The control module refers to the powertrain control module
(PCM). The control module is designed to maintain exhaust emission levels
to 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 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
4 and 7 called one of the following:
The surface mounted quad driver module can independently control up
to 4 outputs (control module) terminals. The output driver modules can independently
control up to 7 outputs. Not all outputs are always used.
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.
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.
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.
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.
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.
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.
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.
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.
A basic understanding of hand tools is necessary in order
to effectively use this information.
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, complete
or not complete, in order to verify that the vehicle meets the criteria which
complies with local area requirements. Using the system status display, any
of the following systems or a combination of the systems may be monitored
for I/M readiness:
Following a DTC info clear, the system status will clear only for the
systems affected by any DTCs stored. Following a battery disconnect or a control
module replacement, all of the system status information will clear.
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 catalyst monitor heated oxygen sensors (HO2S) for the following
functions:
The main function of the
fuel control heated oxygen sensor 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 heated
oxygen sensors 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, connector or terminals are damaged,
replace the entire oxygen 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 provided. 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 could degrade the oxygen sensor performance.
The oxygen sensor heaters are required by catalyst
monitor sensors to maintain a sufficiently high temperature which allows
accurate exhaust oxygen content readings further from the engine.
In order to control emissions of hydrocarbons (HC), carbon monoxide
(CO), and oxides of nitrogen (NOx), the system uses a three-way catalytic
converter. The catalyst within the converter promotes a chemical reaction
which oxidizes the HC and CO present in the exhaust gas, converting them
into harmless water vapor and carbon dioxide. The catalyst also reduces NOx,
converting it to nitrogen.
The control module has the ability to monitor this process using the
heated oxygen sensors (HO2S). The HO2S produces an output signal which indicates
the oxygen storage capacity of the catalyst, this in turn indicates the catalyst's
ability to convert exhaust gases efficiently. If the catalyst is operating
efficiently, the O2S signal will be far more active than that produced by
the HO2S.
The OBD II catalyst monitor diagnostic measures oxygen storage capacity.
In order to do this, the heated sensors are installed before and after the
three-way catalyst (TWC). The voltage variations between the sensors allow
the control module to determine the catalyst emission performance.
As a catalyst becomes less effective in promoting chemical reactions,
the capacity of the catalyst 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, which operates at approximately 95 percent hydrocarbon
conversion efficiency, shows a relatively flat output voltage on the post-catalyst
heated oxygen sensor (HO2S). A degraded catalyst, which may operate at approximately
65 percent hydrocarbon conversion, shows a greatly increased activity in
output voltage from the post catalyst HO2S.
The post-catalyst HO2S 2 is used to measure the oxygen storage
and release capacity of the catalyst. A high oxygen storage capacity indicates
a good catalyst. Low oxygen storage capacity indicates a failing catalyst.
The TWC, the HO2S 2, and the HO2S 3 must be at operating temperature
in order to achieve correct oxygen sensor voltages like those shown in the
post-catalyst HO2S 3 outputs graphic.
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.
The control module must
monitor the three-way catalyst (TWC) system for efficiency. In order to accomplish
this, the control module monitors the pre-catalyst and post-catalyst oxygen
sensors. When the TWC is operating properly, the post-catalyst (2)
oxygen sensor will have significantly less activity than the pre-catalyst (1)
oxygen sensor. The TWC stores the 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 pre-catalyst and post-catalyst
oxygen sensor voltage levels.
Whenever the voltage levels of the post-catalyst (2) oxygen sensor
nears the voltage levels that of the pre-catalyst (1) oxygen sensor,
the efficiency of the catalyst is degraded.
Stepped or staged testing levels allow the control module to statistically
filter the 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 you are directed to do this by the service information.
Three stages are used in order to monitor catalyst efficiency. Failure
of the first stage indicates that the catalyst requires further testing in
order to determine catalyst efficiency. Failure of the second stage indicates
that the catalyst may be degraded. The third stage then looks more closely
at the inputs from the pre and post O2S before determining if the catalyst
is indeed degraded. This further statistical processing is done in order
to increase the accuracy of the oxygen storage capacity type monitoring.
Failing the first (stage 0) or the second (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.
