RAM 2500 Van V8-5.9L VIN Z LDC (2000)
Oxygen Sensor: Description and Operation
OXYGEN SENSOR (O2S) - PCM INPUT
The Oxygen Sensors (O2S) are attached to, and protrude into the vehicle exhaust system. Depending on the emission package, the vehicle may use
a total of either 1 or 2 sensor(s).
For these engines: 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.
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 wire harness. This is why it is important to never solder an 02 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 heating element(s) within the sensor(s) will be supplied voltage from the ASD relay.
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 4.5 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 °F - 1100 °F (500 °C -
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
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
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).
The downstream oxygen sensor also provides an input to determine catalytic convertor efficiency.
