1994 JEEP CHEROKEE sensor

rotating pulse ring (shutter) on the distributor shaft
(Fig. 4). The pulse ring rotates 180 degrees through
the sync signal generator. Its signal is used in con-
junction with the crankshaft position sensor to differ-
entiate between fuel injection and spark events. It is
also used to synchronize the fuel injectors with their
respective cylinders.
When the leading edge of the pulse ring (shutter)
enters the sync signal generator, the following oc-
curs: The interruption of magnetic field causes the
voltage to switch high resulting in a sync signal of
approximately 5 volts.
When the trailing edge of the pulse ring (shutter)
leaves the sync signal generator, the following oc-
curs: The change of the magnetic field causes the
sync signal voltage to switch low to 0 volts.
For component testing, refer to the Diagnostics/Ser-
vice Procedures section of this group.
For removal and installation of this component, re-
fer to the Component Removal/Installation section of
this group.
CRANKSHAFT POSITION SENSOR
The crankshaft position sensor is mounted to the
transmission bellhousing at the left/rear side of the
engine block (Figs. 5, 6 or 7).
Fig. 1 PDCÐXJ Models
Fig. 2 PDCÐYJ Models
Fig. 3 Camshaft Position SensorÐTypical
Fig. 4 DistributorÐTypical
8D - 2 IGNITION SYSTEMSJ

Engine speed and crankshaft position are provided
through the crankshaft position sensor. The sensor
generates pulses that are the input sent to the Pow-
ertrain Control Module (PCM). The PCM interprets
the sensor input to determine the crankshaft posi-
tion. The PCM then uses this position, along with
other inputs, to determine injector sequence and ig-
nition timing.
The sensor is a hall effect device combined with an
internal magnet. It is also sensitive to steel within a
certain distance from it.
SENSOR OPERATION
The flywheel/drive plate has groups of four notches
at its outer edge. On 4.0L 6 cylinder engines there
are three sets of notches (Figs. 9 or 10). On 2.5L 4
cylinder engines there are two sets of notches (Fig.
8).
The notches cause a pulse to be generated when
they pass under the sensor. The pulses are the inputto the PCM. For each engine revolution there are
two groups of four pulses generated on 2.5L 4 cylin-
der engines. There are 3 groups of four pulses gener-
ated on 4.0L 6 cylinder engines.
The trailing edge of the fourth notch, which causes
the pulse, is four degrees before top dead center
(TDC) of the corresponding piston.
The engine will not operate if the PCM does not re-
ceive a crankshaft position sensor input.
For component testing, refer to the Diagnostics/Ser-
vice Procedures section of this group.
For removal and installation of this sensor, refer to
the Component Removal/Installation section of this
group.
DISTRIBUTORS
All engines are equipped with a camshaft driven
mechanical distributor containing a shaft driven dis-
tributor rotor. All distributors are equipped with an
internal camshaft position (fuel sync) sensor. This
sensor provides fuel injection synchronization and
cylinder identification.
The distributors on the 2.5L and 4.0L engines do
not have built in centrifugal or vacuum assisted ad-
vance. Base ignition timing and all timing advance
is controlled by the Powertrain Control Module
(PCM). Because ignition timing is controlled by the
PCM,base ignition timing is not adjustable on
any of these engines.
The distributor is locked in place by a notch on the
distributor housing. The distributor holddown clamp
bolt passes through this notch when installed. Be-
cause the distributor position is locked when in-
Fig. 5 Crankshaft Position SensorÐ2.5L
EngineÐTypical
Fig. 6 Crankshaft Position SensorÐ4.0L EngineÐAll
Except YJ models With Automatic Transmission
Fig. 7 Crankshaft Position SensorÐ4.0L EngineÐYJ
models With Automatic Transmission
JIGNITION SYSTEMS 8D - 3

