Vehicle Control Module Serial Data Communication

Class 2 Serial Data

U.S. Federal regulations require that all automobile manufacturers establish a common communications system. This vehicle utilizes the Class 2 communications system. Each bit of information can have one of two lengths, long or short. This allows the vehicle wiring to be reduced by the transmission and reception of the multiple signals over a single wire. The messages which are carried on Class 2 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 the higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that the regulation provides the scan tool manufacturers with the capability of accessing the data from any make or model vehicle sold in the United States.

The Diagnostic Executive

The Diagnostic Executive is a unique segment of the software which is designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The Diagnostic Executive has the following main responsibilities:

On-Board Diagnostic Tests

A diagnostic test is a series of steps which has a beginning and an end. The result of which is a pass or fail reported to the diagnostic executive. When a diagnostic test reports a pass result, the diagnostic executive records the following data:

When a diagnostic test reports a fail result, the diagnostic executive records the following data:

Trip

The ability for a diagnostic test to run depends largely upon whether or not a trip has been completed. A trip for a particular diagnostic is defined as a key ON and key OFF cycle in which all the enabling criteria for a given diagnostic have been met allowing the diagnostic to run. The requirements for trips vary as they may involve items of an unrelated nature, such as driving style, length of trip, ambient temperature, etc. Some diagnostic tests 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.

Warm-Up Cycle

A warm-up cycle consists of an engine start-up and vehicle operation such that the coolant temperature has risen more than 4°C (40°F) from the start-up temperature and reached a minimum engine coolant temperature of 71°C (160°F). If this condition is not met during the ignition cycle, the diagnostic may not run.

Diagnostic Information

The diagnostic tables and the functional checks are designed in order to locate a poor circuit or a malfunctioning component through a process of logical decisions. The tables are prepared with the assumption that the vehicle functioned correctly at the time of assembly and that there are no multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures which are contained in this manual. The language of communicating the source of the malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code will set and the malfunction indicator lamp (MIL) will illuminate on some applications.

Malfunction Indicator Lamp (MIL)

The malfunction indicator lamp (MIL) is on the instrument panel. The MIL has the following functions:

When the MIL remains ON while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, perform an On-Board Diagnostic (OBD) System Check. The procedures for these checks are given in engine controls. These checks expose faults which the technician may not detect if other diagnostics are performed first.

MIL Requests and History Codes

The diagnostic executive must acknowledge when all the emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Each diagnostic test is separated into 4 types:

When a type A diagnostic test reports a failure, the diagnostic executive immediately requests to have the MIL turn ON for that diagnostic test. When a type B diagnostic test reports a failure during 2 consecutive trips, the diagnostic executive turns on the MIL for that diagnostic test. The diagnostic executive has the option of turning the MIL OFF when the diagnostic test which caused the MIL to illuminate the passes for 3 consecutive trips. In the case of misfire or fuel trim malfunctions, there are additional requirements as follows:

When the diagnostic executive requests the service light to be turned ON or a type C diagnostic fault is reported, a history DTC is also recorded for the diagnostic test. The provision for clearing a history DTC for any diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail, a battery disconnect, or a scan tool clear info command.

Special Cases Of Type A Diagnostic Tests

Unique to the misfire diagnostic, the diagnostic executive has the capability of alerting the driver of potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter, the diagnostic executive will command the MIL to flash at a rate of once per second during the times that the catalyst damaging misfire condition is present.

Special Cases Of Type B Diagnostic Tests

Misfire and fuel trim malfunctions are special cases of type B diagnostics. Each time a fuel trim malfunction is detected, the engine load, the engine speed, and the engine coolant temperatures are recorded.

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 fuel trim malfunction occurs during 2 consecutive trips, the diagnostic executive treats the failure as a normal type B diagnostic. The diagnostic executive does not use the stored conditions. However, if a fuel trim malfunction occurs on 2 non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:

Storing And Erasing Freeze Frame Data

Government regulations require that the engine operating conditions are to be captured whenever the MIL is illuminated. The data that is 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 failure records 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:

Freeze Frame data can only be overwritten with the data associated with a misfire or a fuel trim malfunction. The data from these faults take precedence over data that is associated with any other fault. The Freeze Frame data will not be erased unless the associated history DTC is cleared.

Intermittent Malfunction Indicator Lamp

In the case of an intermittent fault, the malfunction indicator lamp (MIL) may illuminate and then after 3 trips turn OFF. However, the corresponding diagnostic trouble code will store in the memory. When unexpected diagnostic trouble codes appear, check for an intermittent malfunction.

Data Link Connector (DLC)

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

Control Module Learning Ability

The control module has a learning ability which allows the control module to make corrections for minor variations in the fuel system in order to improve driveability. Whenever the battery cable is disconnected, the learning process resets.

The driver may note a change in vehicle performance. In order to allow the PCM to re-learn to drive the vehicle at part throttle with moderate acceleration. The vehicle may also operate at idle conditions until the normal performance returns.

