1993 CHEVROLET PLYMOUTH ACCLAIM heating

resenting throttle blade position. As the position of
the throttle blade changes, the resistance of the TPS
changes. The PCM supplies approximately 5 volts to the
TPS. The TPS output voltage (input signal to the
PCM) represents the throttle blade position. The TPS
output voltage to the PCM varies from approxi-
mately 0.5 volt at minimum throttle opening (idle) to
4 volts at wide open throttle. Along with inputs from
other sensors, the PCM uses the TPS input to deter-
mine current engine operating conditions and adjust
fuel injector pulse width and ignition timing.
VEHICLE SPEED SENSORÐPCM INPUT
The vehicle speed sensor (Fig. 15) is located on the
transaxle extension housing. The sensor input is
used by the PCM to determine vehicle speed and dis-
tance traveled.
The speed sensor generates 8 pulses per sensor rev-
olution. These signals, along with a closed throttle
signal from the TPS, determine if a closed throttle
deceleration or normal idle condition (vehicle
stopped) exists. Under deceleration conditions, the
PCM adjusts the idle air control motor to maintain a desired MAP value. Under idle conditions, the PCM
adjusts the idle air control motor to maintain desired
engine speed.
AIR CONDITIONING CLUTCH RELAYÐPCM
OUTPUT
The PCM operates the air conditioning clutch relay
ground circuit. The radiator fan relay supplies bat-
tery power to the solenoid side of the A/C clutch re-
lay. The air conditioning clutch relay will not
energize unless the radiator fan relay energizes. The
PCM energizes the radiator fan relay when the air
conditioning or defrost switch is put in the ON posi-
tion and the low pressure and high pressure switches
close. When the PCM senses wide open throttle
through the throttle position sensor, or low engine
RPM it will de-energize the A/C clutch relay, open
it's contacts and prevent air conditioning clutch en-
gagement. On AG Body vehicles, the relay is located in the
power distribution center (Fig. 16).
GENERATOR FIELDÐPCM OUTPUT
The PCM regulates the charging system voltage
within a range of 12.9 to 15.0 volts. Refer to Group
8A for charging system information.
AUTO SHUTDOWN (ASD) RELAY AND FUEL PUMP
RELAYÐPCM OUTPUT
The PCM operates the auto shutdown (ASD) relay
and fuel pump relay through one ground path. The
PCM operates the relays by switching the ground
path on and off. Both relays turn on and off at the
same time. The ASD relay connects battery voltage to the fuel
injector and ignition coil. The fuel pump relay con-
nects battery voltage to the fuel pump and oxygen
sensor heating element. The PCM turns the ground path off when the igni-
tion switch is in the Off position. Both relays are off.
When the ignition switch is in the On or Crank po-
Fig. 14 Throttle Position Sensor and Idle Air Control Motor
Fig. 15 Vehicle Speed Sensor
Fig. 16 Power Distribution CenterÐAG Body
Ä FUEL SYSTEMS 14 - 89

sition, the PCM monitors the crankshaft position and
camshaft position sensor signals to determine engine
speed and ignition timing (coil dwell). If the PCM
does not receive the crankshaft position sensor and
camshaft position sensor signals when the ignition
switch is in the Run position, it de-energizes both re-
lays. When the relays are de-energized, battery volt-
age is not supplied to the fuel injector, ignition coil,
fuel pump and oxygen sensor heating element. The ASD relay and fuel pump relay are located in
the power distribution center (Fig. 16).
IDLE AIR CONTROL MOTORÐPCM OUTPUT
The idle air control motor is mounted on the throt-
tle body (Fig. 14). The PCM operates the motor. The
PCM adjusts engine idle speed through the idle air
control motor to compensate for engine load or ambi-
ent conditions. The throttle body has an air bypass passage that
provides air for the engine at idle (the throttle blade
is closed). The idle air control motor pintle protrudes
into the air bypass passage and regulates air flow
through it. The PCM adjusts engine idle speed by moving the
idle air control motor pintle in and out of the bypass
passage. The adjustments are based on inputs the
PCM receives. The inputs are from the throttle posi-
tion sensor, camshaft position sensor, crankshaft po-
sition sensor, coolant temperature sensor, and
various switch operations (brake and air condition-
ing). Deceleration die out is also prevented by in-
creasing airflow when the throttle is closed quickly
after a driving (speed) condition.
