sensor JEEP CHEROKEE 1994 Service Repair Manual

SERVICE PROCEDURES
INDEX
page page
Catalytic Converter........................ 5
Engine Exhaust ManifoldÐ2.5L Engine......... 7
Engine Exhaust ManifoldÐ4.0L Engine......... 7
Exhaust PipeÐXJ Vehicles.................. 3
Exhaust PipeÐYJ Vehicles.................. 4Intake ManifoldÐ2.5L Engine................ 8
Intake ManifoldÐ4.0L Engine................ 9
Muffler and Exhaust TailpipeÐXJ Vehicles...... 5
Muffler and Exhaust TailpipeÐYJ Vehicles...... 6
EXHAUST PIPEÐXJ VEHICLES
WARNING: IF TORCHES ARE USED WHEN WORK-
ING ON THE EXHAUST SYSTEM, DO NOT ALLOW
THE FLAME NEAR THE FUEL LINES.
REMOVAL
(1) Raise and support the vehicle.
(2) Saturate the bolts and nuts with heat valve lu-
bricant (Fig. 1). Allow 5 minutes for penetration.
(3) Remove the oxygen sensor from the exhaust
pipe (Fig. 2).
(4) Disconnect the exhaust pipe from the engine
exhaust manifold. Discard the seal (4.0L engine,
only).
(5) Support the transmission and remove the
transmission support crossmember.
(6) Remove the clamp nuts and clamp (Fig. 3). To
remove the exhaust pipe from the catalytic converter,apply heat until the metal becomes cherry red. Dis-
connect the exhaust pipe from the catalytic converter
(Fig. 3). Remove the exhaust pipe.
CLEANING
Clean the mating surfaces of the engine exhaust
manifold flange and the exhaust pipe.
Fig. 1 Exhaust Pipe-to-Engine Exhaust Manifold
Nuts
Fig. 2 Oxygen Sensor
Fig. 3 Exhaust Pipe-to-Catalytic Converter
Connection
JEXHAUST SYSTEM AND INTAKE MANIFOLD 11 - 3

INSTALLATION
(1) Assemble exhaust pipe to manifold and cata-
lytic converter loosely to permit proper alignment of
all parts.
(2) Use a new clamp at the exhaust pipe to cata-
lytic converter connection and tighten the nuts to 61
NzIm (45 ft. lbs.) torque.
(3) Connect the exhaust pipe to the engine exhaust
manifold using new nuts. Install a new seal between
the exhaust manifold and the exhaust pipe (4.0L en-
gine, only). Tighten the nuts to 31 Nzm (23 ft. lbs.)
torque (Fig. 1).
(4) Align the transmission support crossmember to
the exhaust pipe (Fig. 3) and position on the sill.
Tighten the crossmember-to-sill bolts to 41 Nzm (30
ft. lbs.) torque. Remove the support from the trans-
mission.
(5) Coat the oxygen sensor with anti-seize com-
pound. Install the sensor and tighten the nut to 48
Nzm (35 ft. lbs.) torque.
(6) Lower the vehicle.
(7) Start the engine and inspect for exhaust leaks
and exhaust system contact with the body panels.
Adjust the alignment, if needed.
EXHAUST PIPEÐYJ VEHICLES
WARNING: IF TORCHES ARE USED WHEN WORK-
ING ON THE EXHAUST SYSTEM, DO NOT ALLOW
THE FLAME NEAR THE FUEL LINES.
REMOVAL
(1) Raise and support the vehicle.
(2) Saturate the bolts and nuts with heat valve lu-
bricant (Fig. 4). Allow 5 minutes for penetration.
(3) Remove the oxygen sensor from the exhaust
pipe (Fig. 2).
(4) Disconnect the exhaust pipe from the engine
exhaust manifold. Discard the seal (4.0L engine,
only).
