnosis. This information is referred to as the Freeze Frame/Failure Records. You will see references to the Freeze Frame/Failure Records 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 Freeze Frame/Failure Records data may be retrieved through the DTC menu on the scan tool. If more than one DTC is set review the odometer or the engine run time data located in the Freeze Frame/Failure Records info to determine the most current failure.

Keep in mind that once the Freeze Frame or the Failure Record is selected, the parameter and input data displayed will look just like the normal PCM data except the parameters will not vary since 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/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 the 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 the 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 a component that needs to be investigated further. The Snapshot will remain in the scan tool memory even after the tool is disconnected from a power source. Refer to the scan tool user instructions for more information on the Snapshot function.