engine coolant JEEP WRANGLER 1994 User Guide

DRB SCAN TOOL
For operation of the DRB scan tool, refer to the ap-
propriate Powertrain Diagnostic Procedures service
manual.
DIAGNOSTIC TROUBLE CODE (DTC)
On the following pages, a list of diagnostic trouble
codes is provided for the 2.5L 4 cylinder and 4.0L 6
cylinder engines. A DTC indicates that the power-
train control module (PCM) has recognized an abnor-
mal signal in a circuit or the system. A DTC may
indicate the result of a failure, but never identify the
failed component directly.
DIAGNOSTIC TROUBLE CODE DESCRIPTIONS
Diagnostic
Trouble
CodeDRB Scan Tool
DisplayDescription of Diagnostic Trouble Code
11* .......... NoCrank Reference
Signal at PCMNo crank reference signal detected during engine cranking.
12* ..........Battery Disconnect Direct battery input to PCM was disconnected within the last 50 Key-on
cycles.
13**.......... NoChange in MAP From
Start to RunNo difference recognized between the engine MAP reading and the
barometric (atmospheric) pressure reading at start-up.
14**.......... MAPSensor Voltage Too
LowMAP sensor input below minimum acceptable voltage.
or
MAP Sensor Voltage Too
HighMAP sensor input above maximum acceptable voltage.
15**.......... NoVehicle Speed Sensor
SignalNo vehicle distance (speed) sensor signal detected during road load
conditions.
17* ..........Engine is Cold Too Long Engine coolant temperature remains below normal operating temperatures
during vehicle travel (thermostat).
21**.......... O2SStays at Center Neither rich or lean condition detected from the oxygen sensor input.
or
O2S Shorted to Voltage Oxygen sensor input voltage maintained above the normal operating range.
22**.......... ECTSensor Voltage Too
HighEngine coolant temperature sensor input above maximum acceptable
voltage.
or
ECT Sensor Voltage Too
LowEngine coolant temperature sensor input below minimum acceptable voltage.
* Check Engine Lamp will not illuminate at all times if this Diagnostic Trouble Code was recorded. Cycle Ignition key as
described in manual and observe code flashed by Check Engine lamp.
** Check Engine Lamp will illuminate during engine operation if this Diagnostic Trouble Code was recorded.
Fig. 47 Data Link Connector Schematic
JFUEL SYSTEM 14 - 51

DIAGNOSTIC TROUBLE CODE DESCRIPTIONSÐCONTINUED
Diagnostic
Trouble
CodeDRB Scan Tool
DisplayDescription of Diagnostic Trouble Code
41**..........Generator Field Not
Switching ProperlyAn open or shorted condition detected in the generator field control circuit.
42* ..........Auto Shutdown Relay
Control CircuitAn open or shorted condition detected in the auto shutdown relay circuit.
44* ..........Battery Temp Sensor
Volts out of LimitAn open or shorted condition exists in the engine coolant temperature sensor
circuit or a problem exists in the PCM's battery temperature voltage circuit.
46**..........Charging System Voltage
Too HighBattery voltage sense input above target charging voltage during engine
operation.
47**..........Charging System Voltage
Too LowBattery voltage sense input below target charging during engine operation.
Also, no significant change detected in battery voltage during active test of
generator output.
51**.......... O2SSignal Stays Below
Center (Lean)Oxygen sensor signal input indicates lean air/fuel ratio condition during
engine operation.
52**.......... O2SSignal Stays Above
Center (Rich)Oxygen sensor signal input indicates rich air/fuel ratio condition during
engine operation.
53* ..........Internal PCM Failure PCM Internal fault condition detected.
or
PCM Failure SPI
CommunicationsPCM Internal fault condition detected.
54* .......... NoCamSync Signal at
PCMNo fuel sync (camshaft signal) detected during engine cranking.
55* .......... N/ACompletion of diagnostic trouble code display on the Malfunction Indicator
Lamp (Check Engine Lamp).
62* .......... PCMFailure SPI miles not
storedUnsuccessful attempt to update SPI miles in the PCM EEPROM.
63* .......... PCMFailure EEPROM
Write DeniedUnsuccessful attempt to write to an EEPROM location by the PCM.
* Check Engine Lamp will not illuminate at all times if this Diagnostic Trouble Code was recorded. Cycle Ignition key as
described in manual and observe code flashed by Check Engine lamp.
** Check Engine Lamp will illuminate during engine operation if this Diagnostic Trouble Code was recorded.
JFUEL SYSTEM 14 - 53