(5) Remove the clamp nuts and clamp (Fig. 5). To
remove the exhaust pipe from the catalytic converter,
apply heat until the metal becomes cherry red. Dis-
connect the exhaust pipe from the catalytic converter
and slide the pipe out of the transmission torque arm
insulator (Fig. 5). Remove the exhaust pipe.
CLEANING
Clean the mating surfaces of the engine exhaust
manifold flange and the exhaust pipe.
INSTALLATION
(1) Slide the exhaust pipe into the transmission
torque arm insulator and onto the catalytic converter
(Fig. 5).
(2) Connect the exhaust pipe to the engine exhaust
manifold using new nuts. Install a new seal betweenthe exhaust manifold and the exhaust pipe (4.0L en-
gine, only). DO NOT tighten the nuts at this time.
(3) Align the exhaust pipe.
(4) Use a new clamp at the exhaust pipe to cata-
lytic converter connection and tighten the nuts to 61
Nzm (45 ft. lbs.) torque.
(5) Tighten the exhaust pipe-to-engine exhaust
manifold nuts to 31 Nzm (23 ft. lbs.) torque.
(6) Coat the oxygen sensor with anti-seize com-
pound. Install the sensor and tighten the nut to 48
Nzm (35 ft. lbs.) torque.
(7) Lower the vehicle.
(9) Start the engine and inspect for exhaust leaks
and exhaust system contact with the body panels.
Adjust the alignment, if needed.
Fig. 4 Exhaust Pipe-to-Engine Exhaust Manifold
Nuts
Fig. 5 Exhaust Pipe-to-Catalytic Converter
ConnectionÐYJ Vehicles
11 - 4 EXHAUST SYSTEM AND INTAKE MANIFOLDJ

INTAKE MANIFOLDÐ2.5L ENGINE
REMOVAL
(1) Disconnect the battery negative cable.
(2) Remove the air inlet hose from the throttle
body and air cleaner.
(3) Loosen the accessory drive belt tension and re-
move the belt from the power steering pump.
(4) Remove the power steering pump and brackets
from the water pump and intake manifold. Support
power steering pump and bracket with mechanics
wire attached to the radiator upper crossmember.
(5) Remove the fuel tank filler cap to relieve the
fuel tank pressure.
(6) Install the fuel tank filler cap.
(7) Disconnect fuel supply and return tube from
the fuel rail (refer to Group 14, Fuel System - Quick
Connect Fittings).
(8) Disconnect the accelerator cable from the throt-
tle body and the holddown bracket.
CAUTION: When disconnecting the cruise control
connector at the throttle body, DO NOT pry the con-
nector off with pliers or screwdriver. Use finger
pressure only. Prying the connector off could break
it.
(9) Disconnect the electrical connectors. Pull the
harnesses away from the manifold.
²The throttle position sensor.
²The idle speed control motor.
²The coolant temperature sensor at the thermostat.
²The manifold air temperature sensor at the intake
manifold.
²The fuel injectors.
²The oxygen sensor.
(10) Disconnect the crankcase ventilation (CCV)
vacuum hose and manifold absolute pressure (MAP)
sensor vacuum hose connector at the intake mani-
fold.
(11) Disconnect vacuum hose from vacuum port on
the intake manifold.
(12) Disconnect CCV hose at the cylinder head
cover (Fig. 12).
(13) Remove the molded vacuum harness.
(14) Disconnect the vacuum brake booster hose at
the intake manifold.
(15) Remove bolts 2 through 5 securing the intake
manifold to the cylinder head (Fig. 11). Slightly
loosen bolt No.1 and nuts 6 and 7.
(16) Remove the intake manifold and gaskets.
Drain the coolant from the manifold.
CLEANING
Clean the intake manifold and cylinder head mat-
ing surfaces.DO NOT allow foreign material to
enter either the intake manifold or the ports in
the cylinder head.
INSTALLATION
(1) Install the new intake manifold gasket over the
locating dowels.
(2) Position the manifold in place and finger
tighten the mounting bolts.
