1995 JEEP YJ engine coolant

AIR CONDITIONING (A/C) CLUTCH RELAYÐPCM
OUTPUT
The powertrain control module (PCM) activates the
A/C compressor through the A/C clutch relay. The
PCM regulates A/C compressor operation by switch-
ing the ground circuit for the A/C clutch relay on and
off. The relay is located in the power distribution
center (PDC) (Figs. 17 or 18). For the location of the
relay within the PDC, refer to label on PDC cover.
When the PCM receives a request for A/C from A/C
evaporator switch, it will adjust idle air control (IAC)
motor position. This is done to increase idle speed.
The PCM will then activate the A/C clutch through
the A/C clutch relay. The PCM adjusts idle air control
(IAC) stepper motor position to compensate for in-
creased engine load from the A/C compressor.By switching the ground path for the relay on and
off, the PCM is able to cycle the A/C compressor
clutch. This is based on changes in engine operating
conditions. If, during A/C operation, the PCM senses
low idle speeds or a wide open throttle condition, it
will de-energize the relay. This prevents A/C clutch
engagement. The relay will remain de-energized until
the idle speed increases or the wide open throttle
condition exceeds 15 seconds or no longer exists. The
PCM will also de-energize the relay if coolant tem-
perature exceeds 125ÉC (257ÉF).
AUTO SHUTDOWN (ASD) RELAYÐPCM OUTPUT
The ASD relay is located in the power distribution
center (PDC) (Figs. 17 or 18). For the location of this
relay within the PDC, refer to label on PDC cover.
The ASD supplies battery voltage to the fuel injec-
tors, ignition coil and generator field winding. The
ground circuit for the coil in the ASD relay is con-
trolled by the powertrain control module (PCM). The
PCM operates the relay by switching the ground cir-
cuit on and off.
The fuel pump relay is controlled by the PCM
through same circuit that the ASD relay is con-
trolled.
The powertrain control module (PCM) energizes
the fuel pump through the fuel pump relay. (The
PCM was formerly referred to as the SBEC or engine
controller). Battery voltage is applied to the relay
from the ignition switch. The relay is energized when
a ground is provided by the PCM. The relay is lo-
cated in the power distribution center (PDC) (Figs.
17 or 18). For the location of fuel pump relay within
PDC, refer to label on PDC cover.
For the 1995 model year, the ballast resistor and
ballast resistor bypass relay are no longer used to
control the fuel pump circuit.
Fig. 16 Vehicle Speed SensorÐTypical
Fig. 17 PDCÐYJ Models
Fig. 18 PDCÐXJ Models
14 - 26 FUEL SYSTEM COMPONENT DESCRIPTION/SYSTEM OPERATIONJ

The throttle body has an air control passage that
provides air for the engine at idle (the throttle plate
is closed). The IAC motor pintle protrudes into the
air control passage and regulates air flow through it.
Based on various sensor inputs, the powertrain con-
trol module (PCM) adjusts engine idle speed by mov-
ing the IAC motor pintle in and out of the air control
passage. The IAC motor is positioned when the igni-
tion key is turned to the On position.
A (factory adjusted) set screw is used to mechani-
cally limit the position of the throttle body throttle
plate.Never attempt to adjust the engine idle
speed using this screw.All idle speed functions are
controlled by the PCM.
IGNITION COILÐPCM OUTPUT
System voltage is supplied to the ignition coil pos-
itive terminal. The powertrain control module (PCM)
operates the ignition coil.Base (initial) ignition
timing is not adjustable.The PCM adjusts ignition
timing to meet changing engine operating conditions.
The ignition coil is located near the distributor
(Fig. 22).
Refer to Group 8D, Ignition System for additional
information.
MALFUNCTION INDICATOR LAMPÐPCM OUTPUT
The malfunction indicator lamp illuminates each
time the ignition key is turned on. It will stay on for
approximately three seconds as a bulb test. The lamp
is displayed on the instrument panel as the CHECK
ENGINE lamp (Figs. 23 or 24).
