light DODGE NEON 1999 Service Repair Manual

INSTALLATION
Before installing manifolds. Clean all mating sur-
faces. Replace all gaskets, with new.
(1) Assemble lower manifold to upper (if sepa-
rated) and tighten bolts in sequence shown in (Fig.
29) to 28 N´m (250 in. lbs.).
(2) Install intake manifold onto cylinder head and
tighten fasteners to 28 N´m (250 in. lbs.) in sequence
shown in (Fig. 30).
(3) Remove covering from fuel injector holes and
insure the holes are clean. Install fuel rail assembly
to intake manifold. Tighten screws to 23 N´m (200 in.
lbs.).
(4) Connect PCV and brake booster hoses.
(5) Inspect quick connect fittings for damage,
replace if necessary Refer to Group 14, Fuel System
for procedure. Apply a light amount of clean engine
oil to fuel inlet tube. Connect fuel supply hose to fuel
rail assembly. Check connection by pulling on connec-
tor to insure it locked into position.
(6) Connect heater tube and hose to intake mani-
fold.
(7) Connect upper radiator hose and coolant recov-
ery hose.
(8) Connect coolant temperature sensor wiring con-
nector.
(9) Install throttle body. Tighten fastener to 22
N´m (200 in. lbs.).
(10) Connect Manifold Absolute Pressure/Intake
Air Temperature Sensor (TMAP) wiring connector.(11) Connect knock sensor connector and starter
wires. Connect wiring harness to heater tube tab.
(12) Connect Idle Air Control (IAC) motor and
Throttle Position Sensor (TPS) wiring connectors.
(13) Connect vacuum hoses to throttle body.
(14) Install accelerator, kickdown and speed con-
trol cables to their bracket and connect them to the
throttle lever. Refer to Group 14, Fuel System Throt-
tle Body Installation for procedure.
(15) Loose assemble the EGR tube onto valve and
intake manifold finger tight. Tighten tube fasteners
at the EGR valve first to 11 N´m (95 in. lbs.) then,
tighten the intake manifold side fasteners to 11 N´m
(95 in. lbs.).
(16) Install fresh air duct to air filter housing.
Tighten clamp to 3 N´m (25 in. lbs.).
(17) Connect negative cable to battery.
(18) With the DRB scan tool use ASD Fuel System
Test to pressurize system to check for leaks.
CAUTION: When using the ASD Fuel System Test,
the Auto Shutdown (ASD) relay will remain ener-
gized for 7 minutes or until the ignition switch is
turned to the OFF position, or Stop All Test is
selected.
EXHAUST MANIFOLD
REMOVAL
(1) Remove air cleaner assembly and bracket.
(2) Remove exhaust manifold heat shield (Fig. 31).
(3) Disconnect upstream heated oxygen sensor con-
nector.
Fig. 29 Lower Intake Manifold to Upper Tightening
Sequence
Fig. 30 Intake Manifold To Cylinder Head Tightening
Sequence
Fig. 31 Exhaust Manifold Heat Shield
PLEXHAUST SYSTEM AND INTAKE MANIFOLD 11 - 11
REMOVAL AND INSTALLATION (Continued)

FUEL SYSTEM
CONTENTS
page page
FUEL DELIVERY SYSTEM................... 3
FUEL INJECTION SYSTEM................. 20GENERAL INFORMATION................... 1
GENERAL INFORMATION
INDEX
page page
GENERAL INFORMATION
FUEL REQUIREMENTS.................... 1
GASOLINE/OXYGENATE BLENDS............ 1INTRODUCTION......................... 1
PCM REPLACEMENT..................... 1
GENERAL INFORMATION
INTRODUCTION
Throughout this group, references may be made to
a particular vehicle by letter or number designation.
A chart showing the breakdown of these designations
is included in the Introduction Section at the front of
this service manual.
The Evaporation Control System, is also considered
part of the fuel system. The system reduces the emis-
sion of fuel vapor into the atmosphere.
The description and function of the Evaporation
Control System is found in Group 25 of this manual.
