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

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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

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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 

I 

I 

 

AFU0090