(3) Tighten the fasteners in sequence and to the
specified torque (Fig. 11).
²Fastener No.1ÐTighten to 41 Nzm (30 ft. lbs.)
torque.
²Fasteners Nos.2 through 7ÐTighten to 31 Nzm (23
ft. lbs.) torque.
(4) Connect the fuel return and supply tube to the
connector next to the fuel rail. Push them into the
fitting until a click is heard. Verify that the connec-
tions are complete.
²First, ensure only the retainer tabs protrude from
the connectors.
²Second, pull out on the fuel tubes to ensure they
are locked in place.
(5) Connect the molded vacuum hoses to the vac-
uum port on the intake manifold and the cylinder
head cover.
(6) Connect the electrical connectors.
²The throttle position sensor.
²The automatic idle speed control motor.
²The coolant temperature sensor at the thermostat
housing.
²The fuel injectors.
²The air manifold temperature sensor.
²The oxygen sensor.
(7) Connect the CCV vacuum hose and MAP sen-
sor vacuum hose connectors to the throttle body.
(8) Install the power steering pump and bracket
assembly to the water pump and intake manifold.
(9) Connect the accelerator cable and cruise control
cable to the holddown bracket and the throttle arm.
Fig. 12 Crankcase Ventilation (CCV) Hose
(2.5L Engine)
11 - 8 EXHAUST SYSTEM AND INTAKE MANIFOLDJ

MULTI-PORT FUEL INJECTION (MFI)ÐCOMPONENT DESCRIPTION/SYSTEM
OPERATION
INDEX
page page
Air Conditioning (A/C) Clutch RelayÐPCM Output.24
Air Conditioning (A/C) ControlsÐPCM Input.... 19
Auto Shut Down (ASD) RelayÐPCM Output.... 24
Automatic Shut Down (ASD) SenseÐPCM Input . 19
Battery VoltageÐPCM Input................ 19
Brake SwitchÐPCM Input.................. 20
Camshaft Position SensorÐPCM Input........ 20
Crankshaft Position SensorÐPCM Input....... 20
Data Link ConnectorÐPCM Input............ 20
Data Link ConnectorÐPCM Output........... 24
EMR LampÐPCM Output.................. 24
Engine Coolant Temperature SensorÐPCM Input . 21
Extended Idle SwitchÐPCM Input............ 21
Fuel InjectorsÐPCM Output................ 25
Fuel Pressure Regulator................... 30
Fuel Pump RelayÐPCM Output............. 25
Fuel Rail............................... 30
General Information....................... 17
Generator FieldÐPCM Output............... 25
Generator LampÐPCM Output.............. 25
Idle Air Control (IAC) MotorÐPCM Output...... 25
Ignition Circuit SenseÐPCM Input............ 21
Ignition CoilÐPCM Output.................. 26Intake Air Temperature SensorÐPCM Input.... 20
Malfunction Indicator LampÐPCM Output...... 26
Manifold Absolute Pressure (MAP) SensorÐ
PCM Input............................ 21
Open Loop/Closed Loop Modes of Operation . . . 27
Overdrive/Override Switch.................. 22
Oxygen (O2S) SensorÐPCM Input........... 22
Park/Neutral SwitchÐPCM Input............. 22
Power Ground........................... 22
Power Steering Pressure SwitchÐPCM Input . . . 22
Powertrain Control Module (PCM)............ 18
Radiator Fan RelayÐPCM Output............ 26
SCI ReceiveÐPCM Input.................. 22
SCI TransmitÐPCM Output................. 26
Sensor ReturnÐPCM Input................. 23
Shift IndicatorÐPCM Output................ 26
Speed ControlÐPCM Input................. 23
Speed ControlÐPCM Output................ 27
TachometerÐPCM Output.................. 27
Throttle Body............................ 29
Throttle Position Sensor (TPS)ÐPCM Input..... 23
Torque Converter Clutch RelayÐPCM Output . . . 27
Vehicle Speed SensorÐPCM Input........... 23
GENERAL INFORMATION
All 2.5L 4 cylinder and 4.0L 6 cylinder engines are
equipped with sequential Multi-Port Fuel Injection
(MFI). The MFI system provides precise air/fuel ra-
tios for all driving conditions.
