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TJ
DESCRIPTION AND OPERATION (Continued)
Refer to Open Loop/Closed Loop Modes of Opera-
tion in this section of the group for more information.
ENGINE
COOLANT
TEMPERATURE
SENSOR
ELECTRICAL
CONN15CTORI~
Fig. 4 Engine Coolant Temperature Sensor: Typical
OXYGEN SENSOR (02S)-PCM INPUT
Two heated 02S sensors are used. When the key is
turned ON, and the engine is cold, the sensors will
have an initial voltage of 5 volts. The sensors them-
selves produce voltages from 0 to 1 volt, depending
upon the oxygen content of the exhaust gas in the
exhaust manifold . When a large amount of oxygen is
present (caused by a lean air/fuel mixture), the sen-
sors produces a low voltage . When there is a lesser
amount present (rich air/fuel mixture) it produces a
higher voltage . By monitoring the oxygen content
and converting it to electrical voltage, the sensors act
as a rich-lean switch .
Both oxygen sensors are equipped with a heating
element that reduces the time required for the sen-
sors to reach operating temperature allowing the sys-
tem to enter into closed loop operation sooner.
Maintaining correct sensor temperature at all times
allows the system to remain in closed loop operation
during periods of extended idle . The Automatic Shut-
down (ASD) relay supplies battery voltage to both
the upstream and downstream oxygen sensor heating
elements .
In Closed Loop operation, the Powertrain Control
Module (PCM) monitors both 02S sensor inputs
(along with other inputs) and adjusts the injector
pulse width accordingly. During Open Loop opera-
tion, the PCM ignores the 02 sensor input . The PCM
adjusts injector pulse width based on preprogrammed
(fixed) values and inputs from other sensors .
For more 02S information, refer to Monitored Sys-
tems in Group 25, Emission Control Systems.
UPSTREAM HEATED OXYGEN SENSOR
The upstream 02S sensor is located in the exhaust
downpipe (Fig. 5). It provides an input voltage to the
FUEL SYSTEM
14- 23
PCM . The input tells the PCM the oxygen content of
the exhaust gas. The PCM uses this information to
fine tune the air/fuel ratio by adjusting injector pulse
width.
DOWNSTREAM
OXYGEN SENSOR
Fig. 5 Upstream Oxygen Sensor
CATALYTIC
CONVERTER
Fig. 6 Downstream Oxygen Sensor
UPSTREAM
OXYGEN
SENSOR
80a410c5
80a410c4
DOWNSTREAM HEATED OXYGEN SENSOR
The downstream heated oxygen sensor is located
near the outlet end of the catalytic convertor (Fig. 6).
The downstream heated oxygen sensor input is used
to detect catalytic convertor deterioration and pro-
vide fuel adjustment information .
As the convertor deteriorates, the input from the
downstream sensor begins to match the upstream
sensor input except for a slight time delay. By com-
paring the downstream heated oxygen sensor input
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