The powertrain used in this vehicle consists of a dual overhead cam engine mated to an electronically controlled transaxle. It has 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 Powertrain Control Module is designed to maintain exhaust emission levels to Federal or California 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 PCM controls the following operations:

The Control Module supplies a buffered voltage to various sensors and switches. The input and output devices in the control module include an analog to digital converters, signal buffers, counters, and special drivers. The Control Module controls most components with electronic switches which complete a ground circuit when turned ON. These switches are arranged in groups of 4 and 7 called one of the following:

The surface mounted Quad Driver Module can independently control up to 4 outputs (PCM terminals). The Output Driver Modules can independently control up to 7 outputs. Not all outputs are always used.

Torque Management is a function of the PCM that reduces engine power under certain conditions. Torque management is performed for three 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. It then looks at torque converter 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 fuel injectors for cylinders 1, 4, 6, and 7 will also be disabled for a period of time.

Traction Control is a function of the PCM and the EBTCM that reduces front wheel slip during acceleration by applying the front brakes and reducing engine power. Refer to Brakes for an explanation of the EBTCMs role in traction control. The PCM continuously sends out a 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 it sees the front wheels slipping. The EBTCM may decide to apply the front brakes only or apply the front brakes and request reduced torque output from 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 if the EBTCM is requesting reduced torque output from the powertrain. If the EBTCM requests reduced torque, the PCM will disable between one and seven fuel injectors to achieve this.

Desired Torque will normally be a 90 percent duty cycle signal to the PCM. When the EBTCM decides to request reduced engine power, it decreases the duty cycle of the Desired Torque signal by the amount of torque reduction required (90 percent duty cycle means no torque reduction, 10 percent duty cycle means 100% torque reduction). The PCM responds by shutting off 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 re-enabled one by one as the need for traction control ends.

Several DTC(s) disable traction control when set. They will also trigger a TRACTION OFF light or message. The PCM traction control override also disables traction control and triggers the message. To diagnose a Traction Off light/message, diagnose any DTC(s) set first. Then check the traction control override and, if active, deactivate the override. If the TRACTION OFF light/message is still present, refer to Brakes for further diagnosis.

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 describes the electronic pathway through which serial data travels. The UART receives data in a serial format, converts the data to parallel format, and places them 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 now used to communicate between certain modules within the vehicle.

U.S. Federal regulations require that all automobile manufacturers establish a common communications system. General Motors utilizes the Class II communications system. Each bit of information can have one of two lengths: long or short. This allows vehicle wiring to be reduced by the transmission and reception of multiple signals over a single wire. The messages carried on Class II data streams are also prioritized. In other words, if two messages attempt to establish communications on the data line at the same time, only the message with higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that it provides scan tool manufacturers with the capability of accessing data from any make or model vehicle sold in the United States.

The provision for communicating with the control module is the Data Link Connector (DLC). It 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:

The SERVICE ENGINE SOON Malfunction Indicator Lamp (MIL) is located in the Instrument Panel Cluster (IPC). The SERVICE ENGINE SOON MIL is controlled by the PCM through CKT 419 and is used to indicate that the PCM has detected a problem that affects vehicle emissions, may cause powertrain damage, or severely impacts driveability. Refer to DTC P1641 for diagnosis of Service Engine Soon MIL.

DTCs, when set, indicate that the PCM has detected a malfunction in a particular circuit or system. The PCM is programmed with routines or checks that it follows only under prescribed conditions (called Conditions for Running the DTC). When these conditions exist, the PCM checks certain circuits or systems for a malfunction. These checks are called Conditions for Setting the DTC. When the setting conditions are true, a malfunction is indicated and the DTC is stored as failed. Some DTCs alert the driver through the SERVICE ENGINE SOON MIL or a message. Other DTCs don't trigger a driver warning. Refer to the DTC List table in Specifications for a complete list of PCM DTCs and what driver alerts they trigger. The PCM also saves data and input parameters when most DTCs are set. The saved information is called Freeze Frame and Failure Records.

The ability for a DTC to run depends largely upon whether or not a Trip has been completed. A Trip for a particular DTC is defined as vehicle operation, followed by an engine off period and a driving mode such that any particular DTC has had sufficient time to complete testing. The requirements for trips vary as they may involve items of an unrelated nature; driving style, length of trip, ambient temperature, etc. Some DTCs run only once per trip (e.g. catalyst monitor) while others run continuously (e.g. misfire and fuel system monitors). If the proper enabling conditions are not met during that ignition cycle, the tests may not be complete or the test may not have run.