PCM REPLACEMENT
USE THE DRB SCAN TOOL TO REPROGRAM
THE NEW PCM WITH THE VEHICLES ORIGI-
NAL IDENTIFICATION NUMBER (VIN) AND
THE VEHICLES ORIGINAL MILEAGE. IF THIS
STEP IS NOT DONE A DIAGNOSTIC TROUBLE
CODE (DTC) MAY BE SET.
FUEL REQUIREMENTS
Your vehicle was designed to meet all emission reg-
ulations and provide excellent fuel economy when
using high quality unleaded gasoline.
Use unleaded gasolines having a minimum posted
octane of 87.
If your vehicle develops occasional light spark
knock (ping) at low engine speeds this is not harm-
ful. However; continued heavy knock at high speeds
can cause damage and should be reported to yourdealer immediately. Engine damage as a result of
heavy knock operation may not be covered by the
new vehicle warranty.
In addition to using unleaded gasoline with the
proper octane rating, those that contain detergents,
corrosion and stability additives are recommended.
Using gasolines that have these additives will help
improve fuel economy, reduce emissions, and main-
tain vehicle performance.
Poor quality gasoline can cause problems such as
hard starting, stalling, and stumble. If you experi-
ence these problems, try another brand of gasoline
before considering service for the vehicle.
GASOLINE/OXYGENATE BLENDS
Some fuel suppliers blend unleaded gasoline with
materials that contain oxygen such as alcohol, MTBE
(Methyl Tertiary Butyl Ether) and ETBE (Ethyl Ter-
tiary Butyl Ether). Oxygenates are required in some
areas of the country during winter months to reduce
carbon monoxide emissions. The type and amount of
oxygenate used in the blend is important.
The following are generally used in gasoline
blends:
Ethanol- (Ethyl or Grain Alcohol) properly
blended, is used as a mixture of 10 percent ethanol
and 90 percent gasoline. Gasoline blended with etha-
nol may be used in your vehicle.
MTBE/ETBE- Gasoline and MTBE (Methyl Ter-
tiary Butyl Ether) blends are a mixture of unleaded
gasoline and up to 15 percent MTBE. Gasoline and
ETBE (Ethyl Tertiary Butyl Ether) are blends of gas-
PLFUEL SYSTEM 14 - 1

WARNING: THE FUEL RESERVOIR OF THE FUEL
PUMP MODULE DOES NOT EMPTY OUT WHEN THE
TANK IS DRAINED. THE FUEL IN THE RESERVOIR
WILL SPILL OUT WHEN THE MODULE IS
REMOVED.
(2) Disconnect fuel line from fuel pump module by
depressing quick connect retainers with thumb and
fore finger.
(3) Slide fuel pump module electrical connecter
lock to unlock.
(4) Disconnect the electrical connection from the
fuel pump module, by pushing down on connector
retainer and pulling connector off of module.
(5) Use a transmission jack to support the fuel
tank. remove bolts from fuel tank straps.
(6) Lower tank slightly.
(7) Use Special Tool 6856 to remove fuel pump
module locknut (Fig. 17).
(8) Remove fuel pump and O-ring seal from tank.
Discard old seal.
INSTALLATION
(1) Wipe seal area of tank clean and place a new
seal in position in the tank opening.
(2) Position fuel pump in the tank. Make sure the
aligment tab on the underside of the fuel pump mod-
ule flange sits in the notch on the fuel tank.
(3)
Position the locknut over the fuel pump module.
(4) Tighten the locknut using Special Tool 6856 to
55 N´m (40.5 ft. lbs.) (Fig. 17).
CAUTION: Over tightening the pump lock ring may
result in a leak.
(5) Fill fuel tank. Check for leaks.
FUEL FILTER / PRESSURE REGULATOR
REMOVAL
WARNING: THE FUEL SYSTEM IS UNDER A CON-
STANT PRESSURE, EVEN WITH ENGINE OFF. BEFORE
SERVICING THE FUEL FILTER/FUEL PRESSURE REG-
ULATOR, THE FUEL SYSTEM PRESSURE MUST BE
RELEASED.
(1) Refer to Fuel System Pressure Release in the
Fuel Delivery System section of this group.
The fuel filter/fuel pressure regulator is located on
the top of fuel pump module. Fuel pump module
removal is not necessary.
(2) Raise vehicle on hoist.
(3) Disconnect fuel supply line at the Filter/Regu-
lator nipple (refer to Quick Connect Fittings in this
section).
