TIBURON(GK) >2006 > G 2.7 DOHC > Fuel System > Engine Control System>Description and Operation | ![]() ![]() |
1. OVERVIEW
The California Air
Resources Board (CARB) began regulation of On Board Diagnostics (OBD) for
vehicles sold in california beginning with the 1988 model year. The first
phase, OBD-I, required monitoring of the fuel metering system, Exhust Gas
Recirculation (EGR) system and additional emission related components. The
Malfunction Indicator Lamp (MIL) was required to light and alert the
driver of the fault and the need for repair of the emission control
system. Associated with the MIL was a fault code or Diagnostic Trouble
Code (DTC) idenfying the specific area of the fault.
The OBD system was
proposed by CARB to improve air quality by identifying vehicle exceeding
emission standards. Passage of the Federal Clean Air Act Amendments in
1990 has also prompted the Environmental Protection Agency (EPA) to
develop On Board Diagnostic requirements. CARB OBD-II regulations will be
followed until 1999 when the federal regulations will be
used.
The OBD-II system meets
government regulations by monitoring the emission control system. When a
system or component exceeds emission threshold or a component operates
outside tolerance, a DTC will be stored and the MIL
illuminated.
The diagnostic executive
is computer program in the Engine Control Module (ECM) that coordinates
the OBD-II self-monitoring system. This program controls all the monitors
and interactions, DTC and MIL operation, freeeze frame data and scan tool
interface.
Freeze frame data
describes stored engine conditions, such as state of the engine, state of
fuel control, spark, RPM, load and warm status at the point the first
fault is detected. Previously stored conditions will be replace only if a
fuel or misfire fault is detected. This data is accessible with the scan
tool to assist in repairing the vehicle.
The center of the OBD-II
system is a microprocessor called the Engine Control Module
(ECM).
The ECM receives input
from sensors and other electronic components (switches, relays, and
others) based on information received and programmed into its memory (keep
alive random acess memory, and others), the ECM generates output signals
to control various relays, solenoids and actuators.
2. CONFIGURATION OF HARDWARE AND RELATED
TERMS
The Malfunction
Indicator Lamp (MIL) is connected between ECM-terminal Malfunction
Indicator Lamp and battery supply (open collector amplifier).
In most of car, the MIL
will be installed in the instrument panel. The lamp amplifier can not be
damaged by a short circuit.
Lamps with a power
dissipation much greater than total dissipation of the MIL and lamp in the
tester may cause a fault indication.
▷ At ignition ON and
engine revolution (RPM) < MIN. RPM, the MIL is switched ON for an
optical check by the driver.
When the ECM detects a
malfunction related emission during the first driving cycle, the DTC and
engine data are stored in the freeze frame memory. The MIL is illuminated
only when the ECM detects the same malfunction related the DTC in two
consecutive driving cycles.
● Misfire and Fuel
System Malfunctions:
For misfire or fuel
system malfunctions, the MIL may be eliminated if the same fault does not
reoccur during monitoring in three subsequent sequential driving cycles in
which conditions are similar to those under which the malfunction was
first detected.
● All Other
Malfunctions:
For all other faults,
the MIL may be extinguished after three subsequent sequential driving
cycles during which the monitoring system responsible for illuminating the
MIL functions without detecting the malfunction and if no other
malfunction has been identified that would independently illuminate the
MIL according to the requirements outlined above.
The diagnostic system
may erase a fault code if the same fault is not re-registered in at least
40 engine warm-up cycles, and the MIL is not illuminated for that fault
code.
A driving cycle consists
of engine start up, and engine shut off.
A warm-up cycle means
sufficient vehicle operation such that the engine coolant temperature has
risen by at least 40 degrees Fahrenheit from engine starting and reaches a
minimum has risen by at least 40 degrees Fahrenheit from engine starting
and reaches a minimum temperature of at least 160 degrees
Fahrenheit.
A trip means vehicle
operation (following an engine-off period) of duration and driving mode
such that all components and systems are monitored at least once by the
diagnostic system except catalyst efficiency or evaporative system
monitoring when a steady-speed check is used, subject to the limitation
that the manufacturer-defined trip monitoring conditions shall all be
encountered at least once during the first engine start portion of the
applicable FTP cycle.
The letter of the
beginning of the DTC identifies the function of the monitored device that
has failed. A "P" indicates a powertrain device, "C" indicates a chassis
device. "B" is for body device and "U" indicates a network or data link
code. The first number indicates if the code generic (common to all
manufacturers) or if it is manufacturer specific. A "0" & "2"
indicates generic, "1" indicates manufacturer-specific. The second number
indicates the system that is affected with a number between 1 and
7.
The following is a list
showing what numbers are assign to each system.
The last two numbers of
the DTC indicates the component or section of the system where the fault
is located.
When a freeze frame
event is triggered by an emission related DTC, the ECM stores various
vehicle information as it existed the moment the fault ocurred. The DTC
number along with the engine data can be useful in aiding a technician in
locating the cause of the fault. Once the data from the 1st driving cycle
DTC ocurrence is stored in the freeze frame memory, it will remain there
even when the fault ocurrs again (2nd driving cycle) and the MIL is
illuminated.
3. OBD-II SYSTEM READINESS TESTS
The catalyst efficiency
monitor is a self-test strategy within the ECM that uses the downstream
Heated Oxygen Sensor (HO2S) to determine when a catalyst has fallen below
the minimum level of effectiveness in its ability to control exhaust
emission.
Misfire is defined as
the lack of proper combustion in the cylinder due to the absence of spark,
poor fuel metering, or poor compression. Any combustion that does not
occur within the cylinder at the proper time is also a misfire. The
misfire detection monitor detects fuel, ignition or mechanically induced
misfires. The intent is to protect the catalyst from permanent damage and
to alert the customer of an emission failure or an inspection maintenance
failure by illuminating the MIL . When a misfire is detected, special
software called freeze frame data enabled. The freeze frame data captures
the operational state of the vehicle when a fault is detected from misfire
detection monitor strategy.
The fuel system monitor
is a self-test strategy within the PCM that monitors the adaptive fuel
table The fuel control system uses the adaptive fuel table to compensate
for normal variability of the fuel system components caused by wear or
aging. During normal vehicle operation, if the fuel system appears biased
lean or rich, the adaptive value table will shift the fuel delivery
calculations to remove bias.
The cooling system
monitoring is a self-test strategy within the ECM that monitors ECTS
(Engine Coolant Temperature Sensor) and thermostat about circuit
continuity, output range, rationality faults.
OBD-II regulations
require monitoring of the upstream Heated O2 Sensor (H2OS) to detect if
the deterioration of the sensor has exceeded thresholds. An additional
HO2S is located downstream of the Warm-Up Three Way Catalytic Converter
(WU-TWC) to determine the efficiency of the catalyst.
Although the downstream
H2OS is similar to the type used for fuel control, it functions
differently. The downstream HO2S is monitored to determine if a voltage is
generated. That voltage is compared to a calibrated acceptable
range.
The EVAP. monitoring is
a self-test strategy within the ECM that tests the integrity of the EVAP.
system. The complete evaporative system detects a leak or leaks that
cumulatively are greater than or equal to a leak caused by a 0.040 inch
and 0.020 inch diameter orifice.
The A/C system
monitoring is a self-test strategy within the ECM that monitors
malfunction of all A/C system component at A/C ON.
The comprehensive
components monitoring is a self-test strategy within the ECM that detects
fault of any electronic powertrain components or system that provides
input to the ECM and is not exclusively an input to any other OBD-II
monitor. |