In addition, the execution of a DTC may also be bound by conditions which must comprehend a Warm-up cycle. A Warm-up cycle consists of engine start-up and vehicle operation such that the coolant temperature has risen greater than a certain value (typically 40°F) from start-up temperature and reached a minimum temperature of 160°F. If this condition is not met during the ignition cycle, the DTC may not run.

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.

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

Failure Records save many more relevant data parameters than Freeze Frame records. Which data parameters are saved varies based on the DTC set. Transaxle DTC Failure Records generally contain transaxle related data parameters while fuel DTC Failure Records generally contain fuel related data parameters. If several DTCs are set, Failure Records for the three most recently set DTCs will be saved.

Freeze Frame, Failure Records data may be retrieved through the Diagnostic Trouble Code menu on 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.

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 since it is displaying recorded data.

Important: Always capture the Freeze Frame and Failure Records information with the scan tool BEFORE proceeding with diagnosis. Clearing DTCs, disconnecting the battery, disconnecting 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.

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

DTCs are categorized by type. The type will indicate the action the DTC will take in storing a failure and illuminating the MIL. The following table indicates what action each DTC type will take when a failure is recorded. Type C DTCs that DO NOT display a message were formerly referred to as Type D.

In order for a type B DTC to request MIL illumination the DTC must fail in two consecutive drive trips in which the DTC tests.

Refer to the DTC List table in Specifications for the type of each PCM DTC.

Always refer to the diagnostic support information within each DTC to obtain the Action Taken when the DTC sets and the Conditions for Clearing the DTC. These will indicate any variations from the general type A, B and C actions.

Fuel trim and misfire are special cases of type B DTCs. Each time a fuel trim or misfire malfunction is detected, engine load, engine speed, and engine coolant temperatures are recorded. In order for the fuel trim or misfire DTCs to report a PASS the load conditions must be within 10%, the speed conditions must be within 375 rpm, and the coolant temperatures must be in the same calibratable high or low range at the time the diagnostic test last reported a failure

When the ignition is turned off, the last reported set of conditions remain stored. During subsequent ignition cycles, the stored conditions are used as a reference for similar conditions. If a malfunction occurs during two consecutive trips, the Diagnostic Executive treats the failure as a normal type B diagnostic, and does not use the stored conditions. However, if a malfunction occurs on two non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:

Unique to the misfire diagnostic, the misfire DTC has the ability of alerting the vehicle operator to potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter as a result of high misfire levels, the PCM will command the MIL to flash at a rate of once per second during the time that the catalyst damaging misfire condition is present.

PCM snapshot data may also be taken and retrieved using a diagnostic tool by selecting the Snapshot option. These parameters can then be reviewed at any time.

Keep in mind that once a Snapshot is triggered, the parameter and input data displayed will look just like the normal PCM parameter and input data with the same parameter numbers except it will not vary since it is displaying recorded data.

DTC(s) can only be displayed with the use of a scan tool.

This selection will display any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition cycle. DTC tests which run and pass will cause that DTC number to be removed from this scan tool display.

This selection will display all DTCs that have failed during the present ignition cycle.

This selection will display only DTCs that have failed the last time the test ran. The last test may have ran 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.

This selection will display only DTCsDTCs that are requesting the MIL or a message be displayed. Type C DTCs that do not request a message cannot be displayed using this option. This selection will report type B DTCs only after they have failed twice and requested the MIL.

This selection will display DTCs that have not tested since codes were last cleared. Since any displayed DTCs have not run, their condition (passing or failing) is unknown.

This selection will display all active and history DTCs that have reported a failure since the last time codes were cleared. DTCs that last failed over 40 warm-up cycles will not be displayed.

This selection will display only DTCs that are stored to the PCMs history memory. It will not display type B DTCs that have not requested the MIL. It will display all type A DTCs (and type B DTCs which have requested the MIL) that have failed within the last 40 warm-up cycles. In addition, it will display all type C DTCs that have failed within the last 40 warm-up cycles. Fuel trim and misfire DTCs are stored for 80 warm-up cycles.

Use a scan tool to clear DTCs from the PCM memory. Disconnecting the vehicle battery will also clear the PCM memory, however most other system memories will also be cleared. This is generally undesireable. Disconnecting the PCM soley for clearing DTCs is not recommended since this unnecessarily disturbs the connections. 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, (refer to Capture Info.). Once a problem has been corrected and verified it is a good idea to clear DTCs so that any future service work is not needlessly confused.

Many DTCs have complex running and setting 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.