RAM 3500 Truck 4WD V8-5.9L VIN Z LDC (2000)
Oxygen Sensor: Description and Operation
OXYGEN SENSOR (O2S) - PCM INPUT
DESCRIPTION
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 2 or 4 sensors.
Certain Non-California 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.
Certain California Engines
Four 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.
OPERATION
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 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
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.
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 (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).
The downstream oxygen sensor also provides an input to determine catalytic convertor efficiency.
Upstream Sensors (California Emissions)
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. They provide an