(4) Depress locking spring tab on side of Fuel/Reg-
ulator (Fig. 18) and rotate 90É counter-clockwise and
pull out.
NOTE: Make sure that the upper and lower O-rings
are on the Filter/Regulator assembly.
INSTALLATION
Lightly lubricate the O-rings with engine oil.
(1) Insert Filter/Regulator into the opening in the
fuel pump module, align the two hold down tabs with
the flange.
(2) While applying downward pressure, rotate the
Filter/Regulator clockwise until the the spring tab
engages the locating slot (Fig. 19).
(3) Connect the fuel line to the Filter/Regulator.
(4) Lower vehicle from hoist.
Fig. 16 Fuel Pump Module Removal
Fig. 17 Fuel Tank Locknut
14 - 12 FUEL SYSTEMPL
REMOVAL AND INSTALLATION (Continued)

FUEL PUMP INLET STRAINER
REMOVAL
(1) Remove fuel pump module. Refer to Fuel Pump
Module Removal in this section.
(2) Using a thin straight blade screwdriver, pry
back the locking tabs on fuel pump reservoir and
remove the strainer (Fig. 20).
(3) Remove strainer O-ring from the fuel pump
reservoir body.
(4) Remove any contaminants in the fuel tank by
washing the inside of the fuel tank.
INSTALLATION
(1) Lubricate the strainer O-ring with clean engine
oil.
(2) Insert strainer O-ring into outlet of strainer so
that it sits evenly on the step inside the outlet.
(3) P
ush strainer onto the inlet of the fuel pump
reservoir body. Make sure the locking tabs on the res-
ervoir body lock over the locking tangs on the strainer.
(4) Install fuel pump module. Refer to Fuel Pump
Module Installation in this section.
FUEL LEVEL SENSOR
REMOVAL
Remove fuel pump module. Refer to Fuel Pump
Module in this section.
(1) Depress retaining tab and remove the fuel
pump/level sensor connector from the bottom of the
fuel pump module electrical connector (Fig. 21).
(2) Pull off blue locking wedge (Fig. 22).
(3) Using a small screwdriver lift locking finger
away from terminal and push terminal out of connec-
tor (Fig. 23).
(4) Push level sensor signal and ground terminals
out of the connector (Fig. 24).
(5) Insert a screwdriver between the fuel pump
module and the top of the level sensor housing (Fig.
25). Push level sensor down slightly.
Fig. 18 Locking Spring Tab
Fig. 19 Spring Tab In Locating Slot
Fig. 20 Inlet Strainer Removal
Fig. 21 Fuel Pump/Level Sensor Electrical
Connector
PLFUEL SYSTEM 14 - 13
REMOVAL AND INSTALLATION (Continued)

(5) Remove fuel rail mounting screws.
(6) Lift rail off of intake manifold. Cover the fuel
injector openings in the intake manifold.
(7) Remove fuel injector retainer (Fig. 28).
(8) Pull injector out of fuel rail. Replace fuel injec-
tor O-rings (Fig. 29).
INSTALLATION
(1) Apply a light coating of clean engine oil to the
upper O-ring.(2) Install injector in cup on fuel rail.
(3) Install retaining clip.
(4) Apply a light coating of clean engine oil to the
O-ring on the nozzle end of each injector.
(5) Insert fuel injector nozzles into openings in
intake manifold. Seat the injectors in place. Tighten
fuel rail mounting screws to 22.5 N´m63 N´m
(200630 in. lbs.).
(6) Attach electrical connectors to fuel injectors.
(7) Connect fuel supply tube to fuel rail. Refer to
Quick Connect Fittings in the Fuel Delivery Section
of this Group.
FUEL TANK
REMOVAL
(1) Perform fuel system pressure release.
(2) Drain fuel tank. Refer to Draining Fuel Tank
in this section (Fig. 30).
(3) Raise vehicle on hoist.
WARNING: WRAP SHOP TOWELS AROUND
HOSES TO CATCH ANY GASOLINE SPILLAGE.
(4) Disconnect fuel pump module electrical connec-
tor (Fig. 31).
(5) Disconnect the fuel tube from Fuel Filter/Reg-
ulator. Refer to Quick Connect Fittings in the Fuel
Delivery section of this group.