stalled, its rotational position can not be changed.
Do not attempt to modify the distributor housing
to get distributor rotation. Distributor position
will have no effect on ignition timing.
All distributors contain an internal oil seal that
prevents oil from entering the distributor housing.
The seal is not serviceable.
For component testing, refer to the Diagnostics/Ser-
vice Procedures section of this group.For removal and installation of this component, re-
fer to the Component Removal/Installation section of
this group.
IGNITION COIL
Battery voltage is supplied to the ignition coil pos-
itive terminal from the ASD relay.
The Powertrain Control Module (PCM) opens and
closes the ignition coil ground circuit for ignition coil
operation.
Base ignition timing is not adjustable.By con-
trolling the coil ground circuit, the PCM is able to
set the base timing and adjust the ignition timing
advance. This is done to meet changing engine oper-
ating conditions.
The ignition coil is not oil filled. The windings are
embedded in an epoxy compound. This provides heat
and vibration resistance that allows the ignition coil
to be mounted on the engine.
The ignition coil is mounted to a bracket on the
side of the engine (Fig. 11).
For component testing, refer to the Diagnostics/Ser-
vice Procedures section of this group.
For removal and installation of this component, re-
fer to the Component Removal/Installation section of
this group.
ENGINE COOLANT TEMPERATURE SENSOR
The engine coolant temperature sensor provides an
input voltage to the Powertrain Control Module
(PCM) relating coolant temperature. The PCM uses
this input, along with inputs from other sensors, to
determine injector pulse width and ignition timing.
Fig. 8 Sensor OperationÐ2.5L Engine
Fig. 9 Sensor OperationÐ4.0L EngineÐAll Except
YJ Models With Automatic Transmission
Fig. 10 Sensor OperationÐ4.0L EngineÐYJ Models
With Automatic Transmission
8D - 4 IGNITION SYSTEMSJ

As coolant temperature varies, the sensor resistance
will change, resulting in a different input voltage to
the PCM.
When the engine is cold, the PCM will operate in
the Open Loop Cycle. It will demand slightly richer
air-fuel mixtures and higher idle speeds, until nor-
mal operating temperatures are reached. Refer to
Modes Of Operation in Group 14, Fuel System for a
description of Open and Closed Loop operation.
This sensor is installed in the thermostat housing
(Fig. 12).
For component testing, refer to the Diagnostics/Ser-
vice Procedures section of this group.
For removal and installation of this component, re-
fer to the Component Removal/Installation section of
this group.
INTAKE MANIFOLD AIR TEMPERATURE SENSOR
The sensor element extends into the intake mani-
fold air stream. It provides an input voltage to the
Powertrain Control Module (PCM) indicating intakemanifold air temperature. The input from this sensor
is used along with inputs from other sensors to de-
termine injector pulse width. As the temperature of
the air-fuel stream in the manifold varies, the sensor
resistance will change. This will result in a different
input voltage to the PCM. For more information, re-
fer to Group 14, Fuel System.
This sensor is installed in the intake manifold (Fig.
13, 4.0L engine or Fig. 14, 2.5L engine).
For component testing, refer to the Diagnostics/Ser-
vice Procedures section of this group.
For removal and installation of this component, re-
fer to the Component Removal/Installation section of
this group.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
The MAP sensor reacts to absolute pressure in the
intake manifold and provides an input voltage to the
Powertrain Control Module (PCM). As engine load
changes, manifold pressure varies, causing the MAP
Fig. 11 Ignition CoilÐTypical
Fig. 12 Coolant Temperature SensorÐTypical
Fig. 13 Air Temperature Sensor LocationÐ4.0L
Engine
Fig. 14 Air Temperature Sensor LocationÐ2.5L
Engine
JIGNITION SYSTEMS 8D - 5

