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:
• | The ignition control (IC) |
• | The knock sensor (KS) system |
• | The evaporative emissions (EVAP) system |
• | The secondary air injection (AIR) system (if equipped) |
• | The exhaust gas recirculation (EGR) system |
• | The automatic transmission functions |
• | The cooling fan control |
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:
• | To prevent overstress of the powertrain components |
• | To limit engine power when the brakes are applied |
• | To prevent damage to the vehicle during certain abusive maneuvers |
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:
• | During transaxle upshifts and downshifts |
• | During heavy acceleration from a standing start |
• | If the brakes are applied with moderate to heavy throttle |
• | When the driver is performing abusive or stress-inducing maneuvers
such as shifting into gear at high throttle angles |
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 engine coolant temperature is less than -40°C (-40°F)
or more than 131°C (268°F) |
• | The engine coolant level is low |
• | The engine speed is less than 600 RPM |
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:
• | Identifying stored diagnostic trouble codes (DTCs) |
• | Performing output control tests |
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
• | The MIL informs the driver that a malfunction has occurred and
the vehicle should be taken in for service as soon as possible |
• | The MIL illuminates during a bulb test and a system test |
• | A DTC will be stored if a MIL is requested by the diagnostic |
MIL Illumination
• | The MIL will illuminate with ignition ON and the engine OFF |
• | The MIL will turn OFF when the engine is started |
• | The MIL will remain ON if the self-diagnostic system has detected
a malfunction |
• | The MIL may turn OFF if the malfunction is not present |
• | If the MIL is illuminated and then the engine stalls, the MIL
will remain illuminated if the ignition switch is ON. |
• | If the MIL is not illuminated and the engine stalls, the MIL will
not illuminate until the ignition switch is cycled OFF, then ON. |
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:
• | The engine coolant temperature |
• | The throttle position (TP) angle |
• | The manifold absolute pressure (MAP) |
• | The barometric pressure (BARO) |
• | The injector base pulse width |
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 engine coolant temperature |
• | The throttle position angle |
• | The manifold absolute pressure/barometric pressure |
• | The injector base pulse width |
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.