Reprogramming (Flashing) The Control Module

Some vehicles allow the reprogramming of the control module without removal from the vehicle. This provides a flexible and a cost-effective method of making changes in software and calibrations.

Refer to the latest Techline information on reprogramming or flashing procedures.

Verifying Vehicle Repair

Verification of the vehicle repair will be more comprehensive for vehicles with OBD II system diagnostics. Following a repair, the technician should perform the following steps:

1. Review the fail records and the Freeze Fame data for the DTC which was diagnosed. Record the fail records or Freeze Fame data. The Freeze Frame data will only store for an A or B type diagnostic.
2. Clear the DTCs.
3. Operate the vehicle within the conditions noted in the fail records or the Freeze Frame data.
4. Monitor the DTC status information for the specific DTC which has been diagnosed until the diagnostic test associated with that DTC runs.

Following these steps are very important in verifying repairs on the OBD II systems. Failure to follow these steps could result in an unnecessary repair.

Reading Diagnostic Trouble Codes

Use a diagnostic scan tool in order to read the diagnostic trouble codes. Failure to follow this step could result in unnecessary repairs.

Clearing Diagnostic Trouble Codes

In order to clear Diagnostic Trouble Codes (DTCs), use the diagnostic scan tool clear DTCs or clear info function. When clearing DTCs follow the instructions supplied by the tool manufacturer. When a scan tool is not available, disconnecting one of the following sources for at least thirty (30) seconds can also clear the DTCs:

Notice: Turn off the ignition key when disconnecting or reconnecting battery power in order to prevent system damage.

Disconnecting the negative battery cable may result in the loss of other on-board memory data, such as preset radio tuning.

DTC Modes

The OBD II vehicles have three options available in the scan tool DTC mode in order to display the enhanced information available. A description of the new modes, the DTC Info and the Specific DTC, follows. After selecting the DTC, the following menu appears:

The following is a brief description of each of the sub menus in the DTC Info and the Specific DTC. The order in which they appear here is alphabetical and not necessarily the way they will appear on the scan tool.

DTC Info Mode

Use the DTC Info mode in order to search for a specific type of stored DTC information. There are seven choices. The electronic service information may instruct the technician to test for DTCs in a certain manner. Always follow the published service procedures.

In order to get a complete description of any status, press the Enter key before pressing the desired F-key. For example, pressing enter, then an F key will display a definition of the abbreviated scan tool status.

DTC Status

This selection displays any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition up to a maximum of 33 DTCs. The DTC tests which run and pass removes that DTC number from the scan tool screen.

Fail This Ign. (Fail This Ignition)

This selection displays all of the DTCs that have failed during the present ignition cycle.

History

This selection displays only the DTCs that are stored to the history memory of the control module. The history memory will not display the Type B DTCs that have not requested the MIL. The history memory will display all of the type A and B DTCs that have requested the MIL and have failed within the last 40 warm-up cycles. In addition, the history memory will display all of the type C DTCs that have failed within the last 40 warm-up cycles.

Last Test Fail

This selection displays only the DTCs which have failed during the last time that the test ran. The last test may have ran during a previous ignition cycle if the a type A or B 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

This selection displays only the DTCs that are requesting the MIL. Type C DTCs cannot be displayed by using this option. This selection will report type B DTCs only after the MIL has been requested.

Not Run SCC (Not Run Since Code Clear)

This option displays up to 33 DTCs that have not run since the DTCs were last cleared. Since any displayed DTCs have not run, their condition, passing or failing, is unknown.

Test Fail SCC (Test Failed Since Code Clear)

This selection displays all of the active and history DTCs that have reported a test failure since the last time the DTCs were cleared. The DTCs that last failed over 40 warm-up cycles (before this option is selected) will not be displayed.

Specific DTC Mode

This mode is used in order to check the status of the individual diagnostic tests by the DTC number. This selection can be accessed if a DTC has passed or failed. Many OBD II DTC mode descriptions are possible because of the extensive amount of information that the Diagnostic Executive monitors regarding each test. Some of the many possible descriptions follow with a brief explanation.

This selection only allows the entry of the DTC numbers that are supported by the vehicle that is being tested. If an attempt is made to enter the DTC numbers for tests which the diagnostic executive does not recognize, the requested information will not be displayed correctly and the scan tool may display an error message. The same applies to using the DTC trigger option in the snapshot mode. If an invalid DTC is entered, the scan tool will not trigger.

Failed Last Test

For type A and B DTCs, this message will display during the subsequent ignition cycles until the test passes or the DTCs are cleared. For type C DTCs, this message clears whenever the ignition is cycled.

Failed Since Clear

This message displayed indicates that the diagnostic test failed at least once within the last 40 warm-up cycles since the last time the control module cleared the DTCs.

