The powertrain control module (PCM) is designed to maintain exhaust emission levels while maintaining excellent driveability and fuel efficiency. The PCM controls the following operations:

The powertrain control module (PCM) is located in the engine compartment. The PCM is the control center of the vehicle and controls the following systems:

The PCM constantly monitors the information from various sensors and controls the systems that affect vehicle performance and emissions. The PCM also performs the diagnostic functions for those systems. The PCM can recognize operational problems and alert the driver through the malfunction indicator lamp (MIL) when a malfunction has occurred. When a malfunction is detected, the PCM stores a diagnostic trouble code (DTC) which helps to identify problem areas. This is done to aid the technician in making repairs.

The PCM supplies either 5.0 or 12.0 volts to power various sensors and switches. This is done through resistances in the PCM. The resistance is so high in value that a test lamp does not illuminate when connected to the circuit. In some cases, even an ordinary shop voltmeter does not give an accurate reading because the voltmeters resistance is too low. Therefore, a DMM with a minimum of 10 megaohms input impedance is required to ensure accurate voltage readings.

The PCM controls output circuits such as the fuel injectors, the idle air control (IAC), and the cooling fan relays by controlling the ground or the power feed circuit through transistors or a device called an output driver module (ODM).

Torque management is a function of the PCM that reduces engine power under certain conditions. Torque management is performed for the following reasons:

The PCM monitors the following sensors and engine parameters in order to calculate engine output torque:

The PCM monitors the torque converter status, the transmission gear ratio, and the engine speed in order to determine if torque reduction is required. The PCM retards the spark as appropriate to reduce engine torque output if torque reduction is required. The PCM also shuts OFF the fuel to certain injectors in order to reduce the engine power in the case of an abusive maneuver.

The following are instances when engine power reduction is likely to be experienced:

The driver is unlikely to notice the torque management actions in the first 2 instances. The engine power output is moderate at full throttle in the other cases.

The PCM calculates the amount of spark retard necessary to reduce the engine power by the desired amount. The PCM disables the fuel injectors for cylinders 1, 4, 6, and 7 in the case of an abusive maneuver.

The PCM supplies a buffered voltage to various sensors and switches. The PCM controls most components with electronic switches which complete a ground circuit when turned ON.

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 diagnostic procedures call for probing any connectors.

Without a basic knowledge of electricity, it will be difficult to use the diagnostic procedures contained in this section. You should understand the basic theory of electricity and know the meaning of voltage (volts), current (amps) and resistance (ohms). You should understand what happens in a circuit with an open or a shorted wire. You should be able to read and understand a wiring diagram.

The PCM is designed to withstand normal current draws associated with vehicle operations. Avoid overloading any circuit. When testing for opens or shorts, do not ground any of the PCM circuits unless instructed. When testing for opens or shorts, do not apply voltage to any of the PCM circuits unless instructed. Only test these circuits with a DMM while the PCM connectors remain connected.

Aftermarket, add-on electrical and vacuum equipment is defined as any equipment installed on a vehicle after leaving the factory that connects to the vehicles electrical or vacuum systems. No allowances have been made in the vehicle design for this type of equipment.

Notice: 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.

Notice: Connect any add-on electrically operated equipment to the vehicle's electrical system at the battery (power and ground) in order to prevent damage to the vehicle.

Add-on electrical equipment, even when installed to these strict guidelines, may still cause the powertrain system to malfunction. This may also include equipment not connected to the vehicles electrical system such as portable telephones and radios. Therefore, the first step in diagnosing any powertrain problem is to eliminate all aftermarket electrical equipment from the vehicle. If the problem still exists, diagnose the problem in the normal manner.

Notice: 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.

Electronic components used in the control systems are often designed in order to carry very low voltage. 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. Therefore, it is important to use care when handling and testing electronic components.

The driveability and emissions information describes the function and operation of the powertrain control module (PCM).

The engine controls information contains the following:

The component system includes the following items:

The DTCs also contain diagnostic support information containing circuit diagrams, circuit or system information, and helpful diagnostic information.

Refer to the General Motors Maintenance Schedule of the appropriate service category for the maintenance that the owner or technician should perform in order to retain emission control performance.

Important: This visual and physical inspection is very important. Perform the inspection carefully and thoroughly.

