Dodge dakota 2002 Service Manual page 1740

DR
OXYGEN SENSOR
DESCRIPTION
The Oxygen Sensors (O2S) are attached to, and
protrude into the vehicle exhaust system. Depending
on the engine or emission package, the vehicle may
use a total of either 2 or 4 sensors.
Federal Emission Packages : 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.
California Emission Packages: On this emis-
sions 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 pack-
age, 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-cata-
lytic convertor. The right downstream sensor (2/2) is
located in the right exhaust downpipe just after the
mini-catalytic convertor, and before the main cata-
lytic convertor. The left downstream sensor (1/2) is
located in the left exhaust downpipe just after the
mini-catalytic convertor, and before the main cata-
lytic 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 volt-
age output is high; if the oxygen content is high the
output voltage is low. The PCM uses this information
to adjust injector
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. Cur-
rent 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 cir-
cuit from the sensor back to the PCM to detect sen-
sor operation.
Oxygen Sensor Heater Relay - 5.9L/8.0L: If 4
oxygen sensors are used, a separate heater relay is
used to supply voltage to the sensors heating ele-
ments for only the 1/2 and 2/2 downstream sensors.
Voltage for the other 2 sensor heating elements is
supplied directly from the Powertrain Control Mod-
pulse-width to achieve
ule (PCM) through a Pulse Width Module (PWM)
method.
Pulse Width Module (PWM) - 5.9L/8.0L: Voltage
to the O2 sensor heating elements is supplied
directly from the Powertrain Control Module (PCM)
through two separate Pulse Width Module (PWM)
low side drivers. PWM is used on both the upstream
and downstream O2 sensors if equipped with a Fed-
eral Emissions Package, and only on the 2 upstream
sensors (1/1 and 2/1) if equipped with a California
Emissions Package. The main objective for a PWM
driver is to avoid overheating of the O2 sensor heater
element. With exhaust temperatures increasing with
time and engine speed, it's not required to have a
full-voltage duty-cycle on the O2 heater elements.
To avoid the large simultaneous current surge
needed to operate all 4 sensors, power is delayed to
the 2 downstream heater elements by the PCM for
approximately 2 seconds.
Oxygen Sensor Heater Elements:
The O2 sensor uses a Positive Thermal Co-efficient
(PTC) heater element. As temperature increases,
resistance increases.
around 70°F, the resistance of the heating element is
approximately 13 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 envi-
ronment 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.
the
In Closed Loop operation, the PCM monitors cer-
tain 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 - Federal Emissions Pack-
age : The upstream sensor (1/1) 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 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.
FUEL INJECTION - GAS
At ambient
14 - 43
temperatures