Jeep 1996 Grand Cherokee Service Manual - page 1077
When the engine is cold, the PCM will operate in
Open Loop cycle. It will demand slightly richer air-
fuel mixtures and higher idle speeds. This is done
until normal operating temperatures are reached.
Refer to Open Loop/Closed Loop Modes of Opera-
tion in this section of the group for more information.
OXYGEN SENSOR (O2S)—PCM INPUT
Two heated O2S sensors are used. The sensors pro-
duce 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 sensors pro-
duces 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 con-
verting it to electrical voltage, the sensors act as a
rich-lean switch.
The oxygen sensors are equipped with a heating
element that keeps the sensors at proper operating
temperature during all operating modes. 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 the
O2S sensor input (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.
The Automatic Shutdown (ASD) relay supplies bat-
tery voltage to both the upstream and downstream
heated oxygen sensors. The oxygen sensors are
equipped with a heating element. The heating ele-
ments reduce the time required for the sensors to
reach operating temperature.
UPSTREAM HEATED OXYGEN SENSOR
The upstream O2S sensor is located near the inlet
end of the catalytic converter. It provides an input
voltage to the PCM. The input tells the PCM the oxy-
gen content of the exhaust gas. The PCM uses this
information to fine tune the air/fuel ratio by adjust-
ing injector pulse width.
DOWNSTREAM HEATED OXYGEN SENSOR
The downstream heated oxygen sensor is located
near the outlet end of the catalytic converter. The
downstream heated oxygen sensor input is used to
detect catalytic convertor deterioration. As the con-
vertor deteriorates, the input from the downstream
sensor begins to match the upstream sensor input
except for a slight time delay. By comparing the
downstream heated oxygen sensor input to the input
from the upstream sensor, the PCM calculates cata-
lytic convertor efficiency.
When the catalytic converter efficiency drops below
emission standards, the PCM stores a diagnostic
trouble code and illuminates the Malfunction Indica-
tor Lamp (MIL). For more information, refer to
Group 25, Emission Control Systems.
IGNITION CIRCUIT SENSE—PCM INPUT
The ignition circuit sense input tells the Power-
train Control Module (PCM) the ignition switch has
energized the ignition circuit. Refer to the wiring dia-
grams for circuit information.
INTAKE MANIFOLD AIR TEMPERATURE SENSOR—
5.2L ENGINES—PCM INPUT
The intake manifold air temperature sensor is
installed in the intake manifold with the sensor ele-
ment extending into the air stream (Fig. 8). The sen-
sor provides an input voltage to the powertrain
control module (PCM) indicating intake manifold air
temperature. The input is used along with inputs
from other sensors to determine injector pulse width.
As the temperature of the air-fuel stream in the
manifold varies, the sensor resistance changes. This
results in a different input voltage to the PCM.
INTAKE MANIFOLD AIR TEMPERATURE SENSOR—
4.0L ENGINE—PCM INPUT
The intake manifold air temperature sensor is
installed in the intake manifold with the sensor ele-
ment extending into the air stream (Fig. 9). The sen-
sor provides an input voltage to the powertrain
control module (PCM) indicating intake manifold air
temperature. The input is used along with inputs
from other sensors to determine injector pulse width.
As the temperature of the air-fuel stream in the
manifold varies, the sensor resistance changes. This
results in a different input voltage to the PCM.
Fig. 7 Engine Coolant Temperature Sensor—4.0L
Engine—Typical
14 - 32
FUEL SYSTEM
ZJ
DESCRIPTION AND OPERATION (Continued)