Mitsubishi Eclipse. Technical Information Manual (1994) — part 15
ENGINE <NON-TURBO> Control Svstem
Operation
Once it reaches its operating temperature of
to
the sensor generates a voltage inversely
proportional to the amount of oxygen in the exhaust.
This information is used by the PCM to calculate
the fuel injector pulse width necessary to provide
the critical 14.7 to 1 air/fuel ratio (stoichiometric).
A properly operating sensor must not only be able
to generate an output voltage across its operating
range of 0 to 1 volt as it is exposed to different
levels, it also must be able to detect these
changes quickly. To detect a shift in the air/fuel
mixture (lean or rich) the output voltage must change
beyond a threshold value. A malfunctioning sensor
could have difficulty changing beyond the threshold
value.
While there are existing tests that determine if an
sensor is shorted to voltage, contains an open
circuit, or does not function at all, prior to OBD
II, it was impossible to identify sluggish or marginal
sensors.
A slow response rate or reduced voltage output
are all signs of an improperly operating oxygen sen-
sor. This could mean longer instances of combustion
of less than optimum air/fuel mixtures. This can
significantly increase emissions.
The
sensor must have a voltage output of greater
than
volt in order to pass the test. It must also
switch more times over a 120 second period than
a given threshold (to demonstrate quick response)
or produce a given number of steep slope switches
(demonstrating dynamic range and quick response)
over the same time period. Data is not stored unless
idle time exceeds 10 seconds.
Front
High switching
frequency
fast response
Front
I
Low switching
frequency
slow response
Trigger 1.5 x STD
Switching frequency
AFUOO88
ENGINE <NON-TURBO> Control System
Heater Monitor
Background
In order for the
sensor to function properly, the
sensor must be heated to approximately 572“
662°F. To assist the
sensor in achieving this
temperature, the
sensor is equipped with a Posi-
tive Thermal Coefficient (PTC) heater element. Both
the upstream an downstream’s heater element is
fed battery voltage any time the MFI relay is ener-
gized. The ground for the heater element is routed
through the
sensor’s
connector to an exter-
nal ground source. As current flows through the
element, the temperature of the
sensor in-
creases. As the temperature increases, the resis-
tance of the element increases causing the current
Operation
The resistance of the
sensor changes with tem-
perature and age. It’s resistance normally is between
100 to 4.5
When the temperature of the
sensor is increased, the resistance of the sensor
decreases. Inversely, as the sensor ages, the resis-
tance increases. It is the resistance of the sensor’s
output circuit that is tested for proper heater opera-
tion, not the heater element itself.
The test begins approximately 5 seconds after the
engine has been turned off with the ignition key
and battery voltage greater than 10 volts (the PCM
still operates even though the key is in the off posi-
tion). Once the timer has timed out, the PCM bias
5 volts to the
sensors output wire once every
1.6 seconds, and keeps it biased each time for
35 ms. During this portion of the test, the PCM
monitors the voltage on the output wire of the
sensor. As the sensor cools down, the resistance
should increase, causing the PCM to register an
increase of voltage. The PCM determines a
flow to decrease. The
sensor’s heater maintains
the temperature of the sensor to around 1200°F.
After the engine has started, the upstream
sen-
sor’s information is used by the PCM to assist in
the air/fuel calculation. If the sensor is not up to
operating temperatures, the information given by
the sensor may be inaccurate, possibly causing
an increase in emissions.
The downstream
sensor is used by the PCM
to calculate the efficiency of the catalytic converter.
The sensor must be heated to allow the
sensor
to function normally. Without the
functioning nor-
mally, the catalytic converter test would be invalid.
sor that has cooled enough by detecting an increase
of 0.49 1.56 volts higher what the PCM detected
at the beginning of the test. The maximum amount
of time to perform this portion of the test is 144
seconds.
When the
sensor has cooled enough, the PCM
energizes the MFI relay for the next 48 seconds.
With the relay energized, current should flow through
the heater element causing an increase of tempera-
ture at the sensor. As the heater warms the
sensor, resistance of the
sensors output circuit
should decrease. As the resistance decreases, volt-
age at the PCM decreases. While the MFI relay
is energized, the PCM pulses the 5 volt biased signal
30 times. Each time the biased voltage is activated,
the PCM checks for a voltage drop. The
heater
monitor test passes if the PCM detects at least
0.157 volt decrease in 15 out of the 30 pulsed sig-
nals.
ENGINE <NON-TURBO> - Control System
Catalyst Monitor
Current vehicles use a three-way catalytic converter
to reduce emission of harmful gases. The converters
are referred to as three-way because they specifical-
ly address three pollutants (hydrocarbons, carbon
Operation
02 sensors located above (upstream) and below
(downstream) the catalytic converter are able to
monitor the efficiency of the converter. The dual
02 sensor strategy is based on the fact that as
a catalyst deteriorates, it oxygen storage capacity
and efficiency are both reduced. By monitoring the
U71AK42AA
monoxide, and nitrogen oxide) produced in the com-
bustion chamber. The catalyst monitor uses a pair
of inputs to indirectly measure just how effective
the catalyst is at reducing emissions.
oxygen storage capacity of the catalyst, its efficiency
can be indirectly calculated. The upstream sensor
detects the amount of oxygen in the exhaust gas
before it enters the converter. This sensor should
switch fairly rapidly.
Upstream(primary)
Downstream
Engine
Exhaust
gas
AFUO089
ENGINE <NON-TURBO> - Control System
I-41
A functioning converter stores oxygen so it can be
used for oxidation of hydrocarbons (HC) and carbon
monoxide (CO). The downstream sensor detects
a lower oxygen level in the exhaust than the
stream sensor. It indicates this by switching at a
significantly slower rate than the upstream sensor.
As the converter deteriorates and loses its ability
to store oxygen, the switch rate at the downstream
oxygen sensor approaches that of the upstream
sensor. The system is monitored so that when the
switch rate of the downstream sensor reaches 90%
of the switch rate of the upstream sensor (automatic
transmissions), the MIL is illuminated. The threshold
for manual transmissions is 70%. At this point,
haust emissions are projected to exceed 1
times
the legal limit.
Upstream
Downstream
Low oxygen
storage
Downstream
low
catalyst
eff
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AFU0090
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