Chrysler PT Cruiser 2001 Service Manual Supplement page 1116

PT
DESCRIPTION AND OPERATION (Continued)
As coolant temperature varies, the coolant tempera-
ture sensor resistance changes resulting in a differ-
ent voltage value at the PCM engine coolant sense
circuit.
When the engine is cold, the PCM will provide
slightly richer air-fuel mixtures and higher idle
speeds until normal operating temperatures are
reached.
The combination coolant temperature sensor has
two elements. One element supplies coolant temper-
ature signal to the PCM. The other element supplies
coolant temperature signal to the instrument panel
gauge cluster. The PCM determines engine coolant
temperature from the coolant temperature sensor.
As coolant temperature varies the coolant temper-
ature sensors resistance changes resulting in a differ-
ent input voltage to the PCM and the instrument
panel gauge cluster.
When the engine is cold, the PCM will provide
slightly richer air- fuel mixtures and higher idle
speeds until normal operating temperatures are
reached.
The PCM has a dual temperature range program
for better sensor accuracy at cold temperatures. At
key-ON the PCM sends a regulated five volt signal
through a 10,000 ohm resistor to the sensor. When
the sensed voltage reaches approximately 1.25 volts
the PCM turns on the transistor. The transistor con-
nects a 1,000 ohm resistor in parallel with the 10,000
ohm resistor. With this drop in resistance the PCM
recognizes an increase in voltage on the input circuit.
HEATED OXYGEN SENSORS—PCM INPUT
DESCRIPTION
The upstream oxygen sensor threads into the out-
let flange of the exhaust manifold (Fig. 4). The down-
stream oxygen sensor threads into the side of the
catalytic converter.
OPERATION
The O2 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 produce a voltage below.45 volts. When
there is a lesser amount present (rich air/fuel mix-
ture) it produces a voltage above.45 volts. By moni-
toring 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.
Fig. 4 Oxygen Sensor 1/1 Upstream and 1/2
Downstream
1 – DOWN STREAM O2 SENSOR
2 – UP STREAM O2 SENSOR
Also, it allows the system to remain in closed loop
operation during periods of extended idle. Upstream
O2s (California emission equipped) are Pulse Width
Modulated (PWM). They have a start-up delay to
remove moisture from the heater element on cold
start. This start-up delay may cause the DRB O2
Heater actuator test to be denied. The ground circuit
is controlled by the PCM. This allows the PCM to
duty cycle the heating element for the upstream O2s
In Closed Loop operation the PCM monitors the O2
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 pre-
programmed (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 1/1
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, depend-
ing upon the oxygen content of the exhaust gas in
the exhaust manifold. When a large amount of oxy-
gen is present (caused by a lean air-fuel mixture), the
FUEL SYSTEM 14 - 29