If the powertrain control module (PCM) receives an
incorrect signal, or no signal from certain sensors or
emission related systems, the lamp is turned on. This
is a warning that the PCM has recorded a system or
sensor malfunction. In some cases, when a problem is
declared, the PCM will go into a limp-in mode. This
is an attempt to keep the system operating. It signals
an immediate need for service.The lamp can also be used to display a Diagnostic
Trouble Code (DTC). Cycle the ignition switch On-
Off-On-Off-On within three seconds and any codes
stored in the PCM memory will be displayed. This is
done in a series of flashes representing digits. Refer
to On-Board Diagnostics in the General Diagnosis
section of this group for more information.
RADIATOR FAN RELAYÐPCM OUTPUT
XJ MODELS ONLY
The electric radiator cooling fan used in XJ models
(equipped with 4.0L engine, heavy duty cooling
and/or air conditioning) is controlled by the power-
train control module (PCM) through radiator fan re-
lay. The relay is energized when coolant temperature
is above 103ÉC (217ÉF). It will then de-energize when
Fig. 22 Ignition CoilÐTypical
Fig. 23 Check Engine LampÐXJ ModelsÐTypical
Fig. 24 Check Engine LampÐYJ ModelsÐTypical
14 - 28 FUEL SYSTEM COMPONENT DESCRIPTION/SYSTEM OPERATIONJ

coolant temperature drops to 98ÉC (208ÉF). Refer to
Group 7, Cooling Systems for more information.
The relay is located in the power distribution cen-
ter (PDC) (Fig. 25).
The electric radiator cooling fan is not used on YJ
models.
SCI TRANSMITÐPCM OUTPUT
SCI Transmit is the serial data communication
transmit circuit for the DRB scan tool. The power-
train control module (PCM) transmits data to the
DRB through the SCI Transmit circuit.
SHIFT INDICATORÐPCM OUTPUT
Vehicles equipped with manual transmissions have
an Up-Shift indicator lamp. The lamp is controlled by
the powertrain control module (PCM). The lamp illu-
minates on the instrument panel to indicate when
the driver should shift to the next highest gear for
best fuel economy. The PCM will turn the lamp OFF
after 3 to 5 seconds if the shift of gears is not per-
formed. The up-shift lamp will remain off until vehi-
cle stops accelerating and is brought back to range of
up-shift lamp operation. This will also happen if ve-
hicle is shifted into fifth gear.
The indicator lamp is normally illuminated when
the ignition switch is turned on and it is turned off
when the engine is started up. With the engine run-
ning, the lamp is turned on/off depending upon en-
gine speed and load.
SPEED CONTROLÐPCM OUTPUT
Speed control operation is regulated by the power-
train control module (PCM). The PCM controls the
vacuum to the throttle actuator through the speed
control vacuum and vent solenoids. Refer to Group
8H for speed control information.
TACHOMETERÐPCM OUTPUT
The powertrain control module (PCM) supplies en-
gine rpm values to the instrument cluster tachometer
(if equipped). Refer to Group 8E for tachometer infor-
mation.
TORQUE CONVERTER CLUTCH RELAYÐPCM
OUTPUT
ALL 2.5L 4 CYL. WITH 3-SPEED AUTO. TRANS
4.0L 6 CYL. YJ MODELS WITH 3-SPEED AUTO.
TRANS
The transmission mounted torque converter clutch
(TCC) solenoid is used to control the torque con-
verter. The solenoid is controlled through the power-
train control module (PCM) and by the TCC relay.
This relay is used only on vehicles equipped with a
3-speed automatic transmission.
An electrical output signal is sent from the PCM to
the TCC relay after the PCM receives information
from the vehicle speed, MAP, throttle position and
engine coolant temperature sensors. After the TCC
relay receives this necessary information, it will send
a signal to the torque converter clutch solenoid to
control the torque converter.
On YJ models the TCC relay is located in the en-
gine compartment, on the cowl panel and near the
battery (Fig. 26). On XJ models the TCC relay is lo-
cated in the power distribution center (PDC) (Fig.