(6) Support tank with transmission jack. Loosen
tank mounting straps and lower tank slightly.
(7) Disconnect fuel filler tube and filler vent tube
from filler hose at fuel tank (Fig. 32).
(8) Disconnect fuel filler vapor relief tube from tee
connecting it to the tank vapor relief tube and EVAP
canister tube.
(9) Disconnect vapor line from Evap canister tube.
(10) Remove tank mounting straps and lower tank.Fig. 27 Fuel Rail and Injectors
Fig. 28 Fuel Injector Retainer
Fig. 29 Fuel Injector O-Rings
Fig. 30 Fuel Tank
PLFUEL SYSTEM 14 - 15
REMOVAL AND INSTALLATION (Continued)

(2) Disconnect vapor tube from rollover valve.
(3) Using a straight screwdriver, pry the valve out
of the grommet in the fuel filler tube.
INSTALLATION
(1) Apply a light coating of power steering fluid to
the grommet.
(2) Install valve in grommet.
(3) Attach vapor tube to valve.
(4) Install fuel filler tube cap.
ACCELERATOR PEDAL
CAUTION: When servicing the accelerator pedal,
throttle cable or speed control cable, do not dam-
age or kink the core wire inside the cable sheath-
ing.
REMOVAL
(1) Working from the engine compartment, remove
the throttle control shield.
(2) Hold the throttle body throttle lever in the
wide open position. Remove the throttle cable from
the throttle body cam.
(3) From inside the vehicle, hold up the pedal and
remove the cable retainer and throttle cable from the
upper end of the pedal shaft (Fig. 35) and (Fig. 36).
(4) Working from the engine compartment, remove
nuts from accelerator pedal attaching studs (Fig. 35).
Remove assembly from vehicle.
INSTALLATION
(1) Position accelerator pedal assembly on dash
panel. Install retaining nuts. Tighten retaining nuts
to 12 N´m (105 in. lbs.) torque.(2) From inside the vehicle, hold up the pedal and
install the throttle cable and cable retainer in the
upper end of the pedal shaft.
(3) From the engine compartment, hold the throt-
tle body lever in the wide open position and install
the throttle cable. Install the throttle control shield.
THROTTLE CABLEÐMANUAL TRANSMISSION
REMOVAL
(1) Remove throttle control shield (Fig. 37).
(2) Working from the engine compartment, remove
the throttle cable from the throttle body lever (Fig.
38) and (Fig. 39).
Fig. 35 Accelerator Pedal and Throttle CableÐFront
View
Fig. 36 Accelerator Pedal and Throttle CableÐRear
View
Fig. 37 Throttle Control Shield
PLFUEL SYSTEM 14 - 17
REMOVAL AND INSTALLATION (Continued)

ENGINE START-UP MODE
This is an OPEN LOOP mode. If the vehicle is in
park or neutral (automatic transaxles) or the clutch
pedal is depressed (manual transaxles) the ignition
switch energizes the starter relay. The following
actions occur when the starter motor is engaged.
²If the PCM receives the camshaft position sensor
and crankshaft position sensor signals, it energizes
the Auto Shutdown (ASD) and fuel pump relays. If
the PCM does not receive both signals within approx-
imately one second, it will not energize the ASD
relay and fuel pump relay. The ASD and fuel pump
relays supply battery voltage to the fuel pump, fuel
injectors, ignition coil and heated oxygen sensors.
²The PCM energizes all four injectors (on the 69É
degree falling edge) for a calculated pulse width until
it determines crankshaft position from the camshaft
position sensor and crankshaft position sensor sig-
nals. The PCM determines crankshaft position within
1 engine revolution.
²After determining crankshaft position, the PCM
begins energizing the injectors in sequence. It adjusts
injector pulse width and controls injector synchroni-
zation by turning the individual ground paths to the
injectors On and Off.
²When the engine idles within664 RPM of its
target RPM, the PCM compares current MAP sensor
value with the atmospheric pressure value received
during the Ignition Switch On (zero RPM) mode. If
the PCM does not detect a minimum difference
between the two values, it sets a MAP diagnostic
trouble code into memory.