BAROMETRIC READ SOLENOIDÐPCM OUTPUT
The barometric pressure read solenoid is spliced
into the manifold absolute pressure (MAP) sensor
vacuum hose (Fig. 12). The barometric read solenoid
switches the pressure supply to the MAP sensor from
either barometric pressure (atmospheric) or manifold
vacuum. The PCM operates the solenoid. Atmospheric pressure is periodically supplied to
the MAP sensor to measure barometric pressure.
This occurs at closed throttle, once per throttle clo-
sure but no more often than once every 3 minutes
and within a specified RPM band. Barometric infor-
mation is used primarily for boost control and start
fuel enrichment at various altitudes.
CANISTER PURGE SOLENOIDÐPCM OUTPUT
Vacuum for the Evaporative Canister is controlled
by the Canister Purge Solenoid (Fig. 17). The sole-
noid is controlled by the PCM. The PCM operates the solenoid by switching the
ground circuit on and off. When grounded, the sole-
noid energizes and prevents vacuum from reaching
the evaporative canister. When not energized the so-
lenoid allows vacuum to flow to the canister. During warm-up and for a specified time period after
hot starts the PCM grounds the purge solenoid.
Vacuum does not operate the evaporative canister
valve. The PCM removes the ground to the solenoid when
the engine reaches a specified temperature and the
time delay interval has occurred. When the solenoid is
de-energized, vacuum flows to the canister purge
valve. Vapors are purged from the canister and flow to
the throttle body. The purge solenoid will also be energized during
certain idle conditions, in order to update the fuel
delivery calibration.
MALFUNCTION INDICATOR LAMP (CHECK
ENGINE)ÐPCM OUTPUT
The malfunction indicator lamp (instrument panel
Check Engine lamp) comes on each time the ignition
key is turned ON and stays on for 3 seconds as a bulb
test. The malfunction indicator lamp warns the opera-
tor that the PCM has entered a Limp-in mode. During
Limp-in-Mode, the PCM attempts to keep the system
operational. The malfunction indicator lamp signals
the need for immediate service. In limp-in mode, the
PCM compensates for the failure of certain components
that send incorrect signals. The PCM substitutes for
the incorrect signals with inputs from other sensors. Signals that can trigger the malfunction indi-
cator lamp (Check Engine Lamp).
² Engine Coolant Temperature Sensor
² Manifold Absolute Pressure Sensor
² Throttle Position Sensor
² Battery Voltage Input
² An Emissions Related System
² Charging system
The malfunction indicator lamp can also be used to
display diagnostic trouble codes. Cycle the ignition
switch on, off, on, off, on, within five seconds and any
Fig. 17 EVAP Canister Purge Solenoid and Waste- gate Control Solenoid
14 - 90 FUEL SYSTEMS Ä

noid is energized. The solenoid mounts to the passen-
ger side inner fender panel, next to the strut tower
(Fig. 17).
MODES OF OPERATION
As input signals to the PCM change, the PCM
adjusts its response to the output devices. For example,
the PCM must calculate a different injector pulse
width and ignition timing for idle than it does for wide
open throttle (WOT). There are several different modes
of operation that determine how the PCM responds to
the various input signals. There are two different areas of operation, OPEN
LOOP and CLOSED LOOP. During OPEN LOOP modes, the PCM receives input
signals and responds according to preset PCM pro-
gramming. Input from the oxygen (O
2) sensor is not
monitored during OPEN LOOP modes. During CLOSED LOOP modes, the PCM does moni-
tor the oxygen (O
2) sensor input. This input indicates
to the PCM whether or not the calculated injector pulse
width results in the ideal air-fuel ratio of 14.7 parts air
to 1 part fuel. By monitoring the exhaust oxygen
content through the O
2sensor, the PCM can fine tune
the injector pulse width to achieve optimum fuel
economy combined with low emissions. The 2.2L Turbo III multi-port fuel injection system
has the following modes of operation:
² Ignition switch ON - Zero RPM
² Engine start-up
² Engine warm-up
² Cruise (Idle)
² Acceleration
² Deceleration
² Wide Open Throttle
² Ignition switch OFF
The engine start-up (crank), engine warm-up, and
wide open throttle modes are OPEN LOOP modes. The
acceleration, deceleration, and cruise modes, with the
engine at operating temperature are CLOSED
LOOP modes (under most operating conditions).
IGNITION SWITCH ON (ZERO RPM) MODE
When the ignition switch activates the fuel injection
system the following actions occur:
²
The PCM calculates basic fuel strategy by determining
atmospheric air pressure from the MAP sensor input.