Failed This Ign. (Failed This Ignition)

This message displayed indicates that the diagnostic test has failed at least once during the current ignition cycle. This message will clear when the DTCs are cleared or the ignition is cycled.

History DTC

This message displayed indicates that the DTC has stored to memory as a valid fault. A DTC displayed as a history fault does not necessarily mean that the fault is no longer present. The history description means that all of the conditions necessary for reporting a fault have met.

MIL Requested

This message displayed indicates that the DTC is currently causing the MIL to turn ON. Remember that only type A and B DTCs can request the MIL. The MIL request cannot determine if the DTC fault conditions are currently being experienced. This is because the diagnostic executive requires up to 3 trips during which the diagnostic test passes to turn OFF the MIL.

Not Run Since Cl (Not Run Since Cleared)

This message displayed indicates that the selected diagnostic test has not run since the last time the DTCs were cleared. Therefore, the diagnostic test status, passing or failing, is unknown. After the DTCs are cleared, this message continues to be displayed until the diagnostic test runs.

Not Run This Ign. (Not Run This Ignition)

This message displayed indicates that the selected diagnostic test has not run this ignition cycle.

Test Ran and Passed

This message displayed indicates that the selected diagnostic test has the following items:

Whenever the indicated status of the vehicle is Test Ran and Passed after a repair verification, the vehicle is ready to be released to the customer.

If the indicated status of the vehicle is Failed This Ign after a repair verification, then the repair is incomplete. A further diagnosis is required.

Prior to repairing a vehicle, use the status information in order to evaluate the state of the diagnostic test and to help identify an intermittent problem. The technician can conclude that although the MIL is illuminated, the fault condition that caused the code to set is not present. An intermittent condition must be the cause.

Vehicle Control Module General Description

The VCM is located on the left hand side fenderwell. The VCM is the control center for the fuel, emissions, ignition, and automatic transmission control functions.

The VCM constantly monitors the information from the various sensors. The VCM controls the component systems which affect the engine operation.

The VCM alerts the driver through the Malfunction Indicator Lamp (MIL) or the antilock indicator lamp. The VCM stores the DTCs which identify the problem areas for the technician making repairs. Refer to Vehicle Control Module (Serial Data Communication) , for further information on using the diagnostic function of the VCM for engine operation.

Refer to Transmission for the diagnosis of the automatic transmission. Refer to Section 5E3B in the appropriate service manual for further information on the antilock brakes.

The VCM in this vehicle is programmable. The only services allowed on the VCM is the control module replacement with the KS calibrator PROM transferred or the KS calibrator PROM only.

The KS calibrator contains the up integrated knock sensor calibration. The VCM stores the 4 calibrations in the Electronically Erasable Programmable Read Only Memory (EEPROM).

When replacing the VCM, programming the EEPROM and transferring the KS calibrator PROM to the new VCM is mandatory. Refer to EEPROM Programming .

The 4 calibrations required for the VCM are the Powertrain, the ABS, the VSS buffer, and the A/C. Each calibration has its own part number. Determine the correct calibrations for a particular vehicle based on the VIN number of the vehicle.

Vehicle Control Module (VCM)

The VCM processes the various input information. Then the VCM sends the necessary electrical responses to the control fuel delivery, the spark timing, and the other emission control systems. The input information interrelates to more than one output; therefore, if the one input fails, the failure can affect more than one system's operation.

PROM (KS Module)

This assembly contains an electronic Knock Sensor (KS) module.

Memory

There are two types of memory storage within the VCM, EEPROM and RAM.

EEPROM

Electrically Erasable Programmable Read Only Memory (EEPROM) is a permanent memory that is physically soldered to the circuit boards within the VCM. The EEPROM contains the overall control algorithms. The EEPROM can be reprogrammed by using the scan tool.

RAM

Random Access Memory (RAM) is the microprocessor scratchpad. The processor can write into or read from this memory as needed. This memory is volatile and needs a constant supply of voltage to be retained. If the voltage is lost, the memory is lost.

VCM Learning Ability

Notice: Since the VCM is located under the hood, its connectors are sealed and cannot be backprobed as in the previous model years. Do not attempt to backprobe as a connector or seal damage could occur.

Notice: The VCM must be maintained at a temperature below 85°C (185°F) at all times. This is most critical when the vehicle is put through a paint baking process. The VCM becomes inoperative if its temperature 85°C (185°F). It is recommended that temporary insulation be placed around the VCM or removed from the vehicle during the time the vehicle is in a paint oven or other high temperature process. Do not operate the vehicle if the insulation is on the VCM.

The VCM's learning ability allows it to make corrections for minor variations in the fuel system in order to improve driveability.

When the battery is disconnected for other repairs, the learning process resets. The driver my note a change in the vehicle's performance. In order to teach the vehicle ensure that the engine is at the operating temperature. Drive the vehicle at part throttle with a moderate acceleration and idle conditions until normal performance returns.