The Powertrain Control Module (PCM) operates
the fuel system. The PCM was formerly referred to
as the SBEC or engine controller. The PCM is a pre-
programmed, dual microprocessor digital computer.
It regulates ignition timing, air-fuel ratio, emission
control devices, charging system, speed control, air
conditioning compressor clutch engagement and idle
speed. The PCM can adapt its programming to meet
changing operating conditions.
Powertrain Control Module (PCM) Inputsrep-
resent the instantaneous engine operating conditions.
Air-fuel mixture and ignition timing calibrations for
various driving and atmospheric conditions are pre-
programmed into the PCM. The PCM monitors and
analyzes various inputs. It then computes engine fuel
and ignition timing requirements based on these in-
puts. Fuel delivery control and ignition timing will
then be adjusted accordingly.
Other inputs to the PCM are provided by the brake
light switch, air conditioning select switch and the
speed control switches. All inputs to the PCM are
converted into signals.
Electrically operated fuel injectors spray fuel in
precise metered amounts into the intake port directlyabove the intake valve. The injectors are fired in a
specific sequence by the PCM. The PCM maintains
an air/fuel ratio of 14.7 to 1 by constantly adjusting
injector pulse width. Injector pulse width is the
length of time that the injector opens and sprays fuel
into the chamber. The PCM adjusts injector pulse
width by opening and closing the ground path to the
injector.
Manifold absolute pressure (air density) and engine
rpm (speed) are the primary inputs that determine
fuel injector pulse width. The PCM also monitors
other inputs when adjusting air-fuel ratio.
Inputs That Effect Fuel Injector Pulse Width
²Exhaust gas oxygen content
²Engine coolant temperature
²Manifold absolute pressure (MAP)
²Engine speed
²Throttle position
²Battery voltage
²Air conditioning selection
²Transmission gear selection (automatic transmis-
sions only)
²Speed control
The powertrain control module (PCM) adjusts igni-
tion timing by controlling ignition coil operation. The
ignition coil receives battery voltage when the igni-
tion key is in the run or starter position. The PCM
provides a ground for the ignition coil. The coil dis-
JFUEL SYSTEM 14 - 17

charges when the PCM supplies a ground. By switch-
ing the ground path on and off, the PCM regulates
ignition timing.
The sensors and switches that provide inputs to the
powertrain control module (PCM) comprise the En-
gine Control System. It is also comprised of the PCM
Outputs (engine control devices that the are operated
by the PCM).
SYSTEM DIAGNOSIS
The powertrain control module (PCM) tests many
of its own input and output circuits. If a Diagnostic
Trouble Code (DTC) is found in a major system, this
information is stored in the PCM memory. Refer to
On-Board Diagnostics in the MFI SystemÐGeneral
Diagnosis section of this group for DTC information.
POWERTRAIN CONTROL MODULE (PCM)
The PCM operates the fuel system. The PCM was
formerly referred to as the SBEC or engine control-
ler. The PCM is a pre-programmed, dual micropro-
cessor digital computer. It regulates ignition timing,
air-fuel ratio, emission control devices, charging sys-
tem, speed control, air conditioning compressor
clutch engagement and idle speed. The PCM can
adapt its programming to meet changing operating
conditions.
On XJ models, the PCM is located in the engine
compartment next to the air cleaner (Fig. 1). On YJ
models, the PCM is located in the engine compart-
ment behind the windshield washer fluid reservoir
(Fig. 2).