² The PCM monitors the coolant temperature sensor
and throttle position sensor input. The PCM modifies
fuel strategy based on this input. When the key is in the ON position and the engine is
not running, the auto shutdown (ASD) relay and fuel
pump relay are not energized. Therefore battery volt-
age is not supplied to the fuel pump, ignition coil, fuel
injector or oxygen sensor heating element.
ENGINE START-UP MODE
This is an OPEN LOOP mode. The following actions
occur when the starter motor is engaged. If the PCM receives the camshaft position and crank-
shaft position sensor signals, it energizes the auto
shutdown (ASD) relay and fuel pump relay. These
relays supply battery voltage to the fuel pump, fuel
injectors, ignition coil, and oxygen sensor heating ele-
ment. If the PCM does not receive the camshaft posi-
tion sensor and crankshaft position sensor signals
within approximately one second, it de-energizes the
ASD relay and fuel pump relay. The PCM energizes all injectors until it determines
crankshaft position from the camshaft position sensor
and crankshaft position sensor signals. The PCM de-
termines crankshaft position within 1 engine revolu-
tion. After determining crankshaft position, the PCM be-
gins energizing the injectors in sequence. The PCM
adjusts injector pulse width and controls injector syn-
chronization by turning the individual ground paths to
the injectors On and Off. When the engine idles within 664 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 fault into memory. Once the ASD and fuel pump relays have been
energized, the PCM:
² Determines injector pulse width based on coolant
temperature, manifold absolute pressure (MAP) and
the number of engine revolutions since cranking was
initiated.
² Monitors the coolant temperature sensor, camshaft
position sensor, crankshaft position sensor, MAP sen-
sor, and throttle position sensor to determine correct
ignition timing.
ENGINE WARM-UP MODE
This is a OPEN LOOP mode. The following inputs
are received by the PCM:
² engine coolant temperature
² knock sensor
² manifold absolute pressure (MAP)
² engine speed (crankshaft position sensor)
² throttle position
² A/C switch
² battery voltage
The PCM provides a ground path for the injectors to
precisely control injector pulse width (by switching the
ground on and off). The PCM adjusts engine idle speed
through the idle air control motor. Also, the PCM
regulates ignition timing.
14 - 92 FUEL SYSTEMS Ä

(caused by a lean air-fuel mixture), the sensor pro-
duces a low voltage. When there is a lesser amount
present (rich air-fuel mixture) it produces a higher
voltage. By monitoring the oxygen content and con-
verting it to electrical voltage, the sensor acts as a
rich-lean switch. The oxygen sensor is equipped with a heating ele-
ment that keeps the sensor at proper operating tem-
perature during all operating modes. Maintaining
correct sensor temperature at all times allows the
system to enter into Closed Loop operation sooner.
Also, it allow the system to remain in Closed Loop
operation during periods of extended idle. In Closed Loop operation the PCM monitors the O
2sensor input (along with other inputs) and adjusts
the injector pulse width accordingly. During Open
Loop operation the PCM ignores the O
2sensor input.
The PCM adjusts injector pulse width based on pre-
programmed (fixed) values and from inputs of other
sensors.
SPEED CONTROLÐPCM INPUT
The speed control system provides four separate
voltages (inputs) to the PCM. The voltages corre-
spond to the On/Off, Set, and Resume. The speed control ON voltage informs the PCM
that the speed control system has been activated.
The speed control SET voltage informs the PCM that
a fixed vehicle speed has been selected. The speed
control RESUME voltage indicates the previous fixed
speed is requested. The speed control OFF voltage
tells the PCM that the speed control system has de-
activated. Refer to Group 8H for further speed con-
trol information.
PARK/NEUTRAL SWITCHÐPCM INPUT
The park/neutral switch is located on the transaxle
housing (Fig. 8 or Fig. 9). It provides an input to the
PCM indicating whether the automatic transaxle is
in Park or Neutral. This input is used to determine
idle speed (varying with gear selection), fuel injector
pulse width, and ignition timing advance. The park/
neutral switch is sometimes referred to as the neu-
tral safety switch.
THROTTLE POSITION SENSOR (TPS)ÐPCM INPUT
The Throttle Position Sensor (TPS) is mounted on
the throttle body and connected to the throttle blade
shaft (Fig. 10). The TPS is a variable resistor that
provides the PCM with an input signal (voltage) rep-
resenting throttle blade position. As the position of
the throttle blade changes, the resistance of the TPS
changes.
