An Engine Control Module (ECM controls the exhaust emissions while providing good driveability and fuel efficiency. The sensors and switches located throughout the vehicle gather the data which the functions of the system are based on. The ECM maintains control over the fuel delivery, the ignition, the idle air flow, the fuel pump, and the other system components, while monitoring the system for faulty operation with its diagnostic capabilities.

The ECM's language for communicating the source of the malfunction is a system of diagnostic trouble codes. When the system finds a problem, the Malfunction Indicator Lamp (MIL) lights up on the instrument panel and a Diagnostic Trouble Code (DTC) stores in the ECM memory. This does not mean the engine should be stopped immediately, but check for the cause of the light coming on as soon as possible.

The Vehicle Emissions Control Information label contains important emissions important emission specifications and setting procedures. In the upper left corner is the exhaust emission information which identifies the broadcast code, the engine, and the emission RPO. Also there is an illustrated emission component and vacuum hose schematic. This label is located on the engine air cleaner cover. If this label has been removed, order one from General Motors Service Parts Operation (GMSPO).

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.

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:

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

Notice: 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 system has a computer Engine Control Module (ECM) which controls the fuel delivery, the ignition timing, and some emission control systems.

The system, through the ECM, monitors a number of engine and vehicle functions and controls the following operations:

The Engine Control Module (ECM) (1), located in the passenger compartment, is the control center of the control module system with two connector harnesses (2). The PROM access cover (3) is located on the bottom

The ECM constantly looks at the information from the various sensors, and controls the systems that affect vehicle's performance. The ECM performs the diagnostic function of the system. The ECM recognizes operational problems, alerts the driver through the Malfunction Indicator Lamp (MIL), and stores a DTC which identifies the problem areas to aid the technician in making repairs. Refer to Engine Control Module Diagnosis for more information.

The ECM is designed to process the various input information and then sends the necessary electrical responses to the control fuel delivery, ignition timing and other emission control systems. The input information has an interrelation to more than one output; therefore, a failed input could effect more than one system's operation.

Notice: Maintain the control module at a temperature below 85°C (185°F) at all times. This is most essential if you put the vehicle through a paint baking process. The control module will become inoperative if its temperature exceeds 85°C (185°F). Place temporary insulation around the control module during the time the vehicle is in the paint oven or any other high temperature process.

The ECM has a learning ability which allows it in order to make corrections for minor variations in the fuel system to improve driveability. If the battery is disconnected to clear codes, or for repair, the learning process has to begin all over again.

The driver may note a change in the vehicle's performance. In order to teach the vehicle, make sure the engine is at the operating temperature, and drive at part throttle with moderate acceleration and idle conditions until the normal performance returns.

Notice: In order to prevent possible Electrostatic Discharge damage:

The electronic components used in control systems are often designed to carry very low voltage, and are very susceptible to damage caused by an electrostatic discharge. Less than 100 volts of static electricity can cause damage to some electronic components. By comparison, humans need as much as 4,000 volts 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 in which a charge of as much as 25,000 volts can build up. Charging by induction occurs when a person with well insulated shoes stands near a highly charged object and momentarily touches a ground. Charges of the same polarity are drained off, leaving the person highly charged with the opposite polarity. Static charges of either type can cause damage, therefore, it is important to use, care when handling and testing electronic components.

The ECM supplies a buffered 5 or 12 volts to power various sensors or switches. This is done through resistances in the ECM which are so high in value that a test lamp will not lilluminate when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low. Therefore, the use of a 10 megohm input impedance digital voltmeter is necessary in order to assure accurate voltage readings.

The input/output devices in the ECM include analog to digital converters, signal buffers, counters, and special drivers. The ECM controls output circuits such as the injectors, the IAC, etc. by controlling the ground circuit through transistors or a device called a Quad-Driver Module (QDM) in the ECM.

There are three types of memory storage within the ECM: ROM, RAM and PROM.

ROM

Read Only Memory (ROM) is a permanent memory that is physically soldered to the circuit boards within the ECM. The ROM contains the overall control algorithms. Once programmed the ROM cannot changed. The ROM memory is non-volatile and does not need power in order to be retained.