MULTI-PORT FUEL INJECTION (MFI)ÐCOMPONENT REMOVAL/INSTALLATION
INDEX
page page
Accelerator Pedal and Throttle Cable......... 54
Air Cleaner Housing...................... 54
Air Conditioning (A/C) Clutch Relay........... 54
Air Filter............................... 54
Automatic Shut Down (ASD) Relay........... 54
Brake Switch............................ 54
Camshaft Position Sensor.................. 54
Crankshaft Position Sensor................. 55
Engine Coolant Temperature Sensor.......... 55
Fuel Filter.............................. 55
Fuel Injector............................ 55
Fuel Pump Module....................... 56
Fuel Pump Relay........................ 56
Fuel Rail Assembly....................... 56
Fuel System Pressure Release Procedure...... 56
Fuel Tank Pressure Relief/Rollover Valve...... 56
Fuel Tanks............................. 56Fuel Tubes/Lines/Hoses and Clamps.......... 56
Idle Air Control (IAC) Motor................. 56
Ignition Coil............................. 57
Intake Air Temperature Sensor.............. 54
Intake Manifold.......................... 57
Manifold Absolute Pressure (MAP) Sensor..... 57
Oxygen (O2S) Sensor..................... 57
Park Neutral Switch....................... 58
Power Steering Pressure SwitchÐ2.5L
Engine Only........................... 58
Powertrain Control Module (PCM)............ 58
Quick-Connect Fittings..................... 59
Throttle Body............................ 59
Throttle Position Sensor (TPS).............. 59
Torque Converter Clutch Relay.............. 60
Vehicle Speed Sensor..................... 60
ACCELERATOR PEDAL AND THROTTLE CABLE
Refer to the Accelerator Pedal and Throttle Cable
section of this group for removal/installation proce-
dures.
AIR CONDITIONING (A/C) CLUTCH RELAY
The A/C clutch relay is located in the Power Dis-
tribution Center (PDC) (Figs. 1 or 2). For location of
this relay within the PDC, refer to label on PDC
cover.
AIR CLEANER HOUSING
Refer to Group 25, Emission Control System.
AIR FILTER
Refer to Group 25, Emission Control System.
AUTOMATIC SHUT DOWN (ASD) RELAY
The ASD relay is located in the Power Distribution
Center (Figs. 1 or 2) (PDC). For location of this relay
within the PDC, refer to label on PDC cover.
BRAKE SWITCH
Refer to Group 5, Brakes for removal/installation
procedures.
CAMSHAFT POSITION SENSOR
For removal/installation procedures, refer to Group
8D, Ignition System. See Camshaft Position Sensor.
INTAKE AIR TEMPERATURE SENSOR
The intake manifold air temperature sensor is in-
stalled into the intake manifold plenum (Figs. 3 or
4).
Fig. 1 PDCÐYJ Models
Fig. 2 PDCÐXJ Models
14 - 54 FUEL SYSTEMJ

REMOVAL
(1) Disconnect the electrical connector from the
sensor.
(2) Remove the sensor from the intake manifold.
INSTALLATION
(1) Install the sensor into the intake manifold.
Tighten the sensor to 28 Nzm (20 ft. lbs.) torque.
(2) Connect the electrical connector to the sensor.
CRANKSHAFT POSITION SENSOR
For description, operation and removal/installation
procedures, refer to Group 8D, Ignition Systems in
this manual.
ENGINE COOLANT TEMPERATURE SENSOR
The coolant temperature sensor is installed in the
thermostat housing (Fig. 5).
REMOVAL
(1) Drain cooling system until the coolant level is
below the cylinder head. Observe theWARNINGSin
Group 7, Cooling.(2) Disconnect the coolant temperature sensor wire
connector.
(3) Remove the sensor from the thermostat hous-
ing (Fig. 5).
INSTALLATION
(1) Install coolant temperature sensor into the cyl-
inder block. Tighten to 28 Nzm (21 ft. lbs.) torque.
(2) Connect the wire connector.
(3) Fill the cooling system. Refer to Group 7, Cool-
ing System.
FUEL FILTER
Refer to the Fuel Delivery System section of this
group for removal/installation procedures.
FUEL INJECTOR
REMOVAL
(1) Remove the fuel rail. Refer to Fuel Rail Re-
moval in this section.
(2) Remove the clip(s) that retain the fuel injec-
tor(s) to the fuel rail (Fig. 6).
Fig. 3 Sensor LocationÐ2.5L Engine
Fig. 4 Sensor LocationÐ4.0L Engine
Fig. 5 Engine Coolant Temperature SensorÐTypical
Fig. 6 Injector Retaining Clips
JFUEL SYSTEM 14 - 55

