Engine Control Module Description
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 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 reprogram 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, or the driver information center. 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
|
Caution: 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. |
|
Caution: Connect any add-on electrically operated equipment to the
vehicle's electrical system at the 12 V battery (power and ground)
in order to prevent damage to the vehicle. |
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 powertrain 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 powertrain 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 4
000 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 liters, 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: |
|
- |
Contact with
sharp edges |
|
- |
Contact with hot
exhaust manifolds |
Basic Knowledge Required
|
Caution: 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 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 (V), current (A), and resistance (Ω). 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
(V), the resistance (Ω), the current (A), intermittent
(minimum/maximum), and frequency (Hz). |
• |
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 Adaptor Kit
and use the kit whenever the diagnostic procedures call for front
probing any connector. |
|