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Powertrain Control Module (PCM) Description
Control Module Function
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 7 called output driver modules. The output driver modules can independently control up to 7 outputs. Not all outputs are always used.
Use of Circuit Testing Tools
Do not use a test lamp in order to diagnose the powertrain electrical systems unless specifically instructed by the diagnostic procedures. Use the J 35616-A Connector Test Adapter Kit whenever the diagnostic procedures call for probing any of the connectors.
Control Module Service Precautions
The control module is designed to withstand the normal current draws that are associated with the vehicle operations. Avoid overloading any circuit. When testing for opens or shorts, do not ground any of the control module circuits unless instructed. When testing for opens or shorts, do not apply voltage to any of the control module circuits unless instructed. Only test these circuits with DMM J 39200 , while the control module connectors remain connected to the control module.
Aftermarket (Add-On) Electrical And Vacuum Equipment
The aftermarket (add-on) electrical and vacuum equipment is defined as any equipment that is installed on a vehicle after leaving the factory that connects to the electrical or vacuum systems of the vehicle. No allowances have been made in the vehicle design for this type of equipment.
Do not attach add-on vacuum operated equipment to this vehicle. The use of add-on vacuum equipment may result in damage to vehicle components or systems.
Connect any add-on electrically operated equipment to the vehicle'* electrical system at the battery (power and ground) in order to prevent damage to the vehicle.
The add-on electrical equipment, even when installed to these strict guidelines, may still cause the powertrain system to malfunction. This may also include any equipment which is not connected to the electrical system of the vehicle such as portable telephones and radios. Therefore, the first step in diagnosing any powertrain problem is to remove all of the aftermarket electrical connections from the vehicle. After this is done, if the problem still exists, diagnose the problem in the normal manner.
Electrostatic Discharge Damage
In order to prevent possible Electrostatic Discharge damage to the PCM, Do Not touch the connector pins or the soldered components on the circuit board.
The electronic components used in the control systems are often designed in order to carry very low voltage. The electronic components are susceptible to damage caused by electrostatic discharge. Less than 100 volts of static electricity can cause damage to some electronic components. There are several ways for a person to become statically charged. The most common methods of charging are by friction and by induction. An example of charging by friction is a person sliding across a car seat. Charging by induction occurs when a person with well insulated shoes stands near a highly charged object and momentarily touches ground. Charges of the same polarity are drained off, leaving the person highly charged with the opposite polarity. Static charges can cause damage. Use care when handling and testing the electronic components.
Engine Controls Information
The driveability and emissions information describes the function and operation of the control module. The emphasis is placed on the diagnosis and repair of problems related to the system.
Engine components, wiring diagrams, and diagnostic tables:
The component locations
The wiring diagrams
The control module terminal end view and terminal definitions
The On-Board Diagnostic (OBD) System Check
The diagnostic trouble code (DTC) tables
The Component System includes the following items:
The component and circuit description
The On-vehicle service for each sub-system
The functional checks with the diagnostic tables
How to use electrical systems diagnostic information
The DTCs also contain the diagnostic support information containing the circuit diagrams, the circuit or the system information, and helpful diagnostic information.
Refer to the General Motors Maintenance Schedule in General Information of the appropriate service manual for the maintenance that the owner or technician should perform in order to retain emission control performance.
Visual and Physical Underhood Inspection
Perform a careful visual and physical underhood inspection when performing any diagnostic procedure or diagnosing the cause of an emission test failure. This can often lead to repairing a problem without further steps. Use the following guidelines when performing a visual and physical inspection:
Inspect all of the vacuum hoses for the following conditions:
Inspect the hoses that are difficult to see beneath the air cleaner, the A/C compressor, the generator, etc.
Inspect all of the wires in the engine compartment for the following items:
Burned or chafed spots
Contact with sharp edges
Contact with hot exhaust manifolds
This visual and physical inspection is very important. Perform the inspection carefully and thoroughly.