sensor voltage to change. This change results in a
different input voltage to the PCM. The input volt-
age level supplies the PCM with information. This
relates to ambient barometric pressure during engine
start-up (cranking) and to engine load while the en-
gine is running. The PCM uses this input, along with
inputs from other sensors, to adjust air-fuel mixture.
For more information, refer to Group 14, Fuel Sys-
tem.
The MAP sensor is located in the engine compart-
ment near the rear of engine cylinder head (valve)
cover (Fig. 15). It is connected to the throttle body
with a vacuum hose and to the PCM electrically.
For component testing, refer to the Diagnostics/Ser-
vice Procedures section of this group.
For removal and installation of this component, re-
fer to the Component Removal/Installation section of
this group.
POWERTRAIN CONTROL MODULE (PCM)
The PCM was formerly referred to as the SBEC or
engine controller. On XJ models, the PCM is located
in the engine compartment next to the air cleaner
(Fig. 16). On YJ models, the PCM is located in the
engine compartment behind the windshield washer
fluid reservoir (Fig. 17).
The ignition system is controlled by the PCM.
Base ignition timing by rotation of distributor
is not adjustable.The PCM opens and closes the ig-
nition coil ground circuit to operate the ignition coil.
This is done to adjust ignition timing, both initial
(base) and advance, for changing engine operating
conditions.
The amount of electronic spark advance provided
by the PCM is determined by five input factors: En-
gine coolant temperature, engine rpm, intake mani-
fold air temperature, intake manifold absolute
pressure and throttle position.For removal and installation of this component, re-
fer to the Component Removal/Installation section of
this group.
For diagnostics, refer to the appropriate Powertrain
Diagnostic Procedures service manual for operation
of the DRB scan tool.
THROTTLE POSITION SENSOR
The sensor is mounted on the throttle body (Figs.
18 or 19). It is connected to the throttle blade shaft.
The sensor is a variable resistor. It provides the Pow-
ertrain Control Module (PCM) with an input signal
(voltage) that represents throttle blade position. As
the position of the throttle blade changes, the resis-
tance of the sensor changes.
The PCM supplies approximately 5 volts to the
sensor. The sensor output voltage (input signal to the
PCM) represents the throttle blade position. The
PCM receives an input signal voltage from the sen-
sor. 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 sensor input to determine
Fig. 15 MAP SensorÐTypical
Fig. 16 PCM LocationÐXJ Models
Fig. 17 PCM LocationÐYJ Models
8D - 6 IGNITION SYSTEMSJ

current engine operating conditions. It will also ad-
just fuel injector pulse width and ignition timing.For component testing, refer to the Diagnostics/Ser-
vice Procedures section of this group.
For removal and installation of this component, re-
fer to the Component Removal/Installation section of
this group.
Fig. 18 Throttle Position SensorÐ2.5L EngineFig. 19 Throttle Position SensorÐ4.0L Engine
JIGNITION SYSTEMS 8D - 7

DIAGNOSTICS/SERVICE PROCEDURES
INDEX
page page
Automatic Shut Down (ASD) Relay............ 8
Camshaft Position Sensor Test............... 8
Crankshaft Position Sensor Test.............. 9
Distributor Cap........................... 9
Distributor Rotor......................... 10
DRB Scan Tool.......................... 10
Engine Coolant Temperature Sensor Test...... 12
General Information........................ 8
Ignition Coil............................. 11
Ignition Secondary Circuit Diagnosis.......... 12Ignition Timing........................... 14
Intake Manifold Air Temperature Sensor Test . . . 14
Manifold Absolute Pressure (MAP) Sensor Test . 14
On-Board Diagnostics (OBD)................ 19
Oxygen Sensor Tests..................... 19
Powertrain Control Module (PCM)............ 15
Spark Plug Secondary Cables............... 17
Spark Plugs............................ 16
Throttle Position Sensor Test............... 18
GENERAL INFORMATION
This section of the group, Diagnostics/Service Pro-
cedures, will discuss basic ignition system diagnos-
tics and service adjustments.
For system operation and component identification,
refer to the Component Identification/System Opera-
tion section of this group.
For removal or installation of ignition system com-
ponents, refer to the Component Removal/Installa-
tion section of this group.
For other useful information, refer to On-Board Di-
agnostics in the General Diagnosis sections of Group
14, Fuel System in this manual.
For operation of the DRB Scan Tool, refer to the
appropriate Powertrain Diagnostic Procedures ser-
vice manual.
AUTOMATIC SHUT DOWN (ASD) RELAY
Refer to RelaysÐOperation/Testing in the Group
14, Fuel System section of this service manual.
CAMSHAFT POSITION SENSOR TEST
The camshaft position sensor is located in the dis-
tributor on all engines.
To perform a complete test of this sensor and its
circuitry, refer to the DRB scan tool. Also refer to the
appropriate Powertrain Diagnostics Procedures man-
ual. To test the sensor only, refer to the following:
2.5L OR 4.0L ENGINE
For this test, an analog voltmeter is needed.Do
not remove the distributor connector from the dis-
tributor. Using small paper clips, insert them into
the backside of the distributor wire harness connec-
tor to make contact with the terminals. Be sure that
the connector is not damaged when inserting the pa-
per clips. Attach voltmeter leads to these paper clips.
(1) Connect the positive (+) voltmeter lead into
the sensor output wire. This is at done the distribu-
tor wire harness connector. For wire identification,
refer to Group 8W, Wiring Diagrams.(2) Connect the negative (-) voltmeter lead into the
ground wire. For wire identification, refer to Group
8W, Wiring Diagrams.
(3) Set the voltmeter to the 15 Volt DC scale.
(4) Remove distributor cap. Rotate (crank) engine
with starter until pulse ring (Fig. 1) enters the mag-
netic pickup on camshaft position sensor. Distributor
rotor should be pointed in 9 o'clock position. The
movable pulse ring should now be within the sensor
pickup.
(5) Turn ignition key to ON position. Voltmeter
should read approximately 5.0 volts.
(6) If voltage is not present, check the voltmeter
leads for a good connection.
(7) If voltage is still not present, check for voltage
at the supply wire. For wire identification, refer to
Group 8W, Wiring Diagrams.
Fig. 1 Pulse Ring/Rotor PositionÐTypical
8D - 8 IGNITION SYSTEMSJ