25).
AIR CLEANER
The air cleaner assembly used on all models (Figs.
27 or 28) is open to ambient air. The blend air door
and vacuum motor that was used on engines of pre-
vious model years to supply heated air, is no longer
used. The air cleaner housing contains the engine air
cleaner element.
Fig. 25 PDCÐXJ Models
Fig. 26 TCC Relay LocationÐYJ Models
JFUEL SYSTEM COMPONENT DESCRIPTION/SYSTEM OPERATION 14 - 29

²The powertrain control module (PCM) pre-posi-
tions the idle air control (IAC) motor.
²The PCM determines atmospheric air pressure
from the MAP sensor input to determine basic fuel
strategy.
²The PCM monitors the engine coolant temperature
sensor input. The PCM modifies fuel strategy based
on this input.
²Intake manifold air temperature sensor input is
monitored
²Throttle position sensor (TPS) is monitored
²The auto shutdown (ASD) relay is energized by the
PCM for approximately three seconds.
²The fuel pump is energized through the fuel pump
relay by the PCM. The fuel pump will operate for ap-
proximately three seconds unless the engine is oper-
ating or the starter motor is engaged
²The O2S sensor heater element is energized
through the fuel pump relay. The O2S sensor input is
not used by the PCM to calibrate air-fuel ratio dur-
ing this mode of operation.
²The up-shift indicator lamp is illuminated (manual
transmission only).
ENGINE START-UP MODE
This is an Open Loop mode. The following actions
occur when the starter motor is engaged.
The powertrain control module (PCM) receives in-
puts from:
²Battery voltage
²Engine coolant temperature sensor
²Crankshaft position sensor
²Intake manifold air temperature sensor
²Manifold absolute pressure (MAP) sensor
²Throttle position sensor (TPS)
²Starter motor relay
²Camshaft position sensor signal
The PCM monitors the crankshaft position sensor.
If the PCM does not receive a crankshaft position
sensor signal within 3 seconds of cranking the en-
gine, it will shut down the fuel injection system.
The fuel pump is activated by the PCM through
the fuel pump relay.
Voltage is applied to the fuel injectors with the
PCM. The PCM will then control the injection se-
quence and injector pulse width by turning the
ground circuit to each individual injector on and off.
The PCM determines the proper ignition timing ac-
cording to input received from the crankshaft posi-
tion sensor.
ENGINE WARM-UP MODE
This is an Open Loop mode. During engine warm-
up, the powertrain control module (PCM) receives in-
puts from:
²Battery voltage
²Crankshaft position sensor
²Engine coolant temperature sensor²Intake manifold air temperature sensor
²Manifold absolute pressure (MAP) sensor
²Throttle position sensor (TPS)
²Camshaft position sensor signal (in the distribu-
tor)
²Park/neutral switch (gear indicator signalÐauto.
trans. only)
²Air conditioning select signal (if equipped)
²Air conditioning request signal (if equipped)
Based on these inputs the following occurs:
²Voltage is applied to the fuel injectors with the
powertrain control module (PCM). The PCM will
then control the injection sequence and injector pulse
width by turning the ground circuit to each individ-
ual injector on and off.
²The PCM adjusts engine idle speed through the
idle air control (IAC) motor and adjusts ignition tim-
ing.
²The PCM operates the A/C compressor clutch
through the clutch relay. This is done if A/C has been
selected by the vehicle operator and requested by the
A/C thermostat.
²If the vehicle has a manual transmission, the up-
shift lamp is operated by the PCM.
²When engine has reached operating temperature,
the PCM will begin monitoring O2S sensor input.
The system will then leave the warm-up mode and go
into closed loop operation.