Basic Knowledge Of Tools Required
Lack of basic knowledge of this powertrain when performing diagnostic procedures could result in incorrect diagnostic performance or damage to powertrain components. Do not attempt to diagnose a powertrain problem without this basic knowledge.
A basic understanding of hand tools is necessary in order to effectively use this information.
The control module refers to the powertrain control module (PCM) or the vehicle control module (VCM). The control module is designed to maintain exhaust emission levels at the 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 each specific control module. The control module monitors numerous engine and vehicle functions. The control module controls the following operations:
The fuel control
The ignition control (IC)
The knock sensor (KS) system
The automatic transmission shift functions
The cruise control enable, if equipped
The generator, if equipped
The evaporative emission (EVAP) purge
The A/C clutch control
The cooling fan control
System Status and Drive Cycle For Inspection/Maintenance
The System Status selection is included in the scan tool System Info menu.
Several states require that the I/M 240 (OBD ll system) pass on-board tests for the major diagnostics prior to having a vehicle emission inspection. This is also a requirement to renew license plates in some areas.
Using a scan tool, the technician can observe the System Status in order to verify that the vehicle meets the criteria to comply with the local area requirements. Using the System Status display, any of the following systems, or combination of systems, may be monitored for I/M readiness:
The 3-way catalytic converter
The heated oxygen sensors (HO2S)
The HO2S heaters
The EGR system
The fuel trim system
The System Status display indicates only whether or not the test has been completed. The System Status display does not necessarily mean that the test has passed. If a Failed Last Test indication is present for a DTC associated with one of the above systems, that test failed. Diagnosis and repair is necessary in order to meet the I/M 240 requirement. Verify that the vehicle passes all of the diagnostic tests associated with the displayed System Status prior to returning the vehicle to the customer. Refer to the Typical OBD II Drive Cycle table as a guide to complete the I/M 240 System Status tests.
Following a DTC info clear, a battery disconnect, or a control module replacement, all System Status information will clear.
Oxygen Sensor Diagnosis
The control module diagnoses the following fuel control HO2S conditions:
The heater performance
A slow response
An insufficient switching
The transition time ratio
An inactive sensor
The signal fixed high
The signal fixed low
The control module diagnoses the following 3-way catalyst monitor HO2S conditions:
The heater performance
The signal fixed high
The signal fixed low
An inactive sensor
Fuel Control Heated Oxygen Sensors
The main function of the fuel control HO2S is to provide the control module with exhaust stream information in order to allow proper fueling, and to maintain emissions within the mandated levels. After the sensor reaches the operating temperature, the sensor generates a voltage inversely proportional to the amount of oxygen present in the exhaust gases.
The control module uses the signal voltage from the fuel control HO2S in a closed loop in order to adjust the fuel injector pulse width. While in closed loop, the control module can adjust fuel delivery in order to maintain an air-to-fuel ratio which allows the best combination of emission control and driveability.
If the oxygen sensor pigtail wiring, the connector, or the terminal are damaged, replace the entire sensor assembly. Do not attempt to repair the wiring, the connector, or the terminals. In order for the sensor to function properly, the sensor must have a clean air reference. This clean air reference is obtained by way of the oxygen sensor wires. Any attempt to repair the wires, the connectors, or the terminals could result in the obstruction of the air reference. Any attempt to repair the wires, the connectors, or the terminals degrades the oxygen sensor performance.
Catalyst Monitor Heated Oxygen Sensors
In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), the system uses a 3-way catalytic converter. The catalyst within the converter promotes a chemical reaction which converts the HC and the CO present in the exhaust gas into harmless water vapor and carbon dioxide. The catalyst also converts the NOx to nitrogen.
The control module monitors this process using the post--catalyst heated oxygen sensors. The pre-catalyst oxygen sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the 3-way catalytic converter. The post-catalyst oxygen sensor produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst'* ability to convert the exhaust gases efficiently. If the catalyst is operating efficiently, the pre-catalyst HO2S signal will be far more active than that produced by the post-catalyst HO2S.