Once the ASD and fuel pump relays have been
energized, the PCM determines injector pulse width
based on the following:
²Battery voltage
²Engine coolant temperature
²Engine RPM
²Intake air temperature (IAT)
²Throttle position
²The number of engine revolutions since cranking
was initiated
During Start-up the PCM maintains ignition tim-
ing at 9É BTDC.
ENGINE WARM-UP MODE
This is an OPEN LOOP mode. The following inputs
are received by the PCM:
²Engine coolant temperature
²Manifold Absolute Pressure (MAP)
²Intake air temperature (IAT)
²Crankshaft position (engine speed)
²Camshaft position
²Knock sensor
²Throttle position
²A/C switch
²Battery voltage²Power steering pressure switch
²Vehicle speed
²Speed control
²Both O2 sensors
²All diagnostics
The PCM adjusts injector pulse width and controls
injector synchronization by turning the individual
ground paths to the injectors On and Off.
The PCM adjusts ignition timing and engine idle
speed. Engine idle speed is adjusted through the idle
air control motor.
CRUISE OR IDLE MODE
When the engine is at operating temperature this
is a CLOSED LOOP mode. During cruising or idle
the following inputs are received by the PCM:
²Intake air temperature
²Engine coolant temperature
²Manifold absolute pressure
²Crankshaft position (engine speed)
²Camshaft position
²Knock sensor
²Throttle position
²Exhaust gas oxygen content
²A/C control positions
²Power steering pressure switch
²Battery voltage
²Vehicle speed
The PCM adjusts injector pulse width and controls
injector synchronization by turning the individual
ground paths to the injectors On and Off.
The PCM adjusts engine idle speed and ignition
timing. The PCM adjusts the air/fuel ratio according
to the oxygen content in the exhaust gas (measured
by the upstream and downstream heated oxygen sen-
sor).
The PCM monitors for engine misfire. During
active misfire and depending on the severity, the
PCM either continuously illuminates or flashes the
malfunction indicator lamp (Check Engine light on
instrument panel). Also, the PCM stores an engine
misfire DTC in memory.
The PCM performs several diagnostic routines.
They include:
²Oxygen sensor monitor
²Downstream heated oxygen sensor diagnostics
during open loop operation (except for shorted)
²Fuel system monitor
²EGR monitor
²Purge system monitor
²All inputs monitored for proper voltage range.
²All monitored components (refer to Group 25 for
On-Board Diagnostics).
The PCM compares the upstream and downstream
heated oxygen sensor inputs to measure catalytic
convertor efficiency. If the catalyst efficiency drops
14 - 22 FUEL SYSTEMPL
GENERAL INFORMATION (Continued)

CRANKSHAFT POSITION SENSORÐPCM INPUT
The PCM determines what cylinder to fire from the
crankshaft position sensor input and the camshaft
position sensor input. The second crankshaft counter-
weight has two sets of four timing reference notches
including a 60 degree signature notch (Fig. 7). From
the crankshaft position sensor input the PCM deter-
mines engine speed and crankshaft angle (position).
The notches generate pulses from high to low in
the crankshaft position sensor output voltage. When
a metal portion of the counterweight aligns with the
crankshaft position sensor, the sensor output voltage
goes low (less than 0.5 volts). When a notch aligns
with the sensor, voltage goes high (5.0 volts). As a
group of notches pass under the sensor, the outputvoltage switches from low (metal) to high (notch)
then back to low.
If available, an oscilloscope can display the square
wave patterns of each voltage pulses. From the width
of the output voltage pulses, the PCM calculates
engine speed. The width of the pulses represent the
amount of time the output voltage stays high before
switching back to low. The period of time the sensor
output voltage stays high before switching back to
low is referred to as pulse width. The faster the
engine is operating, the smaller the pulse width on
the oscilloscope.
By counting the pulses and referencing the pulse
from the 60 degree signature notch, the PCM calcu-
lates crankshaft angle (position). In each group of
timing reference notches, the first notch represents
69 degrees before top dead center (BTDC). The sec-
ond notch represents 49 degrees BTDC. The third
notch represents 29 degrees. The last notch in each
set represents 9 degrees before top dead center
(TDC).