IDLE MODE
When the engine is at operating temperature, this
is a Closed Loop mode. At idle speed, the powertrain
control module (PCM) receives inputs from:
²Air conditioning select signal (if equipped)
²Air conditioning request signal (if equipped)
²Battery voltage
²Crankshaft position sensor
²Engine coolant temperature sensor
²Intake manifold air temperature sensor
²Manifold absolute pressure (MAP) sensor
²Throttle position sensor (TPS)
²Camshaft position sensor signal (in the distribu-
tor)
²Battery voltage
²Park/neutral switch (gear indicator signalÐauto.
trans. only)
²Oxygen sensor
Based on these inputs, the following occurs:
²Voltage is applied to the fuel injectors with the
powertrain control module (PCM). The PCM will
then control injection sequence and injector pulse
width by turning the ground circuit to each individ-
ual injector on and off.
²The PCM monitors the O2S sensor input and ad-
justs air-fuel ratio by varying injector pulse width. It
also adjusts engine idle speed through the idle air
control (IAC) motor.
JFUEL SYSTEM COMPONENT DESCRIPTION/SYSTEM OPERATION 14 - 31

²The PCM adjusts ignition timing by increasing
and decreasing spark advance.
²The PCM operates the A/C compressor clutch
through the clutch relay. This happens if A/C has
been selected by the vehicle operator and requested
by the A/C thermostat.
The optional Extended Idle Switch is used to raise
the engine idle speed to approximately 1000 rpm.
This is when the shifter is in either the Park or Neu-
tral position. 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.
CRUISE MODE
When the engine is at operating temperature, this
is a Closed Loop mode. At cruising speed, the power-
train control module (PCM) receives inputs from:
²Air conditioning select signal (if equipped)
²Air conditioning request signal (if equipped)
²Battery voltage
²Engine coolant temperature sensor
²Crankshaft position sensor
²Intake manifold air temperature sensor
²Manifold absolute pressure (MAP) sensor
²Throttle position sensor (TPS)
²Camshaft position sensor signal (in the distribu-
tor)
²Park/neutral switch (gear indicator signalÐauto.
trans. only)
²Oxygen (O2S) sensor
Based on these inputs, the following occurs:
²Voltage is applied to the fuel injectors with the
PCM. The PCM will then adjust the injector pulse
width by turning the ground circuit to each individ-
ual injector on and off.
²The PCM monitors the O2S sensor input and ad-
justs air-fuel ratio. It also adjusts engine idle speed
through the idle air control (IAC) motor.
²The PCM adjusts ignition timing by turning the
ground path to the coil on and off.
²The PCM operates the A/C compressor clutch
through the clutch relay. This happens if A/C has
been selected by the vehicle operator and requested
by the A/C thermostat.
ACCELERATION MODE
This is an Open Loop mode. The powertrain control
module (PCM) recognizes 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 in-
creased throttle opening.
DECELERATION MODE
When the engine is at operating temperature, this
is an Open Loop mode. During hard deceleration, the
powertrain control module (PCM) receives the follow-
ing inputs.
²Air conditioning select signal (if equipped)
²Air conditioning request signal (if equipped)
²Battery voltage
²Engine coolant temperature sensor
²Crankshaft position sensor
²Intake manifold air temperature sensor
²Manifold absolute pressure (MAP) sensor
²Throttle position sensor (TPS)
²Camshaft position sensor signal (in the distribu-
tor)
²Park/neutral switch (gear indicator signalÐauto.
trans. only)
If the vehicle is under hard deceleration with the
proper rpm and closed throttle conditions, the PCM
will ignore the oxygen sensor input signal. The PCM
will enter a fuel cut-off strategy in which it will not
supply battery voltage to the injectors. If a hard de-
celeration does not exist, the PCM will determine the
proper injector pulse width and continue injection.
Based on the above inputs, the PCM will adjust en-
gine idle speed through the idle air control (IAC) mo-
tor.
The PCM adjusts ignition timing by turning the
ground path to the coil on and off.
The PCM opens the ground circuit to the A/C
clutch relay to disengage the A/C compressor clutch.
This is done until the vehicle is no longer under de-
celeration (if the A/C system is operating).
WIDE OPEN THROTTLE MODE
This is an Open Loop mode. During wide open
throttle operation, the powertrain control module
(PCM) receives the following inputs.