The PCM supplies approximately 5 volts to the
TPS. The TPS output voltage (input signal to the
PCM) represents throttle blade position. The TPS
output voltage to the PCM varies from approxi-
mately 0.5 volt at minimum throttle opening (idle) to
3.5 volts at wide open throttle. The wide open throt-
tle input is approximately 3 volts more than the min-
imum throttle opening value. Along with inputs from other sensors, the PCM
uses the TPS input to determine current engine op-
erating conditions. After determining the current op-
erating conditions, the PCM adjust fuel injector pulse
width and ignition timing.
Fig. 8 Park Neutral SwitchÐ3-Speed Automatic Transaxle
Fig. 9 Park Neutral SwitchÐ4-Speed ElectronicAutomatic Transaxle
Fig. 10 Throttle Position Sensor
Ä FUEL SYSTEMS 14 - 117

AUTO SHUTDOWN (ASD) RELAY AND FUEL PUMP
RELAYÐPCM OUTPUT
The PCM operates the auto shutdown (ASD) relay
and fuel pump relay through one ground path. The
PCM operates the relays by switching the ground
path on and off. Both relays turn on and off at the
same time. The ASD relay connects battery voltage to the fuel
injector and ignition coil. The fuel pump relay con-
nects battery voltage to the fuel pump and oxygen
sensor heating element. The PCM turns the ground path off when the igni-
tion switch is in the Off position. Both relays are off.
When the ignition switch is in the On or Crank po-
sition, the PCM monitors the distributor pick-up sig-
nal to determine engine speed and ignition timing (coil dwell). If the PCM does not receive a distributor
signal when the ignition switch is in the Run posi-
tion, it will de-energize both relays. When the relays
are de-energized, battery voltage is not supplied to
the fuel injector, ignition coil, fuel pump and oxygen
sensor heating element. On AA body vehicles, the relays are located next to
the drivers side strut tower (Fig. 13). On AC, AG and AJ body vehicles, the relays are lo-
cated in the power distribution center (Fig. 12 or Fig.
14).
IDLE AIR CONTROL MOTORÐPCM OUTPUT
The idle air control motor is mounted on the throt-
tle body and is controlled by the PCM (Fig. 10). The
PCM adjusts engine idle speed through the idle air
control motor to compensate for engine load or ambi-
ent conditions. The throttle body has an air bypass passage that
provides air for the engine at idle (the throttle blade
is closed). The idle air control motor pintle protrudes
into the air bypass passage and regulates air flow
through it. The PCM adjusts engine idle speed by moving the
idle air control motor pintle in and out of the bypass
passage. The adjustments are based on inputs the
PCM receives. The inputs are from the throttle posi-
tion sensor, engine speed sensor (distributor pick-up
coil), coolant temperature sensor, and various switch
operations (brake, park/neutral, air conditioning).
Deceleration die out is also prevented by increasing
airflow when the throttle is closed quickly after a
driving (speed) condition.
DUTY CYCLE EVAP CANISTER PURGE
SOLENOIDÐPCM OUTPUT
The duty cycle EVAP purge solenoid regulates the
rate of vapor flow from the EVAP canister to the
throttle body. The powertrain control module oper-
ates the solenoid. During the cold start warm-up period and the hot
start time delay, the PCM does not energize the so-
lenoid. When de-energized, no vapors are purged.
The PCM de-energizes the solenoid during open loop
operation. The engine enters closed loop operation after it
reaches a specified temperature and the time delay
ends. During closed loop operation, the PCM ener-
gizes and de-energizes the solenoid approximately 5
to 10 times per second, depending upon operating
conditions. The PCM varies the vapor flow rate by
changing solenoid pulse width. Pulse width is the
amount of time the solenoid energizes. A rubber boot covers the EVAP purge solenoid.
The solenoid and bracket attach to the EVAP canis-
ter mounting studs (Fig. 15). The top of the solenoid
has the word TOP on it. The solenoid will not oper-
ate unless it is installed correctly.
Fig. 13 Relay Identification (AA Body)
Fig. 14 Relay Identification (AG and AJ Body)
Ä FUEL SYSTEMS 14 - 119

The engine start-up (crank), engine warm-up, and
wide open throttle modes are OPEN LOOP modes. The
acceleration, deceleration, and cruise modes, with the
engine at operating temperature are CLOSED
LOOP modes (under most operating conditions).
IGNITION SWITCH ON (ZERO RPM) MODE
When the multi-port fuel injection system is acti-
vated by the ignition switch, the following actions
occur:
² The PCM determines atmospheric air pressure from
the MAP sensor input to determine basic fuel strategy.