The PCM receives input signals from various
switches and sensors. Based on these inputs, the
PCM regulates various engine and vehicle operations
through different system components. These compo-
nents are referred to as PCM Outputs. The sensors
and switches that provide inputs to the PCM are con-
sidered PCM Inputs.The PCM adjusts ignition timing based upon in-
puts it receives from sensors that react to: engine
rpm, manifold absolute pressure, coolant tempera-
ture, throttle position, transmission gear selection
(automatic transmission), vehicle speed and the
brake switch.
The PCM adjusts idle speed based on inputs it re-
ceives from sensors that react to: throttle position,
vehicle speed, transmission gear selection, coolant
temperature and from inputs it receives from the air
conditioning clutch switch and brake switch.
Based on inputs that it receives, the PCM adjusts
ignition coil dwell. The PCM also adjusts the gener-
ator charge rate through control of the generator
field and provides speed control operation.
Powertrain Control Module (PCM) Inputs:
²Generator output
²A/C request (if equipped with factory A/C)
²A/C select (if equipped with factory A/C)
²Auto shut down (ASD) sense
²Intake air temperature sensor
²Battery voltage
²Brake switch
²Engine coolant temperature sensor
²Crankshaft position sensor
²Ignition circuit sense (ignition switch in run posi-
tion)
²Manifold absolute pressure sensor
²Overdrive/override switch
²Oxygen sensor
²Park/neutral switch (auto. trans. only)
²SCI receive (DRB scan tool connection)
²Speed control resume switch
²Speed control set switch
²Speed control on/off switch
²Camshaft position sensor signal
²Throttle position sensor
²Vehicle speed sensor
²Sensor return
²Power ground
Fig. 1 PCM LocationÐXJ Models
Fig. 2 PCM LocationÐYJ Models
14 - 18 FUEL SYSTEMJ

²Signal ground
Powertrain Control Module (PCM) Outputs
²A/C clutch relay
²Idle air control (IAC) motor
²Auto shut down (ASD) relay
²Generator field
²Malfunction indicator lamp
²Fuel injectors
²Fuel pump relay
²Ignition coil
²SCI transmit (DRB scan tool connection)
²Shift indicator lamp (manual transmission only)
²Speed control vacuum solenoid
²Speed control vent solenoid
²Tachometer (on instrument panel, if equipped)
²Torque converter clutch relay (3-speed auto. trans.
only)
The PCM contains a voltage convertor. This con-
verts battery voltage to a regulated 8.0 volts. It is
used to power the crankshaft position sensor and
camshaft position sensor. The PCM also provides a
five (5) volt supply for the Manifold Absolute Pres-
sure (MAP) sensor and Throttle Position Sensor
(TPS).
AIR CONDITIONING (A/C) CONTROLSÐPCM INPUT
The A/C control system information applies to fac-
tory installed air conditioning units only.
A/C SELECT SIGNAL:When the A/C switch is
in the ON position and the A/C low pressure switch
is closed, an input signal is sent to the powertrain
control module (PCM). The signal informs the PCM
that the A/C has been selected. The PCM adjusts idle
speed to a pre-programmed rpm through the idle air
control (IAC) motor to compensate for increased en-
gine load.
A/C REQUEST SIGNAL:Once A/C has been se-
lected, the PCM receives the A/C request signal from
the evaporator switch. The input indicates that the
evaporator temperature is in the proper range for
A/C application. The PCM uses this input to cycle
the A/C compressor clutch (through the A/C relay). It
will also determine the correct engine idle speed
through the IAC motor position.
If the A/C low pressure switch opens (indicating a
low refrigerant level), the PCM will not receive an
A/C select signal. The PCM will then remove the
ground from the A/C relay. This will deactivate the
A/C compressor clutch.
If the evaporator switch opens, (indicating that
evaporator is not in proper temperature range), the
PCM will not receive the A/C request signal. The
PCM will then remove the ground from the A/C re-
lay, deactivating the A/C compressor clutch.
