Chrysler neon 1997 Service Manual page 1316

ENGINE TEMPERATURE
PL
DESCRIPTION AND OPERATION (Continued)
SOHC
The coolant sensor threads into the rear of the cyl-
inder head, next to the camshaft position sensor (Fig.
9). New sensors have sealant applied to the threads.
Fig. 9 Engine Coolant Temperature Sensor—SOHC
DOHC
The coolant sensor threads into the intake mani-
fold next to the thermostat housing (Fig. 10). New
sensors have sealant applied to the threads.
Fig. 10 Engine Coolant Temperature Sensor—DOHC
FUEL LEVEL SENSOR—PCM INPUT
The fuel level sensor (fuel gauge sending unit)
sends a signal to the PCM to indicate fuel level. The
purpose of this feature is to prevent a false setting of
misfire and fuel system monitor trouble codes if the
fuel level is less than approximately 15 percent of its
rated capacity. It is also used to send a signal for fuel
gauge operation via the CCD bus circuits.
HEATED OXYGEN SENSOR (O2S SENSOR)—PCM
INPUT
As vehicles accumulate mileage, the catalytic con-
vertor deteriorates. The deterioration results in a
POSITION CAMSHAFT COOLANT SENSOR SENSOR COOLANT TEMPER- ATURE SENSOR
FUEL RAIL
less efficient catalyst. To monitor catalytic convertor
deterioration, the fuel injection system uses two
heated oxygen sensors. One sensor upstream of the
catalytic convertor, one downstream of the convertor.
The PCM compares the reading from the sensors to
calculate the catalytic convertor oxygen storage
capacity and converter efficiency. Also, the PCM uses
the upstream heated oxygen sensor input when
adjusting injector pulse width.
When the catalytic converter efficiency drops below
emission standards, the PCM stores a diagnostic
trouble code and illuminates the malfunction indica-
tor lamp (MIL).
The O2S 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) it pro-
duces a higher voltage. By monitoring the oxygen
content and converting it 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, it allows the system to remain in closed loop
operation during periods of extended idle.
In Closed Loop operation the PCM monitors the
O2S sensor input (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.
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 OXYGEN SENSOR
The input from the upstream heated oxygen sensor
tells the PCM the oxygen content of the exhaust gas.
Based on this input, the PCM fine tunes the air-fuel
ratio by adjusting injector pulse width.
The sensor input switches 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 sensor
produces voltage as low as 0.1 volt. When there is a
lesser amount of oxygen present (rich air-fuel mixture)
the sensor produces a voltage as high as 1.0 volt. By
monitoring the oxygen content and converting it to elec-
trical voltage, the sensor acts as a rich-lean switch.
FUEL SYSTEM 14 - 29