Cherokee 4WD LHD L6-4.0L VIN S (2001)
Oxygen Sensor: Description and Operation
OXYGEN SENSOR
The Oxygen Sensors (O2S) are attached to, and protrude into the vehicle exhaust system. Four different sensors are used: 2 upstream (referred to
as 1/1 and 2/1) and 2 downstream (referred to as 1/2 and 2/2).
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.
An 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 wire harness. This is why it is important to never solder an O2 sensor connector, or pack the connector with
grease.
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
The O2S relays are controlled by the PCM on all emission packages. The heaters 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 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.
The downstream oxygen sensor also provides an input to determine catalyst efficiency.
Upstream Sensors
Two upstream sensors are used (1/1 and 2/1). The 1/1 sensor 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 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 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
Two downstream sensors are used (1/2 and 2/2). The downstream sensors are located in the exhaust downpipes 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.