AUTOMATIC SHUT DOWN (ASD) SENSEÐPCM
INPUT
A 12 volt signal at this input indicates to the PCM
that the ASD has been activated. The ASD relay is
located in the power distribution center (PDC) in the
engine compartment (Figs. 3 or 4). It is used to con-
nect oxygen sensor heater element, ignition coil, gen-
erator field winding and fuel injectors to 12 volt +
power supply. Also refer to Automatic Shut Down
RelayÐPCM Output.
This input is used only to sense that the ASD relay
is energized. If the PCM does not see 12 volts at this
input when the ASD should be activated, it will set a
Diagnostic Trouble Code (DTC).
BATTERY VOLTAGEÐPCM INPUT
The battery voltage input provides power to the
powertrain control module (PCM). It also informs the
PCM what voltage level is supplied to the ignition
coil and fuel injectors.
If battery voltage is low, the PCM will increase in-
jector pulse width (period of time that the injector is
Fig. 3 Power Distribution CenterÐYJ Models
Fig. 4 Power Distribution CenterÐXJ Models
JFUEL SYSTEM 14 - 19

energized). This is done to compensate for the re-
duced flow through injector caused by the lowered
voltage.
BRAKE SWITCHÐPCM INPUT
When the brake light switch is activated, the pow-
ertrain control module (PCM) receives an input indi-
cating that the brakes are being applied. After
receiving this input, the PCM maintains idle speed
to a scheduled rpm through control of the idle air
control (IAC) motor. The brake switch input is also
used to operate the speed control system.
CAMSHAFT POSITION SENSORÐPCM INPUT
A sync signal is provided by the camshaft position
sensor located in the ignition distributor (Fig. 5). The
sync signal from this sensor works in conjunction
with the crankshaft position sensor to provide the
powertrain control module (PCM) with inputs. This
is done to establish and maintain correct injector fir-
ing order.
Refer to Camshaft Position Sensor in Group 8D, Ig-
nition System for more information.
DATA LINK CONNECTORÐPCM INPUT
The data link connector (diagnostic scan tool con-
nector) links the DRB scan tool with the powertrain
control module (PCM). The data link connector is lo-
cated in the engine compartment (Figs. 6 or 7). For
operation of the DRB scan tool, refer to the appropri-
ate Powertrain Diagnostic Procedures service man-
ual.
The data link connector uses two different pins on
the PCM. One is for Data Link Transmit and the
other is for Data Link Receive.
INTAKE AIR TEMPERATURE SENSORÐPCM INPUT
The intake manifold air temperature sensor is in-
stalled in the intake manifold with the sensor ele-
ment extending into the air stream (Figs. 8 or 9).
The sensor provides an input voltage to the power-
train control module (PCM) indicating intake mani-
fold air temperature. The input is used along with
inputs from other sensors to determine injector pulse
width. As the temperature of the air-fuel stream in
the manifold varies, the sensor resistance changes.
This results in a different input voltage to the PCM.
CRANKSHAFT POSITION SENSORÐPCM INPUT
This sensor is a Hall Effect device that detects
notches in the flywheel (manual transmission), or
flexplate (automatic transmission).
This sensor is used to indicate to the powertrain
control module (PCM) that a spark and or fuel injec-
tion event is to be required. The output from this
sensor, in conjunction with the camshaft position
sensor signal, is used to differentiate between fuel in-
jection and spark events. It is also used to synchro-
nize the fuel injectors with their respective cylinders.
Fig. 5 Camshaft Position Sensor
Fig. 6 Data Link ConnectorÐYJ ModelsÐTypical
Fig. 7 Data Link ConnectorÐXJ ModelsÐTypical
14 - 20 FUEL SYSTEMJ

Refer to Group 8D, Ignition System for more crank-
shaft position sensor information.
The engine will not operate if the PCM does not re-
ceive a crankshaft position sensor input.
