Powertrain

The powertrain used in this vehicle consists of a 3.1L pushrod 60° V-6 engine mated to a 4T45-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 which 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 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 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 spark as appropriate in order to reduce engine torque output. In the case of abusive maneuvers, the PCM may also shut off fuel to certain cylinders in order to reduce engine power.

There 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 situations, the engine power output will be moderate at full throttle.

When the PCM determines that engine power reduction is required, the 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 momentarily disables fuel injectors in order to obtain the necessary amount of torque reduction.

UART Serial Data

Two methods of data transmission are used. One method involves a universally asynchronous receiving/transmitting (UART) protocol. UART is an interfacing device that allows the on-board computer to send and receive serial data. Serial data refers to information which 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 into a 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/scanner since 1981. UART is sometimes used to communicate between modules within the vehicle.

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. Large message content lowers the baud rate, while 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/malfunction indicator lamp (MIL) is located in the instrument panel (IP) cluster. 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.

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, 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 increases 22°C (40°F) from start-up. The engine coolant must also achieve a minimum temperature. 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 monitor 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 whether or not 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.

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, it is important 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.

There are different types of DTCs and the action they take when 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 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.

Freeze Frame Data

The data captured is called Freeze Frame data. 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.

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

The PCM may store Failure Records data when a DTC reports a failure. Failure Records data can be stored by DTC's that DO NOT illuminate the MIL.

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.

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. The captured information will contain any additional DTCs and related data that was stored originally (if multiple DTCs were stored).

Freeze Frame/Failure Records

Freeze Frame and Failure Records data may be retrieved through the DTC menu on 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.

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.

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.

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. Should the PCM or battery need to be disconnected the technician will still be able to review of the stored information.

PCM Snapshot

The scan tool can take a snapshot of the PCM input and output parameters. The snapshot data can be reviewed for diagnostic purposes while not driving the vehicle. The snapshot can be used in two ways at the time a symptom is noticed.