In addition to catalyst monitoring, the post-catalyst heated oxygen sensor has a limited role in controlling fuel delivery. If the post-catalyst HO2S signal indicates a high oxygen content or a low oxygen content for an extended period of time while in a closed loop, the control module adjusts the fuel delivery slightly in order to compensate.
Catalyst Monitor Diagnostic Operation
The OBD II catalyst monitor diagnostic measures oxygen storage capacity. In order to do this, the heated oxygen sensors are installed before and after the 3-way catalyst (TWC). Voltage variations between the sensors allow the control module to determine the catalyst emission performance.
As the catalyst becomes less effective in promoting chemical reactions, the catalyst'* capacity to store and release oxygen generally degrades. The OBD II catalyst monitor diagnostic is based on a correlation between conversion efficiency and oxygen storage capacity.
A good catalyst shows a relatively flat output voltage on the post-catalyst heated oxygen sensor (HO2S). A degraded catalyst shows a greatly increased activity in output voltage from the post-catalyst HO2S.
The post-catalyst HO2S is used to measure the oxygen storage and release capacity of the catalyst. A high oxygen storage capacity indicates a good catalyst. A low oxygen storage capacity indicates a failing catalyst. The TWC and the HO2S must be at operating temperature in order to achieve the correct oxygen sensor voltages like those shown in the post-catalyst HO2S outputs graphic.
The catalyst monitor diagnostic is sensitive to the following conditions:
Any exhaust leaks
An HO2S contamination
Any alternate fuels
Exhaust system leaks may cause the following results:
Prevent a degraded catalyst from failing the diagnostic
Cause a false failure for a normally functioning catalyst
Prevent the diagnostic from running
Some of the contaminants that may be encountered are phosphorus, lead, silica, and sulfur. The presence of these contaminants prevents the TWC diagnostic from functioning properly.
Three-Way Catalyst Oxygen Storage Capacity
The control module must monitor the 3-way catalyst (TWC) system for efficiency. In order to accomplish this, the control module monitors the pre-catalyst oxygen sensor and the post-catalyst oxygen sensor. When the TWC is operating properly, the post-catalyst oxygen sensor will have significantly less activity (2) than the pre-catalyst oxygen sensor (1) . The TWC stores oxygen as needed during the normal reduction and oxidation process. The TWC releases oxygen as needed during the normal reduction and oxidation process. The control module calculates the oxygen storage capacity using the difference between the voltage levels of the pre-catalyst oxygen sensor and the post-catalyst oxygen sensor .
If the voltage level of the post-catalyst oxygen sensor (2) approximates the voltage level of the pre-catalyst oxygen sensor (1), the catalyst efficiency is degraded.
Stepped or staged testing levels allow the control module to statistically filter test information. This prevents falsely passing or falsely failing the oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. For this reason, do not use post catalyst oxygen sensor activity in order to determine the oxygen storage capacity unless directed by the electronic service information.
Three stages are used in order to monitor the catalyst efficiency. Failure of the first stage indicates that the catalyst requires further testing in order to determine the catalyst efficiency. Failure of the second stage indicates that the catalyst may be degraded. The third stage then monitors the inputs from both HO2 sensors more closely before determining if the catalyst is actually degraded. This further statistical processing is done in order to increase the accuracy of oxygen storage capacity type monitoring. Failing the stage 0 test or the stage 1 test does not indicate a failed catalyst. The catalyst may be marginal or the fuel sulfur content could be very high.
Aftermarket HO2S characteristics may be different from the original equipment manufacturer sensor. This may lead to a false pass or a false fail of the catalyst monitor diagnostic. Similarly, if an aftermarket catalyst does not contain the same amount of cerium as the original part, the correlation between oxygen storage and conversion efficiency may be altered enough to set a false DTC.
Misfire Monitor Diagnostic Operation
The misfire monitor diagnostic is based on the crankshaft rotational variations, or reference period. The control module determines the crankshaft rotational velocity using the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor. When a cylinder misfires, the crankshaft slows down momentarily. By monitoring the crankshaft and camshaft position sensor signals, the control module can calculate when a misfire occurs.
For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 1000-3200 engine revolutions.