ENGINE COOLANT TEMPERATURE SENSORÐPCM
INPUT
The coolant temperature sensor is installed in the
thermostat housing (Fig. 10) and protrudes into the
water jacket. The sensor provides an input voltage to
the powertrain control module (PCM) relating cool-
ant temperature. The PCM uses this input along
with inputs from other sensors to determine injector
pulse width and ignition timing. As coolant temper-
ature varies, the coolant temperature sensor's resis-
tance changes. The change in resistance results in a
different input voltage to the PCM.
When the engine is cold, the PCM will operate in
Open Loop cycle. It will demand slightly richer air-
fuel mixtures and higher idle speeds. This is done
until normal operating temperatures are reached.
Refer to Open Loop/Closed Loop Modes of Opera-
tion in this section of the group for more information.
EXTENDED IDLE SWITCHÐPCM INPUT
OPTIONAL POLICE PACKAGE ONLY
The extended idle switch is used to raise the en-
gine idle speed to approximately 1000 rpm. This is
when the shifter is in either the Park or Neutral po-
sition. A rocker-type 2-wire switch (extended idle
switch) is mounted to the instrument panel. This
switch will supply a ground circuit to the powertrain
control module (PCM).The switch is available
only with 4.0L engine when supplied with the
optional police package.
For testing and diagnosis of this switch and its cir-
cuit, refer to the MFI SystemÐGeneral Diagnosis
section of this group.
IGNITION CIRCUIT SENSEÐPCM INPUT
The ignition circuit sense input tells the powertrain
control module (PCM) the ignition switch has ener-
gized the ignition circuit. Refer to the wiring dia-
grams for circuit information.
MANIFOLD ABSOLUTE PRESSURE (MAP)
SENSORÐPCM INPUT
The MAP sensor reacts to absolute pressure in the
intake manifold. It provides an input voltage to the
powertrain control module (PCM). As engine load
changes, manifold pressure varies. The change in
manifold pressure causes MAP sensor voltage to
change. The change in MAP sensor voltage results in
a different input voltage to the PCM. The input volt-
age level supplies the PCM with information about
ambient barometric pressure during engine start-up
(cranking) and engine load while the engine is run-
ning. The PCM uses this input along with inputs
from other sensors to adjust air-fuel mixture.
The MAP sensor is mounted on the dash panel.
The sensor is connected to the throttle body with a
vacuum hose and to the PCM electrically.
Fig. 8 Sensor LocationÐ4.0L Engine
Fig. 9 Sensor LocationÐ2.5L Engine
Fig. 10 Coolant Temperature SensorÐTypical
JFUEL SYSTEM 14 - 21

OVERDRIVE/OVERRIDE SWITCH
On vehicles equipped with overdrive, the power-
train control module (PCM) regulates the 3-4 over-
drive up-shift and down-shift through the overdrive
solenoid.
Refer to Group 21 for more information.
OXYGEN (O2S) SENSORÐPCM INPUT
The O2S sensor is located in the exhaust down pipe
(Fig. 11). It provides an input voltage to the power-
train control module (PCM) relating the oxygen con-
tent of the exhaust gas. The PCM uses this
information to fine tune the air-fuel ratio by adjust-
ing injector pulse width.
The O2S sensor produces voltages from 0 to 1 volt.
This voltage will depend 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 produces 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
correct sensor temperature at all times allows the
system to enter into closed loop operation sooner.
In Closed Loop operation, the powertrain control
module (PCM) monitors the O2S sensor input (along
with other inputs). It then adjusts the injector pulse
width accordingly. During Open Loop operation, the
PCM ignores the O2S sensor input and adjusts injec-
tor pulse width to a preprogrammed value (based on
other sensor inputs).
PARK/NEUTRAL SWITCHÐPCM INPUT
The park/neutral switch is located on the transmis-
sion housing and provides an input to the powertrain
control module (PCM). This will indicate that the au-
tomatic transmission is in Park, Neutral or a drivegear selection. This input is used to determine idle
speed (varying with gear selection), fuel injector
pulse width and ignition timing advance. Refer to
Group 21, Transmissions, for testing, replacement
and adjustment information.