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

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

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 Distribu-
tion 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 con-
trol (IAC) stepper motor position to compensate for
increased 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, itwill de-energize the relay. This prevents A/C clutch
engagement. The relay will remain de-energized un-
til 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 SHUT DOWN (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 pump,
fuel injector, ignition coil, generator field winding
and oxygen (O2S) sensor heating element. 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 1994 model year, the ballast resistor and
ballast resistor bypass relay are no longer used to
control the fuel pump circuit.
DATA LINK CONNECTORÐPCM OUTPUT
Refer to the previous paragraphs on Data Link
ConnectorÐPCM Input for information.
EMR LAMPÐPCM OUTPUT
The EMR lamp is not used for the 1994 model
year.
Fig. 16 Vehicle Speed SensorÐTypical
Fig. 17 PDCÐYJ Models
Fig. 18 PDCÐXJ Models
14 - 24 FUEL SYSTEMJ

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 ignition distrib-
utor (Fig. 22).
Refer to Group 8D, Ignition System for additional
information.
MALFUNCTION INDICATOR LAMPÐPCM OUTPUT
The Malfunction Indicator Lamp (formerly referred
to as the Check Engine Lamp) illuminates on the in-
strument panel each time the ignition key is turned
on. It will stay on for three seconds as a bulb test.
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
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. 23).
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
illuminates 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 light will remain off until vehi-
cle stops accelerating and is brought back to range of
up-shift light 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.
Fig. 23 PDCÐXJ Models
Fig. 22 Ignition CoilÐTypical
14 - 26 FUEL SYSTEMJ

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 tachome-
ter (if equipped). Refer to Group 8E for tachometer
information.
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. 24). On XJ models the TCC relay is lo-
cated in the power distribution center (PDC) (Fig.
23).
OPEN LOOP/CLOSED LOOP MODES OF
OPERATION
As input signals to the powertrain control module
(PCM) change, the PCM adjusts its response to the
output devices. For example, the PCM must calculate
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 deter-
mine how the PCM responds to the various input sig-
nals.
MODES
²Open Loop
²Closed Loop
During Open Loop modes, the powertrain control
module (PCM) receives input signals and responds
only according to preset PCM programming. Input
from the oxygen (O2S) sensor is not monitored dur-
ing Open Loop modes.
During Closed Loop modes, the PCM will monitor
the oxygen (O2S) sensor input. This input indicates
to the PCM whether or not the calculated injector
pulse width results in the ideal air-fuel ratio. This
ratio is 14.7 parts air-to-1 part fuel. By monitoring
the exhaust oxygen content through the O2S sensor,
the PCM can fine tune the injector pulse width. This
is done to achieve optimum fuel economy combined
with low emission engine performance.
The fuel injection system has the following modes
of operation:
²Ignition switch ON
²Engine start-up (crank)
²Engine warm-up
²Idle
²Cruise
²Acceleration
²Deceleration
²Wide open throttle (WOT)
²Ignition switch OFF
The ignition switch On, engine start-up (crank),
engine warm-up, acceleration, deceleration and wide
open throttle modes are Open Loop modes. The idle
and cruise modes, (with the engine at operating tem-
perature) are Closed Loop modes.
IGNITION SWITCH (KEY-ON) MODE
This is an Open Loop mode. When the fuel system
is activated by the ignition switch, the following ac-
tions occur:
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.
Fig. 24 TCC Relay LocationÐYJ Models
JFUEL SYSTEM 14 - 27