For 1990-2009 cars

The engine control module (ECM) interacts with many emission related components and systems, and monitors the emission related components and systems for deterioration. OBD II diagnostics monitor the system performance and a diagnostic trouble code (DTC) sets if the system performance degrades.

The malfunction indicator lamp (MIL) operation and the DTC storage are dictated by the DTC type. A DTC is ranked as a type A or type B if the DTC is emissions-related. Type C is a non-emissions- related DTC.

The ECM is located in the engine compartment. The ECM is the control center of the engine controls system. The ECM controls the following components:

• | The fuel injection system

• | The ignition system

• | The emission control systems

• | The on-board diagnostics

• | The A/C and fan systems

• | The throttle body motor system

The ECM constantly monitors the information from various sensors and other inputs, and controls the systems that affect the vehicle performance and the emissions. The ECM also performs diagnostic tests on various parts of the system. The ECM can recognize operational conditions and alert the driver via the MIL. When the ECM detects a malfunction, the ECM stores a DTC. The condition area is identified by the particular DTC that is set. This helps the technician in making repairs.

### ECM Function

The engine control module (ECM) can supply 5 V or 12 V to the various sensors or switches. This is done through pull-up resistors to the regulated power supplies within the ECM. In some cases, even an ordinary shop voltmeter will not give an accurate reading because the resistance is too low. Therefore, a DMM with at least 10 MΩ input impedance is required in order to ensure accurate voltage readings.

The ECM controls the output circuits by controlling the ground or the power feed circuit through the transistors or a device called an output driver module.

### EEPROM

The programmable read only memory (EEPROM), which can be erased electronically, is a permanent memory that is physically part of the engine control module (ECM). The EEPROM contains program and calibration information that the ECM needs in order to control the powertrain operation.

Special equipment, as well as the correct program and calibration for the vehicle, are required in order to re-program the ECM.

### ECM Default Actions

When a malfunction occurs within the engine control system, the engine control module (ECM) maintains control of the system with default actions. Default actions are calculated values, and/or calibrated default values, that are stored within the ECM. A certain level of engine performance is possible when a malfunction occurs dependant on the default actions taken. The ECM default actions prevent a complete loss of engine performance.

### ECM Output Controls

The scan tool can control certain solenoids, valves, motors, and relays. The output controls can be found under the special functions selection of the scan tool. Some output controls may be disabled by the engine control module (ECM) during certain types of vehicle operation.

### Data Link Connector (DLC)

The data link connector (DLC) is a 16-pin connector that provides the technician with a means of accessing serial data for aid in the diagnosis. This connector allows the technician to use a scan tool in order to monitor the various serial data parameters, and display the DTC information. The DLC is located inside the drivers compartment, underneath the dash.

### Malfunction Indicator Lamp (MIL)

The malfunction indicator lamp (MIL) is located on the instrument panel cluster (IPC), or the driver information center (DIC). The MIL is controlled by the engine control module (ECM) and illuminates when the ECM detects a condition that affects the vehicle emissions.

### ECM Service Precautions

The engine control module (ECM), by design, can withstand the normal current draws that are associated with the vehicle operations. However, care must be taken in order to avoid overloading any of these circuits. When testing for opens or shorts, do not ground or apply voltage to any of the ECM circuits unless the diagnostic procedure instructs you to do so. These circuits should only be tested with a DMM.

### Aftermarket (Add-On) Electrical And Vacuum Equipment

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.

Add-on electrical equipment, even when installed to these strict guidelines, may still cause the power-train system to malfunction. This may also include equipment not connected to the vehicle electrical system, such as portable telephones and radios. Therefore, the first step in diagnosing any power-train condition is to make sure all of the after-market electrical equipment is removed or disconnected from the vehicle. After this has been done and if the condition still exists, the condition may be diagnosed in the normal manner.

### Electrostatic Discharge (ESD) Damage

Note: In order to prevent possible electrostatic discharge damage to the ECM, do not touch the connector pins on the ECM.

The electronic components that are used in the control systems are often designed to carry very low voltage. The electronic components are susceptible to damage caused by electrostatic discharge. Less than 100 V of static electricity can cause damage to some electronic components. By comparison, it takes as much as 4000 V for a person to even feel the zap of a static discharge.

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 exercise great care when handling and testing electronic components.

### Emission Control Information Label

The underhood vehicle emissions control information label contains important emission specifications and setting procedures. This identifies the year, the manufacturing division of the engine, the displacement of the engine in litres, the class of the vehicle, and type of fuel metering system. There is also an illustrated emission components and vacuum hose schematic.

This label is located in the engine compartment of every General Motors vehicle. If the label has been removed, it can be ordered from GM service parts operations (GMSPO).

### Underhood Inspection

Note: This inspection is very important and must be done carefully and thoroughly.

Perform a careful underhood inspection when performing any diagnostic procedure or diagnosing the cause of an emission test failure. This can often lead to repairing a condition without further steps. Use the following guidelines when performing an inspection:

• | Inspect all of the vacuum hoses for correct routing, pinches, cuts, or disconnects.

• | Inspect any hoses that are difficult to see.

• | Inspect all of the wires in the engine compartment for the following conditions:

• | Burned or chafed spots

• | Pinched wires

• | Contact with sharp edges

• | Contact with hot exhaust manifolds

### Basic Knowledge Required

A basic understanding of hand tools is necessary in order to effectively use this section of the service manual.

You must be familiar with some of the basics of engine operation and electrical diagnosis in order to use this section of the service manual.

• | Basic electrical circuits--You should have an understanding of basic 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, and you should be able to identify a shorted or open circuit by using a digital multimeter (DMM). You should be able to read and understand a wiring diagram.

• | Use of digital multimeter, You should be familiar with the DMM, particularly the essential tool. You should be able to use the meter in order to measure the voltage (volts), the resistance (ohms), the current (amps), intermittent (min/max), and frequency (Hertz).

• | Use of circuit testing tools--You should not use a test lamp to diagnose the engine controls system unless you are specifically instructed to do so. You should know how to use jumper wires in order to test the components and allow DMM readings without damaging the terminals. You should know how to use the J 35616 connector test adapter kit and use the kit whenever the diagnostic procedures call for front probing any connector.