² The PCM monitors the coolant temperature sensor
and throttle position sensor input. The PCM modifies
fuel strategy based on these inputs. When the key is in the ON position and the engine is
not running (zero rpm), the auto shutdown (ASD) relay
and fuel pump relay are not energized. Therefore
battery voltage is not supplied to the fuel pump,
ignition coil, fuel injectors or oxygen sensor heating
element.
ENGINE START-UP MODE
This is an OPEN LOOP mode. The following actions
occur when the starter motor is engaged. If the PCM receives a distributor signal, it energizes
the auto shutdown (ASD) relay and fuel pump relay.
These relays supply battery voltage to the fuel pump,
fuel injectors, ignition coil, and oxygen sensor heating
element. If the PCM does not receive a distributor
input, the ASD relay and fuel pump relay will be
de-energized after approximately one second. The PCM energizes all six injectors until it deter-
mines crankshaft position from the distributor pick-up
signals. The PCM determines crankshaft position
within 2 engine revolutions. After determining crankshaft position, the PCM be-
gins energizing the injectors in sequence. The PCM
adjusts injector pulse width and controls injector syn-
chronization by turning the individual ground paths to
the injectors On and Off. When the engine idles within 664 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 fault into memory. Once the ASD and fuel pump relays have been
energized, the PCM:
² determines injector pulse width based on coolant
temperature, manifold absolute pressure (MAP) and
the number of engine revolutions since cranking was
initiated. ²
monitors the coolant temperature sensor, distribu-
tor pick-up, MAP sensor, and throttle position sensor
to determine correct ignition timing.
ENGINE WARM-UP MODE
This is a OPEN LOOP mode. The following inputs
are received by the PCM:
² engine coolant temperature
² crankshaft position (distributor pick-up)
² manifold absolute pressure (MAP)
² engine speed (distributor pick-up)
² throttle position
² A/C switch
² battery voltage
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 by regulating
the idle air control motor and ignition timing.
CRUISE OR IDLE MODE
When the engine is at operating temperature this
is a CLOSED LOOP mode. During cruising speed the
following inputs are received by the PCM:
² engine coolant temperature
² crankshaft position (distributor pick-up)
² manifold absolute pressure
² engine speed (distributor pick-up)
² throttle position
² exhaust gas oxygen content
² A/C control positions
² battery voltage
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 controls the air/fuel ratio according
to the oxygen content in the exhaust gas.
ACCELERATION MODE This is a CLOSED LOOP mode. The PCM recog-
nizes an abrupt increase in throttle position or MAP
pressure as a demand for increased engine output
and vehicle acceleration. The PCM increases injector
pulse width in response to increased fuel demand.
DECELERATION MODE This is a CLOSED LOOP mode. During decelera-
tion the following inputs are received by the PCM:
² engine coolant temperature
² crankshaft position (distributor pick-up)
² manifold absolute pressure
² engine speed (distributor pick-up)
² throttle position
² exhaust gas oxygen content
² A/C control positions
² battery voltage
14 - 122 FUEL SYSTEMS Ä

² transaxle gear selection (park/neutral switch)
The PCM also adjusts engine idle speed through
the idle air control motor based on the following in-
puts.
² brake switch
² engine coolant temperature
² engine speed (crankshaft position sensor)
² throttle position
² transaxle gear selection (park/neutral switch)
² vehicle speed
The auto shutdown (ASD) and fuel pump relays are
mounted externally, but turned on and off by the
PCM through the same circuit. The camshaft position sensor and crankshaft posi-
tion sensor signals are sent to the PCM. If the PCM
does not receive both signals within approximately
one second of engine cranking, it deactivates the
ASD and fuel pump relays. When these relays are
deactivated, power is shut off to the fuel injector, ig-
nition coil, oxygen sensor heating element and fuel
pump. The PCM contains a voltage converter that
changes battery voltage to a regulated 8.0 volts. The
8.0 volts power the camshaft position sensor, crank-
shaft position sensor and vehicle speed sensor. The
PCM also provides a 5.0 volts supply for the coolant
temperature sensor, manifold absolute pressure sen-
sor and throttle position sensor.
AIR CONDITIONING SWITCH SENSEÐPCM INPUT
When the air conditioning or defrost switch is put
in the ON position and the low pressure, high pres-
sure and ambient temperature switches are closed,
the PCM receives an input for air conditioning. After
receiving this input, the PCM activates the A/C com-
pressor clutch by grounding the A/C clutch relay.
