Dodge ram truck 3500 1996 Service Manual page 1266

1 4 - 3 2 FUEL SYSTEM BR
DESCRIPTION AND OPERATION.
(Continued)
Fig. 7 Engine Coolant Temperature Sensor—3.9U
5.2U5.9L Engines—Typical
coolant temperature. The PCM uses this input along
with inputs from other sensors to determine injector
pulse width and ignition timing. As coolant tempera­
ture varies, the coolant temperature sensor resis­
tance will change. This change in resistance results
in a different input voltage to the PCM.
When the engine is cold, the PCM will operate in
Open Loop cycle. I t will demand slightly richer air-
fuel mixtures and higher idle speeds. This is done
until normal operating temperatures are reached.
Fig. 8 Engine Coolant Temperature Sensor—8.0L
Engine—Typical
HEATED OXYGEN SENSOR (02S)—3.9L/5.2L/5.9L
LDC ENGINES—PCM INPUT
Two heated 02S sensors are used (upstream and
downstream). The sensors produce voltages from 0 to
1 volt, depending upon the oxygen content of the
exhaust gas in the exhaust manifold. When a large
amount of oxygen is present (caused by a lean air/
fuel mixture), the sensors produces a low voltage.
When there is a lesser amount present (rich air/fuel
mixture) i t produces a higher voltage. By monitoring
the oxygen content and converting i t to electrical
voltage, the sensors act as a rich-lean switch.
The oxygen sensors are equipped with a heating
element that keeps the sensors at proper operating
temperature during all operating modes. Maintaining
correct sensor temperature at all times allows the
system to enter into closed loop operation sooner.
Also, i t allows the system to remain in closed loop
operation during periods of extended idle.
In Closed Loop operation, the PCM monitors the
02S sensor input (along with other inputs) and
adjusts the injector pulse width accordingly. During
Open Loop operation, the PCM ignores the 02 sensor
input. The PCM adjusts injector pulse width based
on preprogrammed (fixed) values and inputs from
other sensors.
The Automatic Shutdown (ASD) relay supplies bat­
tery voltage to both the upstream and downstream
heated oxygen sensors. The oxygen sensors are
equipped with a heating element. The heating ele­
ments reduce the time required for the sensors to
reach operating temperature.
UPSTREAM HEATED OXYGEN SENSOR
The upstream 02S sensor is located i n the exhaust
downpipe (Fig. 9). I t 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 sen­
sor 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 catalyst efficiency. Refer to Group 25
Emissions, On-Board Diagnostics, Catalyst Monitor
for more information.
DOWNSTREAM HEATED OXYGEN SENSOR
The downstream heated oxygen sensor is located
near the outlet end of the catalytic convertor (Fig. 9).
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 sensor also provides an
input to determine catalyst efficiency. Refer to Group
25 Emissions Control Systems, On-Board Diagnos­
tics, Catalyst Monitor for more information.