Grand Cherokee 2WD V8-4.7L VIN J (2004)
Oxygen Sensor: Description and Operation
The Oxygen Sensors (02S) are attached to, and protrude into the vehicle exhaust system. Depending on the emission package, the vehicle may use a total
of either 2 or 4 sensors.
Federal Emissions Package: Two sensors are used: upstream (referred to as 1/1) and downstream (referred to as 1/2). With this emission package, the
upstream sensor (1/1) is located just before the main catalytic convertor. The downstream sensor (1/2) is located just after the main catalytic convertor.
4.7L V-8 With California Emissions Package:
On this emissions package, 4 sensors are used: 2 upstream (referred to as 1/1 and 2/1) and 2 downstream (referred to as 1/2 and 2/2). With this emission
package, the right upstream sensor (2/1) is located in the right exhaust downpipe just before the mini-catalytic convertor. The left upstream sensor (1/1)
is located in the left exhaust downpipe just before the mini-catalytic convertor. The right downstream sensor (2/2) is located in the right exhaust
downpipe just after the mini-catalytic convertor, and before the main catalytic convertor. The left downstream sensor (1/2) is located in the left exhaust
downpipe just after the mini-catalytic convertor, and before the main catalytic convertor.
4.0L 6-Cylinder With California Emissions Package: On this emissions package, 4 sensors are used: 2 upstream (referred to as 1/1 and 2/1) and 2
downstream (referred to as 1/2 and 2/2). With this emission package, the rear/upper upstream sensor (2/1) is located in the exhaust downpipe just before
the rear mini-catalytic convertor. The front/upper upstream sensor (1/1) is located in the exhaust down- pipe just before the front mini-catalytic
convertor. The rear/lower downstream sensor (2/2) is located in the exhaust downpipe just after the rear mini-catalytic convertor, and before the main
catalytic convertor. The front/lower downstream sensor (1/2) is located in the exhaust downpipe just after the front mini-catalytic convertor, and before
the main catalytic convertor.
An O2 sensor is a galvanic battery that provides the PCM with a voltage signal (0-1 volt) inversely proportional to the amount of oxygen in the exhaust.
In other words, if the oxygen content is low, the voltage output is high if the oxygen content is high the
output voltage is low. The PCM uses this information to adjust injector pulse-width to achieve the 14.7-to-1 air/fuel ratio necessary for proper engine
operation and to control emissions.
The O2 sensor must have a source of oxygen from outside of the exhaust stream for comparison. Current O2 sensors receive their fresh oxygen (outside
air) supply through the O2 sensor case housing.
Four wires (circuits) are used on each O2 sensor: a 12-volt feed circuit for the sensor heating element; a ground circuit for the heater element; a
low-noise sensor return circuit to the PCM, and an input circuit from the sensor back to the PCM to detect sensor operation.
Oxygen Sensor Heaters/Heater Relays:
Depending on the emissions package, the heating elements within the sensors will be supplied voltage from either the ASD relay, or 2 separate oxygen
sensor relays. Refer to Wiring Diagrams to determine which relays are used.
The O2 sensor uses a Positive Thermal Co-efficient (PTC) heater element. As temperature increases, resistance increases. At ambient temperatures
around 700F, the resistance of the heating element is approximately 4.5 ohms on 4.0L engines. It is approximately 13.5 ohmson the 4.7L engine. 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 environment 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 certain 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 sensor (1/1) 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 catalytic convertor efficiency.
Downstream Sensor (Non-California Emissions):
The downstream oxygen sensor (1/2) is also used to determine the correct air-fuel ratio. As the oxygen content changes at the downstream sensor, 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).