The timing reference notches are machined at 20É
increments. From the voltage pulse width the PCM
tells the difference between the timing reference
notches and the 60 degree signature notch. The 60
degree signature notch produces a longer pulse width
than the smaller timing reference notches. If the
camshaft position sensor input switches from high to
low when the 60 degree signature notch passes under
the crankshaft position sensor, the PCM knows cylin-
der number one is the next cylinder at TDC.
The crankshaft position sensor mounts to the
engine block behind the alternator, just above the oil
filter (Fig. 8).
ENGINE COOLANT TEMPERATURE SENSORÐPCM
INPUT
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.
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. 5 Target MagnetÐTypical
Fig. 6 Target Magnet Polarity
14 - 26 FUEL SYSTEMPL
DESCRIPTION AND OPERATION (Continued)

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, 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
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 electrical voltage, the sensor acts as
a rich-lean switch.The heating element in the sensor provides heat to
the sensor ceramic element. Heating the sensor
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, the PCM adjusts injector pulse
width based on the upstream heated oxygen sensor
input along with other inputs. In Open Loop, the
PCM adjusts injector pulse width based on prepro-
grammed (fixed) values and inputs from other sen-
sors.
The upstream oxygen sensor threads into the out-
let flange of the exhaust manifold (Fig. 11).
DOWNSTREAM OXYGEN SENSOR
The downstream heated oxygen sensor threads into
the outlet pipe at the rear of the catalytic convertor
(Fig. 12). The downstream heated oxygen sensor
input is used to detect catalytic convertor deteriora-
tion. As the convertor deteriorates, the input from
the downstream sensor begins to match the upstream
sensor input except for a slight time delay. By com-
paring the downstream heated oxygen sensor input
to the input from the upstream sensor, the PCM cal-
culates catalytic convertor efficiency.
IGNITION CIRCUIT SENSEÐPCM INPUT
The ignition circuit sense input tells the Power-
train Control Module (PCM) the ignition switch has
energized the ignition circuit. Refer to the wiring dia-
grams for circuit information.
INTAKE AIR TEMPERATURE SENSORÐPCM INPUT
The Intake Air Temperature (IAT) sensor measures
the temperature of the intake air as it enters the
engine. The sensor supplies one of the inputs the
PCM uses to determine injector pulse width and
spark advance.
Fig. 11 Upstream Heated Oxygen Sensor
14 - 28 FUEL SYSTEMPL
DESCRIPTION AND OPERATION (Continued)

The vehicle speed sensor generates 8 pulses per
sensor revolution. These signals, in conjunction with
a closed throttle signal from the throttle position sen-
sor, indicate a closed throttle deceleration to the
PCM. Under deceleration conditions, the PCM
adjusts the Idle Air Control (IAC) motor to maintain
a desired MAP value.
When the vehicle is stopped at idle, a closed throt-
tle signal is received by the PCM (but a speed sensor
signal is not received). Under idle conditions, the
PCM adjusts the IAC motor to maintain a desired
engine speed.
The vehicle speed sensor signal is also used to
operate the following functions or systems:
²Speedometer
²Speed control²Daytime Running Lights (Canadian Vehicles
only).AIR CONDITIONING CLUTCH RELAYÐPCM
OUTPUT
The PCM controls the air conditioning clutch relay
ground circuit. The A/C clutch relay coil side contains
a 10 amp fuse between the buss bar in the Power
Distribution Center (PDC) and the relay. The power
side of this relay is fused with a 40 amp fuse. When
the PCM receives an air conditioning input, it
grounds the A/C compressor clutch relay and the
radiator fan relay.
When the PCM senses low idle speeds or wide open
throttle through the throttle position sensor, it
removes the ground for the A/C compressor clutch
relay. When the relay de-energizes, the contacts open
preventing air conditioning clutch engagement. Also,
if the PCM senses a part throttle launch condition, it
Fig. 18 Throttle Position Sensor and Idle Air Control
MotorÐSOHC
Fig. 19 Throttle Position Sensor and Idle Air Control
MotorÐDOHC
Fig. 20 Vehicle Speed SensorÐAutomatic
Transmission
Fig. 21 Vehicle Speed SensorÐManual
Transmission
PLFUEL SYSTEM 14 - 31
DESCRIPTION AND OPERATION (Continued)