(8) If voltage is not present at supply wire, check
for voltage at pin 7 of Powertrain Control Module
(PCM) 60-way connector. Leave the PCM connector
connected for this test.
(9) If voltage is still not present, perform vehicle
test using the DRB scan tool.
(10) If voltage is present at pin 7, but not at the
supply wire:
(a) Check continuity between the supply wire.
This is checked between the distributor connector
and pin 7 at the PCM. If continuity is not present,
repair the harness as necessary.
(b) Check for continuity between the camshaft
position sensor output wire and pin 44 at the PCM.
If continuity is not present, repair the harness as
necessary.
(c) Check for continuity between the ground cir-
cuit wire at the distributor connector and ground.
If continuity is not present, repair the harness as
necessary.
(11) While observing the voltmeter, crank the en-
gine with ignition switch. The voltmeter needle
should fluctuate between 0 and 5 volts while the en-
gine is cranking. This verifies that the camshaft po-
sition sensor in the distributor is operating properly
and a sync pulse signal is being generated.
If sync pulse signal is not present, replacement of
the camshaft position sensor is necessary.
For removal or installation of ignition system com-
ponents, refer to the Component Removal/Installa-
tion section of this group.
For system operation and component identification,
refer to the Component Identification/System Opera-
tion section of this group.
CRANKSHAFT POSITION SENSOR TEST
To perform a complete test of this sensor and its
circuitry, refer to the DRB scan tool. Also refer to the
appropriate Powertrain Diagnostics Procedures man-
ual. To test the sensor only, refer to the following:
The sensor is located on the transmission bellhous-
ing at the left/rear side of the engine block (Figs. 2, 3
or 4).
(1) Near the rear of intake manifold, disconnect
sensor pigtail harness connector from main wiring
harness.
(2) Place an ohmmeter across terminals B and C
(Fig. 5). Ohmmeter should be set to 1K-to-10K scale
for this test. The meter reading should be open (no
resistance). Replace sensor if a low resistance is indi-
cated.
For removal or installation of ignition system com-
ponents, refer to the Component Removal/Installa-
tion section of this group.
DISTRIBUTOR CAP
INSPECTION
Remove the distributor cap and wipe it clean with
a dry lint free cloth. Visually inspect the cap for
cracks, carbon paths, broken towers, or damaged ro-
tor button (Figs. 6 and 7). Also check for white de-
posits on the inside (caused by condensation entering
the cap through cracks). Replace any cap that dis-
plays charred or eroded terminals. The machined
surface of a terminal end (faces toward rotor) will in-
dicate some evidence of erosion from normal opera-
tion. Examine the terminal ends for evidence of
mechanical interference with the rotor tip.
If replacement of the distributor cap is necessary,
transfer spark plug cables from the original cap to
the new cap. This should be done one cable at a time.
Each cable is installed onto the tower of the new cap
that corresponds to its tower position on the original
cap. Fully seat the cables onto the towers. If neces-
Fig. 2 Crankshaft Position SensorÐ2.5L
EngineÐTypical
Fig. 3 Crankshaft Position SensorÐ4.0L EngineÐAll
Except YJ models With Auto. Trans.
JIGNITION SYSTEMS 8D - 9