For a catalyst damage misfire, the diagnostic responds to the misfire within 200 engine revolutions.
Rough roads may cause a false misfire detection. A rough road applies torque to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity. The control module detects this as a false misfire.
On the automatic transmission equipped vehicles, the torque converter clutch (TCC) will disable whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the effect of the drive wheel inputs on the crankshaft rotation.
When the TCC has disabled as a result of misfire detection, the TCC will re-enable after approximately 3200 engine revolutions if no misfire is detected. The TCC remains disabled whenever the misfire is detected, with or without a DTC set. This allows the misfire diagnostic to re-evaluate the system.
During a transmission high temperature condition, the misfire diagnostic will disable and the TCC will operate normally. This avoids further increasing the temperature of the transmission.
Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time the misfire occurred. These misfire counters are basically a file on each engine cylinder.
A current and a history misfire counter is maintained for each cylinder. The misfire current counters, Misfire Cur #1 -8, indicate the number of firing events in the last 200 cylinder firing events which were misfires. The misfire current counters displays real time data without a misfire DTC stored. The misfire history counters, Misfire Hist #1 - 8, indicate the total number of cylinder firing events which were misfires. The misfire history counters displays 0 until the misfire diagnostic has failed and DTC P0300 is set. Once the misfire DTC sets, the misfire history counters are updated every 200 cylinder firing events.
The misfire counters graphic illustrates how these misfire counters are maintained. If the misfire diagnostic reports a failure, the diagnostic executive reviews all of the misfire counters before reporting a DTC. This way, the diagnostic executive reports the most current information.
When the crankshaft rotation is erratic, the control module detects a misfire condition. Because of this erratic condition, the data that is collected by the diagnostic can sometimes incorrectly identify which cylinder is misfiring. The misfire counters graphic shows that misfires are counted from more than one cylinder. Cylinder #1 has the majority of the counted misfires. In this case, the misfire counters would identify cylinder #1 as the misfiring cylinder. The misfires in the other counters were just background noise caused by the erratic rotation of the crankshaft. If the number of misfires accumulated is sufficient for the diagnostic to identify a true misfire, the diagnostic will set DTC P0300.
Use Techline equipment to monitor the misfire counter data on OBD ll compliant vehicles. Knowing which cylinders misfired can lead to the root cause, even when dealing with a multiple cylinder misfire. Using the information in the misfire counters, identify which cylinders are misfiring. If the counters indicate cylinders number 1 and 4 misfired, look for a circuit or component common to both cylinders number 1 and 4 such as an open ignition coil in an electronic ignition system.
The misfire counter information is located in the Misfire Data menu of the data list.
The misfire diagnostic may indicate a fault due to a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples include the following items:
A contaminated fuel
Running out of fuel
Any fuel-fouled spark plugs
A basic engine fault
Fuel Trim System Monitor Diagnostic Operation
This system monitors the averages of short-term and long-term fuel trim values. If these fuel trim values stay at their limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares the averages of short-term fuel trim values and long-term fuel trim values to rich and lean thresholds. If either value is within the thresholds, a pass is recorded. If either value is outside their thresholds, a rich or lean DTC will set.
In order to meet OBD ll requirements, the control module uses weighted fuel trim cells in order to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if fuel trim counts in the weighted fuel trim cells exceed specifications. This means that the vehicle could have a fuel trim problem which is causing a concern under certain conditions (i.e. the engine could be idling high due to a small vacuum leak or rough due to a large vacuum leak) while the engine operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or HO2S DTC may set). Remember, use a scan tool in order to observe fuel trim counts while the problem is occurring.
Remember, a fuel trim DTC may be triggered by a list of vehicle faults. Make use of all information available (other DTCs stored, rich or lean condition, etc.) when diagnosing a fuel trim fault.
Comprehensive Component Monitor Diagnostic
The comprehensive component monitoring diagnostics are required to monitor emissions-related input and output powertrain components. The CARB OBD II comprehensive component monitoring list of components Intended to illuminate the malfunction indicator lamp (MIL) is a list of components, features or functions that could fall under this requirement.