RAM

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

PROM

The Programmable Read Only Memory (PROM) (1) in the PROM carrier (2) is the portion of the ECM (4) that contains the different engine calibration information that is specific to the year, the model, and the emissions. Another PROM (CAL-PAK) (3) (with specific calibration information allows fuel delivery if the other parts of the ECM are damaged. The PROM is a non-volatile memory that is read only by the ECM. While one ECM part number can be used by many vehicle lines, a PROM is very specific and must be used for the right vehicle. Check the latest parts book and service bulletin information for the correct part number when replacing a PROM. An ECM used for service comes without a PROM. Retain the PROM with the vehicle information following the ECM replacement. Carefully remove the PROM from the old ECM and install into the new ECM.

The short term fuel trim (integrator) is an ECM volatile memory register that contains a number between 0 and 255. The neutral value for the short term fuel trim is 128; any deviation from this value indicates the short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 128. The short term fuel trim changes the pulse width by varying the Closed Loop factor of the base pulse width equation. As the ECM monitors the Oxygen Sensor (O2S) input and constantly varyies the short term fuel trim's value. The value is updated very quickly; therefore, the short term fuel trim only corrects for short term mixture trends. The correction of long term mixture trends is the function of the long term fuel trim.

The long term fuel trim (block learn) is a matrix of cells arranged by RPM and MAP. Each cell of the long term fuel trim is a register like the short term fuel trim. As the engine operating conditions change, the ECM will switch from cell to cell to determine what long term fuel trim factor to use in the base pulse width equation. While in any given cells, the ECM also monitors the short term fuel trim. If the short term fuel trim is far enough from 128, the ECM will change the long term fuel trim value. Once the long term fuel trim value is changed, it should force the short term fuel trim back toward 128.

If the mixture is still not correct (as judged by the O2S), the short term fuel trim will continue to have a large deviation from the ideal 128. In this case, the long term fuel trim value will continue to change until the short term fuel trim becomes balanced.

Both the short term fuel trim and long term fuel trim have limits which will vary with each PROM. If the mixture is off enough so that long term fuel trim reaches the limit of its control and still cannot correct the condition, the short term fuel trim would also go to its limit of control in the same direction.

If the mixture is still not corrected by both short term fuel trim and long term fuel trim at their extreme values, a DTC 44, or DTC 45 will likely result. Under the conditions of power enrichment, the ECM sets the short term fuel trim to 128 and freezes it there until power enrichment is no longer in effect. This is done so the Closed Loop factor and the long term fuel trim will not try to correct for the commanded richness of power enrichment.

The Throttle Body Injection (TBI) system is a speed and air density system. The system is based on the speed density fuel management.

Three specific data sensors provide the ECM with the basic information for the fuel management portion of its operation. That is, three specific signals to the ECM establish the engine speed and air density factors.

The engine speed signal comes from the distributor ignition control module to the ECM on the reference high circuit. The ECM uses this information in order to determine the speed or RPM factor for fuel and ignition management.

Two sensors contribute to the density factor, the Engine Coolant Temperature (ECT) and the Manifold Absolute Pressure (MAP) sensors.

The ECT sensor is a 2-wire sensor that measures the temperature of the engine coolant. The ECT sensor is a thermistor that changes its resistance based on temperature. When the temperature is low, the resistance is high, and when the temperature is high, the resistance is low.

The MAP sensor is a 3-wire sensor that monitors the changes in intake manifold pressure which result from changes in engine load. These pressure changes are supplied to the ECM in the form of analog electrical signals.

As intake manifold pressure increases, the air density in the intake manifold also increases, and additional fuel is required. The MAP sensor sends this pressure information to the ECM, and the ECM increases the amount of fuel injected, by increasing the injector pulse width. Conversely, as manifold pressure decreases, the amount of fuel is decreased.

These three inputs MAP, ECT, and RPM are the major determinants of the air/fuel mixture delivered by the fuel injection system.

The remaining sensors and switches provide the electrical inputs to the ECM which uses the inputs for the modification of the air/fuel mixture, as well as for other ECM control functions, such as Idle Air Control (IAC).