Dodge Dakota Sport 2001 Service Manual page 1374

AN
O2 HEATER RELAY
DESCRIPTION
The oxygen (O2) sensor heater relay is located in
the Powertrain Distribution Center (PDC).
OPERATION
Refer to Oxygen Sensor for oxygen sensor relay
information.
REMOVAL
The oxygen sensor heater relay is located in the
Power Distribution Center (PDC) (Fig. 33). Refer to
label on PDC cover for relay location.
(1) Remove PDC cover.
(2) Remove relay from PDC.
(3) Check condition of relay terminals and PDC
connector terminals for damage or corrosion. Repair
if necessary before installing relay.
(4) Check for pin height (pin height should be the
same for all terminals within the PDC connector).
Repair if necessary before installing relay.
INSTALLATION
The oxygen sensor heater relay is located in the
Power Distribution Center (PDC) (Fig. 33). Refer to
label on PDC cover for relay location.
(1) Install relay to PDC.
(2) Install cover to PDC.
O2 SENSOR
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.
NAA 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.
NAS or NAE Emissions Package: 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 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 cir-
cuit from the sensor back to the PCM to detect sen-
sor operation.
Oxygen Sensor Heater Relay: If the vehicle is
equipped with 4 oxygen sensors, a separate oxygen
sensor relay is used to supply voltage to the sensor
heating elements. This particular relay is used only
for the 1/2 and 2/2 downstream sensors. Voltage for
the other 2 sensor heating elements is supplied
directly from the ASD relay. Refer to 8, Wiring Dia-
grams to determine which relay is used.
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 4.5 ohms on 2.5/3.9/5.2 and 5.9L
engines. It is approximately 13.5 ohms on the 4.7L
engine. As the sensor's temperature increases, resis-
tance in the heater element increases. This allows
the heater to maintain the optimum operating tem-
perature 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 inter-
changeable.
Maintaining correct sensor temperature at all
times allows the system to enter into closed loop
operation sooner. Also, it allows the system to remain
FUEL INJECTION
pulse-width to achieve
At ambient temperatures
14 - 47
the