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:

Important: Lack of basic knowledge of this powertrain when performing diagnostic procedures could result in incorrect diagnosis 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.

There are primary system-based diagnostics which evaluate the system operation and their effect on vehicle emissions. The primary system-based diagnostics are listed below, with a brief description of the diagnostic functionality.

Diagnose the fuel control heated oxygen sensors (HO2S) for the following conditions:

Diagnose the catalyst monitor heated oxygen sensors for the following functions:

The misfire monitor diagnostic is based on crankshaft rotational velocity, aka reference period, variations. The PCM determines crankshaft rotational velocity using the crankshaft position (CKP) sensor and camshaft position (CMP) sensor. When a cylinder misfires, the crankshaft slows down momentarily. By monitoring the crankshaft and camshaft position sensor signals, the PCM can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 1,000-3,200 engine revolutions.

For catalyst damage misfire, the diagnostic responds to the misfire within 200 engine revolutions.

Rough roads may cause false misfire detection. A rough road applies sudden torque variations to the drive wheels and drivetrain. This torque can intermittently decrease the crankshaft rotational velocity.

Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time the misfire occurred.

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 out of 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 display 0 until the misfire diagnostic has failed and a DTC P0300 is set. Once the misfire DTC sets, the misfire history counters will be updated every 200 cylinder firing events. The Misfire counters graphic illustrates how these misfire counters are maintained.

When crankshaft rotation is erratic, the PCM 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 there are misfires counted from more than one cylinder. Cylinder 1 has the majority of 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 accumulated misfires is sufficient for the diagnostic to identify a true misfire, the diagnostic will set DTC P0300--Misfire Detected. The illustration depicts an accumulation in the history buffers.

If two cylinders in sequential firing order are both misfiring, the first misfiring cylinder will accumulate misfires in its buffer, but the second misfiring cylinder will not. This is because the PCM compares a misfiring cylinder with the cylinder 90 degrees prior to that cylinder in the firing order. Therefore the PCM would be comparing crankshaft speed of the second misfiring cylinder to an already suspect cylinder. The PCM however, will be able to detect both misfiring cylinders after the engine exceeds 2,000 RPM. This is because the PCM then starts to compare misfires to the opposing cylinder rather than the previous cylinder in the firing order.

Use Techline® equipment to monitor the misfire counter data on applicable vehicles. Knowing which specific cylinders misfire can lead to the root cause. Using the information in the misfire counters identifies which cylinders are misfiring. If the counters indicate cylinders number 1 and 4 misfired, look for a circuit or component common to both cylinders.

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:

Comprehensive component monitoring diagnostics are required to monitor emissions-related input and output powertrain components.

The PCM 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, such as a throttle position (TP) sensor that indicates high throttle position at low engine loads or manifold absolute pressure (MAP) voltage. The input components may include, but are not limited to, the following sensors:

In addition to the circuit continuity and rationality check, the ECT sensor is monitored for ECT sensors ability to achieve a steady state temperature to enable Closed Loop fuel control.

Diagnose the output components for the proper response to PCM 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:

Replace the wire harnesses with the proper part number replacement. When splicing signal wires into a harness, use the wiring that has high temperature insulation.

Consider the low amperage and voltage levels utilized in the powertrain control systems. Make the best possible bond at all splices. Use rosin-core solder in these areas.

Molded-on connectors require complete replacement of the connector. Splice a new connector into the harness. Replacement connectors and terminals are listed in Group 8.965 in the Standard Parts Catalog.

For wiring repair, refer to the appropriate procedures in Wiring Systems.

In order to prevent shorting between opposite terminals, use care when probing a connector and when replacing terminals. Damage to the components could result.

Always use jumper wires between connectors for circuit checking.

Never probe through Weather-Pack seals.

The J 35616-A Connector Test Adapter Kit, or the equivalent, contains an assortment of flexible connectors used to probe terminals during diagnosis. BT-8616 Fuse Remover and Test Tool is used for removing a fuse and to adapt the fuse holder to a DMM for diagnosis.

Open circuits are often difficult to locate by sight because oxidation or terminal misalignment are hidden by the connectors. Merely wiggling a connector on a sensor, or in the wiring harness, may temporarily correct the open circuit. Oxidized or loose connections may cause intermittent problems.

Be certain of 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.