BATTERY VOLTAGEÐPCM INPUT
The PCM monitors the battery voltage input to de-
termine fuel injector pulse width and generator field
control. If battery voltage is low the PCM will in-
crease injector pulse width.
BRAKE SWITCHÐPCM INPUT
When the brake switch is activated, the PCM re-
ceives an input indicating that the brakes are being
applied. the brake signal cancels speed control and
unlocks the torque convertor. The brake switch is
mounted on the brake pedal support bracket.
CAMSHAFT POSITION SENSORÐPCM INPUT
The camshaft position sensor provides cylinder
identification to the powertrain control module
(PCM) (Fig. 3). The sensor generates pulses as
groups of notches on the camshaft sprocket pass un-
derneath it (Fig. 4). The PCM keeps track of crank-
shaft rotation and identifies each cylinder by the pulses generated by the notches on the camshaft
sprocket. Four crankshaft pulses follow each group of
camshaft pulses.
When the PCM receives two camshaft pulses fol-
lowed by the long flat spot on the camshaft sprocket,
it knows that the crankshaft timing marks for cylin-
der one are next (on driveplate). When the PCM re-
ceives one camshaft pulse after the long flat spot on
the sprocket, cylinder number two crankshaft timing
marks are next. After 3 camshaft pulses, the PCM
knows cylinder four crankshaft timing marks follow.
One camshaft pulse after the three pulses indicates
cylinder five. The two camshaft pulses after cylinder
5 signals cylinder six (Fig. 4). The PCM can synchro-
nize on cylinders 1 or 4.
When metal aligns with the sensor, voltage goes
low (less than 0.5 volts). When a notch aligns with
the sensor, voltage spikes high (5.0 volts). As a group
of notches pass under the sensor, the voltage
switches from low (metal) to high (notch) then back
Fig. 3 Camshaft Position Sensor
Fig. 4 Camshaft Sprocket
Ä FUEL SYSTEMS 14 - 147

tion timing from the crankshaft position sensor. Once
crankshaft position has been determined, the PCM
begins energizing the injectors in sequence. The crankshaft position sensor is located in the
transaxle housing, above the vehicle speed sensor (Fig.
9). The bottom of the sensor is positioned next to the
drive plate. The distance between the bottom of
sensor and the drive plate is critical to the op-
eration of the system. When servicing the crank-
shaft position sensor, refer to the Multi-Port Fuel
Injection Service ProceduresÐ3.3L Engine sec-
tion in this Group.
MANIFOLD ABSOLUTE PRESSURE (MAP)
SENSORÐPCM INPUT
The PCM supplies 5 volts to the MAP sensor. The
MAP sensor converts intake manifold pressure into
voltage. The PCM monitors the MAP sensor output
voltage. As vacuum increases, MAP sensor voltage
decreases proportionately. Also, as vacuum decreases,
MAP sensor voltage increases proportionately. During cranking, before the engine starts running,
the PCM determines atmospheric air pressure from
the MAP sensor voltage. While the engine operates, the
PCM determines intake manifold pressure from the
MAP sensor voltage. Based on MAP sensor voltage and
inputs from other sensors, the PCM adjusts spark
advance and the air/fuel mixture. The MAP sensor (Fig. 10) mounts to the side of the
intake manifold, below the positive crankcase ventila-
tion (PCV) valve. The sensor connects electrically to
the PCM.
HEATED OXYGEN SENSOR (O2SENSOR)ÐPCM
INPUT
The O2sensor is located in the exhaust manifold and
provides an input voltage to the PCM. The input tells
the PCM the oxygen content of the exhaust gas (Fig.
11). The PCM uses this information to fine tune the
air-fuel ratio by adjusting injector pulse width. The O
2sensor produces 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 sensor 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
sensor acts as a rich-lean switch. The oxygen sensor is equipped with a heating ele-
ment that keeps the sensor at proper operating tem-
perature during all operating modes. Maintaining cor-
rect 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 O
2
sensor input (along with other inputs) and adjusts the
injector pulse width accordingly. During Open Loop
operation the PCM ignores the O
2sensor input. The
PCM adjusts injector pulse width based on prepro-
grammed (fixed) values and inputs from other sensors.
Fig. 9 Crankshaft Position Sensor Location
Fig. 10 Map Sensor
Fig. 11 Heated Oxygen SensorÐ3.3L Engine
Ä FUEL SYSTEMS 14 - 149