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S R
FUEL SYSTEM
14 - 23
D E S C R I P T I O N A N D O P E R A T I O N ( C o n t i n u e d )
Fig. 5 Crankshaft
Position
Sensor
Location
ENGINE COOLANT TEMPERATURE SENSOR—PCM
INPUT
The engine coolant temperature
sensor threads
into the front of the driver side cylinder head (Fig. 6 ) .
The sensor provides an i n p u t to the P C M . As coolant
temperature varies, the sensor resistance changes,
resulting i n a different i n p u t voltage to the P C M .
U n t i l the engine reaches operating
temperature,
the P C M demands slightly richer air-fuel mixtures
and higher idle speeds.
This sensor is also used for cooling fan control and
A/C cutoff at h i g h coolant temperatures.
This 3 - w a y sensor is also used for the water tem-
perature gauge and dashboard w a r n i n g light.
Fig,
6 Engine
Coolant
Temperature
Sensor
FUEL LEVEL SENSOR—PCM INPUT
The fuel level sensor (fuel , gauge sending u n i t )
sends a signal to the P C M to indicate fuel level. The
purpose of this feature is to prevent a false setting of
misfire and fuel system monitor trouble codes i f the
fuel level is less t h a n approximately 15 percent of its
rated capacity. I t is also used to send a signal for fuel
gauge operation via the P C I bus circuits.
HEATED OXYGEN SENSOR (02S SENSOR)—PCM
INPUT
As vehicles accumulate mileage, the catalytic con-
verter deteriorates.
The deterioration results i n a
less efficient catalyst. To monitor catalytic convertor
deterioration, the fuel injection system
uses two
heated oxygen sensors i n each side of the exhaust
system (4 total). One sensor upstream of the catalytic
convertor, one downstream
of the convertor.
The
PCM compares the reading from the sensors to cal-
culate the catalytic convertor oxygen storage capacity
and converter efficiency. Also, the P C M uses the
upstream heated oxygen sensor i n p u t when adjusting
injector pulse w i d t h .
When the catalytic converter efficiency drops below
emission standards, the P C M stores a
diagnostic
trouble code and illuminates the Malfunction Indica-
tor L a m p ( M I L ) .
The 0 2 sensors produce voltages from 0 to 1 volt,
depending upon the oxygen content of the exhaust
gas i n 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 pro-
duces 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 w i t h a heating
element t h a t keeps the sensors at proper operating
temperature d u r i n g a l l operating modes. M a i n t a i n i n g
correct sensor temperature
at a l l times allows the
system to enter into closed loop operation sooner.
Also, i t allows the system to remain i n closed loop
operation d u r i n g periods of extended idle.
I n Closed Loop operation the P C M monitors the 0 2
sensor i n p u t (along w i t h other inputs) and adjusts
the injector pulse w i d t h accordingly. D u r i n g Open
Loop operation the P C M ignores the 0 2 sensor input.
The P C M adjusts injector pulse w i d t h based on pre-
programmed (fixed) values and inputs from
other
sensors.
The Automatic Shutdown (ASD) relay supplies bat-
tery voltage to a l l upstream and downstream heated
oxygen sensors. The oxygen sensors are
equipped
w i t h a heating element. The heating elements reduce
the t i m e required for the sensors to reach operating
temperature.
The i n p u t from the upstream heated oxygen sensor
tells the P C M the oxygen content of the exhaust gas.
Based on this input, the P C M fine tunes the air-fuel
ratio by adjusting injector pulse w i d t h .
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Troubleshooting
