Jeep Cherokee 2000, Service Manual

Page: 859 / 1528

nia emission package, the heaters on both sensors
are fed battery voltage from the two O2S heater
relays. The O2S relays are also controlled by the
PCM. On the California emission package, the heat-
ers on all 4 sensors are fed battery voltage from the
two O2S Heater Relays.
The O2 sensor uses a Positive Thermal Co-efficient
(PTC) heater element. As temperature increases,
resistance
increases. At ambient temperatures
around 70°F, the resistance of the heating element is
approximately 6 ohms. As the sensor’s temperature
increases, resistance in the heater element increases.
This allows the heater to maintain the optimum
operating temperature of approximately 930°-1100°F
(500°-600° C). Although the sensors operate the
same, there are physical differences, due to the envi-
ronment that they operate in, that keep them from
being interchangeable.
Maintaining correct sensor temperature at all
times allows the system to enter into closed loop
operation sooner. Also, it allows the system to remain
in closed loop operation during periods of extended
idle.
In Closed Loop operation, the PCM monitors cer-
tain O2 sensor input(s) along with other inputs, and
adjusts the injector pulse width accordingly. During
Open Loop operation, the PCM ignores the O2 sensor
input. The PCM adjusts injector pulse width based
on preprogrammed (fixed) values and inputs from
other sensors.
Upstream Sensor (Non-California Emissions):
The upstream O2S sensor (1/1 sensor) is located in
the exhaust downpipe before the catalytic convertor.
It provides an input voltage to the PCM. The input
tells the PCM the oxygen content of the exhaust gas.
The PCM uses this information to fine tune fuel
delivery to maintain the correct oxygen content at
the downstream oxygen sensor. The PCM will change
the air/fuel ratio until the upstream sensor inputs a
voltage that the PCM has determined will make the
downstream sensor output (oxygen content) correct.
The upstream oxygen sensor also provides an input
to determine catalyst efficiency.
Downstream Sensor (Non-California Emis-
sions): The downstream heated oxygen sensor (1/2
sensor) is located near the outlet end of the catalytic
convertor. The downstream sensor is also used to
determine the correct air fuel ratio. As the oxygen
content changes at the downstream the PCM calcu-
lates how much air fuel ratio change is required. The
PCM then looks at the upstream oxygen sensor volt-
age and changes fuel delivery until the upstream
sensor voltage changes enough to correct the down-
stream sensor voltage (oxygen content).
The downstream oxygen sensor also provides an
input to determine catalyst efficiency.
Upstream Sensors (California Emissions): Two
upstream sensors are used (1/1 and 2/1). The 1/1 sen-
sor is the first sensor to receive exhaust gases from
the #1 cylinder. Both of the upstream O2S sensors
are located in the exhaust manifold just before the
mini-catalytic convertors. They provide an input volt-
age to the PCM. The input tells the PCM the oxygen
content of the exhaust gas. The PCM uses this infor-
mation to fine tune fuel delivery to maintain the cor-
rect oxygen content at the downstream oxygen
sensors. The PCM will change the air/fuel ratio until
the upstream sensors input a voltage that the PCM
has determined will make the downstream sensors
output (oxygen content) correct.
The upstream oxygen sensors also provide an input
to determine mini-catalyst efficiency.
Downstream Sensors (California Emissions):
Two downstream sensors are used (1/2 and 2/2). The
downstream sensors are located in the exhaust down-
pipes just after the mini-catalytic convertors. The
downstream is also used to determine the correct air
fuel ratio. As the oxygen content changes at the
downstream the PCM calculates how much air fuel
ratio change is required. The PCM then looks at the
upstream oxygen sensor voltage and changes fuel
delivery until the upstream sensor voltage changes
enough to correct the downstream sensor voltage
(oxygen content).
The downstream oxygen sensors also provide an
input to determine mini-catalyst efficiency.
IGNITION CIRCUIT SENSE—PCM INPUT
DESCRIPTION
This circuit ties the ignition switch to the Power-
train Control Module (PCM).
OPERATION
The ignition circuit sense input tells the PCM the
ignition switch has energized the ignition circuit.
Battery voltage is also supplied to the PCM
through the ignition switch when the ignition is in
the RUN or START position. This is referred to as
the “ignition sense” circuit and is used to “wake up”
the PCM. Voltage on the ignition input can be as low
as 6 volts and the PCM will still function. Voltage is
supplied to this circuit to power the PCM’s 8-volt reg-
ulator and to allow the PCM to perform fuel, ignition
and emissions control functions. The battery voltage
on this line is supplied to the 8-volt regulator which
then passes on a power-up supply to the 5-volt regu-
lator.
XJ
FUEL SYSTEM 14 - 31
DESCRIPTION AND OPERATION (Continued)