²Battery voltage
²Crankshaft position sensor
²Engine coolant temperature sensor
²Intake manifold air temperature sensor
²Manifold absolute pressure (MAP) sensor
²Throttle position sensor (TPS)
²Camshaft position sensor signal (in the distribu-
tor)
During wide open throttle conditions, the following
occurs:
²Voltage is applied to the fuel injectors with the
powertrain control module (PCM). The PCM will
then control the injection sequence and injector pulse
width by turning the ground circuit to each individ-
ual injector on and off. The PCM ignores the oxygen
sensor input signal and provides a predetermined
amount of additional fuel. This is done by adjusting
injector pulse width.
²The PCM adjusts ignition timing by turning the
ground path to the coil on and off.
14 - 32 FUEL SYSTEM COMPONENT DESCRIPTION/SYSTEM OPERATIONJ

MULTI-PORT FUEL INJECTION (MFI)ÐGENERAL DIAGNOSIS
INDEX
page page
Automatic Shutdown (ASD) Relay Testing....... 46
Camshaft Position Sensor Test............... 46
Crankshaft Position Sensor Test.............. 47
Diagnostic Trouble Code (DTC)............... 54
DRB Scan Tool........................... 54
Engine Coolant Temperature Sensor Test....... 46
Extended Idle Switch Test................... 48
Fuel Injector Test......................... 51
Fuel Pump Relay Testing................... 47
Fuel System Pressure Test.................. 51
General Information....................... 35
Idle Air Control Motor Test................... 49
Intake Manifold Air Temperature Sensor Test..... 46Manifold Absolute Pressure (Map) Sensor Test . . . 47
On-Board Diagnostics (OBD)................. 51
Oxygen Sensor (O2S) Heating Element Test..... 48
Pcm System Schematics.................... 41
Powertrain Control Module (PCM) 60-Way
Connector............................. 40
RelaysÐOperation/Testing.................. 50
Starter Motor Relay Test.................... 51
Throttle Position Sensor (TPS) Test............ 48
Torque Converter Clutch Relay Test............ 48
Vehicle Speed Sensor Test.................. 48
Visual Inspection.......................... 35
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.
VISUAL INSPECTION
A visual inspection for loose, disconnected, or incor-
rectly routed wires and hoses should be made. This
should be done before attempting to diagnose or ser-
vice the fuel injection system. A visual check will
help spot these faults and save unnecessary test and
diagnostic time. A thorough visual inspection will in-
clude the following checks:
(1) Verify that the 60-way connector is fully in-
serted into the connector of the powertrain control
module (PCM) (Figs. 1 or 2). Verify that the connec-
tor mounting bolt is tightened to 4 Nzm (35 in. lbs.)
torque.(2) Inspect the battery cable connections. Be sure
they are clean and tight.
(3) Inspect fuel pump relay and air conditioning
compressor clutch relay (if equipped). Inspect ASD
relay and radiator fan relay (if equipped) connec-
tions. Inspect starter motor relay connections. In-
spect relays for signs of physical damage and
corrosion. The relays are installed in the power dis-
tribution center (PDC) (Figs. 3 or 4).
(4) Inspect ignition coil connections. Verify that coil
secondary cable is firmly connected to coil (Figs. 5 or
6).
(5) Verify that distributor cap is correctly attached
to distributor. Be sure that spark plug cables are
firmly connected to the distributor cap and the spark
plugs in their correct firing order. Be sure that coil
cable is firmly connected to distributor cap and coil.
Be sure that camshaft position sensor wire connector
is firmly connected to harness connector (Figs. 7 or
8). Inspect spark plug condition. Refer to Group 8D,
Fig. 1 PCMÐYJ Models
Fig. 2 PCMÐXJ Models
JFUEL SYSTEM GENERAL DIAGNOSIS 14 - 35

(18) Inspect engine ground strap connections at
dash panel and rear cylinder head bolt (Fig. 20). For
ground locations, refer to Group 8, Wiring.