POWER GROUND
The power ground is used to control ground circuits
for the following powertrain control module (PCM)
loads:
²Generator Field Winding
²8 volt (PCM) power supply
²Fuel Injectors
²Ignition Coil
POWER STEERING PRESSURE SWITCHÐPCM
INPUT
A pressure sensing switch is included in the power
steering system (mounted on the high-pressure line).
This switch will be on vehicles equipped with a 2.5L
engine and power steering. The switch (Fig. 12 YJ
Models or Fig. 13 XJ Models) provides an input to
the PCM. This input is provided during periods of
high pump load and low engine rpm; such as during
parking maneuvers. The PCM will then increase the
idle speed through the idle air control (IAC) motor.
This is done to prevent the engine from stalling un-
der the increased load.
When steering pump pressure exceeds 1896 kPa6
172 kPa (275625 psi) the PCM will increase the en-
gine idle speed. This will prevent the engine from
stalling.
SCI RECEIVEÐPCM INPUT
SCI Receive is the serial data communication re-
ceive circuit for the DRB scan tool. The powertrain
control module (PCM) receives data from the DRB
through the SCI Receive circuit.
Fig. 11 Heated Oxygen Sensor LocationÐTypical
Fig. 12 Power Steering Pump Pressure SwitchÐYJ
Models
14 - 22 FUEL SYSTEMJ

SPEED CONTROLÐPCM INPUT
The speed control system provides three separate
inputs to the powertrain control module (PCM); On/
Off, Set and Resume. The On/Off input informs the
PCM that the speed control system has been acti-
vated. The Set input informs the PCM that a fixed
vehicle speed has been selected. The Resume input
indicates to the PCM that the previous fixed speed is
requested.
The speed control operating range is from 50 km/h
to 142 km/h (35 to 85 mph). Inputs that effect speed
control operation are:
²Park/neutral switch
²Vehicle speed sensor
²Throttle position sensor
Refer to Group 8H for further speed control infor-
mation.
SENSOR RETURNÐPCM INPUT
Sensor Return provides a low noise ground refer-
ence for all system sensors.
THROTTLE POSITION SENSOR (TPS)ÐPCM INPUT
The Throttle Position Sensor (TPS) is mounted on
the throttle body (Figs. 14 or 15). The TPS is a vari-
able resistor that provides the powertrain control
module (PCM) with an input signal (voltage) that
represents throttle blade position. The sensor is con-
nected to the throttle blade shaft. 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
PCM receives an input signal voltage from the TPS.
This will vary in an approximate range of from 1
volt at minimum throttle opening (idle), to 4 volts at
wide open throttle. Along with inputs from other sen-
sors, the PCM uses the TPS input to determine cur-rent engine operating conditions. In response to
engine operating conditions, the PCM will adjust fuel
injector pulse width and ignition timing.
VEHICLE SPEED SENSORÐPCM INPUT
The vehicle speed sensor (Fig. 16) is located in the
extension housing of the transmission (2 wheel drive)
or on the transfer case extension housing (4 wheel
drive). The sensor input is used by the powertrain
control module (PCM) to determine vehicle speed and
distance traveled.
The speed sensor generates 8 pulses per sensor rev-
olution. These signals, in conjunction with a closed
throttle signal from the throttle position sensor, indi-
cate a closed throttle deceleration to the PCM. When
the vehicle is stopped at idle, a closed throttle signal
is received by the PCM (but a speed sensor signal is
not received).
Under deceleration conditions, the PCM adjusts the
idle air control (IAC) motor to maintain a desired
MAP value. Under idle conditions, the PCM adjusts
the IAC motor to maintain a desired engine speed.
Fig. 13 Power Steering Pump Pressure SwitchÐXJ
Models
Fig. 14 Throttle Position SensorÐ2.5L Engine
Fig. 15 Throttle Position SensorÐ4.0L Engine
JFUEL SYSTEM 14 - 23