The control module monitors the input components for circuit continuity and out-of-range values. This includes performance checking. Performance checking refers to indicating a fault when the signal from a sensor does not seem reasonable, for example, a throttle position (TP) sensor that indicates high TP at low engine loads or MAP voltage. The input components may include but are not limited to the following sensors:
The vehicle speed sensor (VSS)
The crankshaft position (CKP) sensor
The knock sensor (KS)
The throttle position (TP) sensor
The engine coolant temperature (ECT) sensor
The camshaft position (CMP) sensor
The manifold absolute pressure (MAP) sensor
The mass air flow (MAF)
In addition to the circuit continuity and rationality check, the ECT sensor is monitored for the ability to achieve a steady state temperature in order to enable a closed loop fuel control.
Diagnose the output components for the proper response to control module commands. Components where functional monitoring is not feasible will be monitored for circuit continuity and out-of-range values if applicable.
Output components to be monitored include, but are not limited to the following circuits:
The Idle Air Control (IAC) Motor
The Control Module controlled EVAP Canister Purge Valve
The Electronic transaxle/transmission controls
The A/C relay
The Cooling Fan(*) Relay(*)
The VSS output
The MIL control
The Cruise Control inhibit
The AIR system pump
California Air Resources Board (CARB) OBD II Comprehensive Component Monitoring List of Components Intended to Illuminate MIL
Not all vehicles have these components.
The following components are intended to illuminate the malfunction indicator lamp (MIL).
Transaxle/Transmission Range (TR) Mode Pressure Switch
Transaxle/Transmission Turbine Speed Sensor (HI/LO)
Transaxle/Transmission Vehicle Speed Sensor (HI/LO)
Transaxle/Transmission Vehicle Speed Sensor (HI/LO)
Ignition Sensor (Cam Sync, Diag)
Ignition Sensor Hi Res (7x)
Knock Sensor (KS)
Engine Coolant Temperature (ECT) Sensor
Intake Air Temperature (IAT) Sensor
Throttle Position (TP) Sensor A, B
Manifold Absolute Pressure (MAP) Sensor
Mass Air Flow (MAF) Sensor
Components Intended to Illuminate MIL
Automatic Transaxle/Transmission Temperature Sensor
Transaxle/Transmission Torque Converter Clutch (TCC) Control Solenoid
Transaxle/Transmission TCC Enable Solenoid
Transaxle/Transmission Shift Solenoid A
Transaxle/Transmission Shift Solenoid B
Transaxle/Transmission 3/2 Shift Solenoid
Ignition Control (IC) System
Idle Air Control (IAC) Coil
Evaporative Emission Purge Vacuum Switch
Evaporative Emission Canister Purge (EVAP Canister Purge)
Wiring Harness Service
The control module harness electrically connects the control module to the various solenoids, switches, and sensors in the engine compartment and the passenger compartment.
Replace the wiring harnesses with the proper part number replacements. When splicing signal wires into a harness, use a wire that has a high-temperature insulation.
Consider the low amperage and the voltage levels utilized in the powertrain control system. Make the best possible bond at all splices. Use rosin-core solder in the areas.
Molded-on connectors require complete replacement of the connector. Splice a new connector into the harness. The replacement connectors and terminals are listed in Group 8.965 in the Standard Parts Catalog.
For wiring repair procedures, refer to Wiring Repairs in Wiring Systems.
Connectors and Terminals
In order to prevent shorting between opposing terminals, use care when probing a connector and when replacing terminals. Damage to the components could result.
Always use fused jumper wires between connectors when testing circuits.
Never probe through Weather-Pack seals.
Open circuits are often difficult to locate by sight because oxidation or terminal misalignment are hidden by the connectors. Wiggling a connector or the wiring harness may temporarily correct the open circuit. Oxidized or loose connections may cause intermittent problems.
Be certain the type of connector and terminal before making any connector or terminal repair. Weather-Pack and Com-Pack III terminals look similar, but are serviced differently.