(19) Verify that MAP sensor electrical connector is
firmly connected to MAP sensor (Fig. 21). Verify that
vacuum hose is firmly connected to MAP sensor and
to the intake manifold.
(20) Verify that fuel injector wire harness connec-
tors are firmly connected to the fuel injectors in the
correct order. Each harness connector is tagged with
the number of its corresponding fuel injector (Fig.
22).
(21) Verify that harness connectors are firmly con-
nected to idle air control (IAC) motor and throttle po-
sition sensor (TPS) (Figs. 18, 19 or 23).
(22) Verify that wire harness connector is firmly
connected to the engine coolant temperature sensor
(Fig. 24).
(23) Verify that oxygen sensor wire connector is
firmly connected to the sensor. Inspect sensor and
connector for damage (Fig. 25).
(24) Raise and support the vehicle.(25) Inspect for pinched or leaking fuel tubes. In-
spect for pinched cracked or leaking fuel hoses.
(26) Inspect for exhaust system restrictions such
as pinched exhaust pipes, collapsed muffler or
Fig. 20 Engine Ground Strap ConnectionsÐTypical
Fig. 21 MAP SensorÐTypical
Fig. 22 Fuel Injector Wire HarnessÐTypical
Fig. 23 IAC Motor and TPSÐ2.5L Engine
Fig. 24 Engine Coolant Temperature SensorÐ
Typical
JFUEL SYSTEM GENERAL DIAGNOSIS 14 - 39

AUTOMATIC SHUTDOWN (ASD) RELAY TESTING
To perform a complete test of the ASD relay and its
circuitry, refer to the DRB scan tool and appropriate
Powertrain Diagnostics Procedures manual. To test
the relay only, refer to RelaysÐOperation/Testing in
this section of the group.
CAMSHAFT POSITION SENSOR TEST
Refer to Group 8D, Ignition Systems, for Camshaft
Position Sensor testing.
ENGINE COOLANT TEMPERATURE SENSOR TEST
To perform a complete test of the engine coolant
temperature sensor and its circuitry, refer to DRB
scan tool and appropriate Powertrain Diagnostics
Procedures manual. To test the sensor only, refer to
the following:
Disconnect wire harness connector from engine
coolant temperature sensor (Fig. 33).
Test the resistance of the sensor with a high input
impedance (digital) volt-ohmmeter. The resistance
should be less than 1000 ohms with the engine at its
correct operating temperature. Refer to the Coolant
Temperature Sensor/Manifold Air Temperature Sen-
sor resistance chart. Replace the sensor if it is not
within the range of resistance specified in the chart.
Test continuity of the wire harness. Do this be-
tween the Powertrain Control Module (PCM) wire
harness connector terminal-2 and the sensor connec-
tor terminal. Also test continuity of wire harness ter-
minal-4 to the sensor connector terminal. Repair the
wire harness if an open circuit is indicated.
INTAKE MANIFOLD AIR TEMPERATURE SENSOR
TEST
To perform a complete test of the sensor and its cir-
cuitry, refer to DRB scan tool and appropriate Pow-ertrain Diagnostics Procedures manual. To test the
sensor only, refer to the following:
Disconnect the wire harness connector from the in-
take manifold air temperature sensor (Figs. 34 or
35).
Test the resistance of the sensor with an input im-
pedance (digital) volt-ohmmeter. The resistance
should be less than 4000 ohms with the engine at op-
erating temperature. The longer the engine idles, the
warmer the intake manifold temperature will be-
come. Refer to the Coolant Temperature Sensor/Man-
ifold Air Temperature Sensor resistance chart.
Replace the sensor if it is not within the range of re-
sistance specified in the chart.
Fig. 33 Engine Coolant Temperature SensorÐ
Typical
SENSOR RESISTANCE (OHMS)ÐCOOLANT
TEMPERATURE SENSOR/MANIFOLD AIR
TEMPERATURE
Fig. 34 Air Temperature SensorÐ2.5L Engine
14 - 46 FUEL SYSTEM GENERAL DIAGNOSISJ