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IGNITION SYSTEMS
8D - 1
IGNITION SYSTEMS
CONTENTS
page
page
COMPONENT IDENTIFICATION/SYSTEM
OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
COMPONENT REMOVAL/INSTALLATION . . . . . . 17
DIAGNOSTICS/SERVICE PROCEDURES . . . . . . . 6
IGNITION SWITCH—XJ MODELS . . . . . . . . . . . 26
IGNITION SWITCH—YJ MODELS . . . . . . . . . . . 29
SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . 32
COMPONENT IDENTIFICATION/SYSTEM OPERATION
INDEX
page
Automatic Shutdown (ASD) Relay . .
Camshaft Position Sensor . . . . . . . .
Crankshaft Position Sensor . . . . . . .
Distributors . . . . . . . . . . . . . . . . . . .
Engine Coolant Temperature Sensor
General Information . . . . . . . . . . . .
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3
4
5
1
Ignition Coil . . . . . . . . . . . . . . . . . . . . . .
Intake Manifold Air Temperature Sensor . .
Manifold Absolute Pressure (MAP) Sensor
Oxygen (O2S) Sensor . . . . . . . . . . . . . . .
Powertrain Control Module (PCM) . . . . . .
Throttle Position Sensor . . . . . . . . . . . . .
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4
5
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5
GENERAL INFORMATION
Throughout this group, references are made to par-
ticular vehicle models by alphabetical designation
(XJ or YJ) or by the particular vehicle nameplate. A
chart showing a breakdown of alphabetical designa-
tions is included in the Introduction group at the be-
ginning of this manual.
This section of the group, Component Identifica-
tion/System Operation, will discuss ignition system
operation and will identify ignition system compo-
nents.
For diagnostic procedures and adjustments, refer to
the Diagnostics/Service Procedures section of this
group.
For removal and installation of ignition system
components, refer to the Component Removal/Instal-
lation 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.
An Ignition specifications section is included at the
end of this group. A general Maintenance Schedule
(mileage intervals) for ignition related items can be
found in Group 0, Lubrication and Maintenance. This
schedule can also be found in the Owners Manual.
IGNITION SYSTEMS
A multi-port, fuel injected engine is used on all
models. The ignition system is controlled by the pow-
ertrain control module (PCM) on all engines. The
PCM was formerly referred to as the SBEC or engine
controller.
The ignition system consists of:
•
Spark plugs
•
Ignition coil
•
Secondary ignition cables
•
Distributor (contains rotor and camshaft position
sensor)
•
Powertrain control module (PCM)
•
Crankshaft position sensor
AUTOMATIC SHUTDOWN (ASD) RELAY
The automatic shutdown (ASD) relay is located in
the power distribution center (PDC) near the battery
(Fig. 1 or 2). As one of its functions, it will supply
battery voltage to the ignition coil.
The ground circuit for the ASD relay is controlled
by the powertrain control module (PCM). This is
done through pin/cavity number 51 of the PCM 60-
way connector. The PCM then regulates ASD relay
operation by switching this ground circuit on-and-off.
Also refer to Ignition Coil for additional informa-
tion.
8D - 2
IGNITION SYSTEMS
J
Fig. 1 PDC—XJ Models
Fig. 4 Distributor Assembly—Typical
The camshaft position sensor contains a hall effect
device called a sync signal generator to generate a
fuel sync signal. This sync signal generator detects a
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 occurs:
The interruption of magnetic field causes the voltage
to switch high resulting in a sync signal of approxi-
mately 5 volts.
When the trailing edge of the pulse ring (shutter)
leaves the sync signal generator, the following occurs:
The change of the magnetic field causes the sync sig-
nal 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.
Fig. 2 PDC—YJ Models
CAMSHAFT POSITION SENSOR
The camshaft position sensor is located in the dis-
tributor (Figs. 3 or 4) on all engines.
Fig. 3 Camshaft Position Sensor—Typical
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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).
IGNITION SYSTEMS
8D - 3
Fig. 5 Crankshaft Position Sensor—2.5L 4-Cyl.
Engine—Typical
Fig. 7 Crankshaft Position Sensor—4.0L 6-Cyl.
Engine—YJ models With Automatic Transmission
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).
Fig. 6 Crankshaft Position Sensor—4.0L 6-Cyl.
Engine—All Except YJ models With Automatic
Transmission
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.
Fig. 8 Sensor Operation—2.5L 4-Cyl. Engine
The notches cause a pulse to be generated when
they pass under the sensor. The pulses are the input
to the PCM. For each engine revolution there are two
8D - 4
IGNITION SYSTEMS
J
groups of four pulses generated on 2.5L 4-cylinder
engines. There are 3 groups of four pulses generated
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.
Fig. 10 Sensor Operation—4.0L 6-Cyl. Engine—YJ
Models With Automatic Transmission
tributor housing to get distributor rotation.
Distributor position will have no effect on igni-
tion timing. The position of the distributor will
determine fuel synchronization only.
All distributors contain an internal oil seal that
prevents oil from entering the distributor housing.
The seal is not serviceable.
Distributor removal and installation procedures
have changed for the 1995 model year. Refer to Dis-
tributor in the Component Removal/Installation sec-
tion of this group.
Fig. 9 Sensor Operation—4.0L 6-Cyl. Engine—All
Except YJ Models With Automatic Transmission
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 both the 2.5L 4-cylinder and
the 4.0L-6 cylinder engines do not have built in cen-
trifugal or vacuum assisted advance. 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 tim-
ing is not adjustable on any of these engines.
The distributor is locked in place by a fork with a
slot located on the distributor housing base. The dis-
tributor holddown clamp bolt passes through this slot
when installed. Because the distributor position is
locked when installed, its rotational position can not
be changed.
Do not attempt to modify the dis-
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. This is done through pin/cavity number 19
of the PCM 60-way connector.
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 ad-
vance. This is done to meet changing engine operat-
ing 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.
On the 2.5L 4-cylinder engine, the ignition coil is
mounted to a bracket on the side of the engine (to
the rear of the distributor).
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On the 4.0L 6-cylinder engine, the ignition coil is
mounted to a bracket on the side of the engine (to
the front of the distributor) (Fig. 11).
IGNITION SYSTEMS
8D - 5
Fig. 12 PCM Location—XJ Models
Fig. 11 Ignition Coil—Typical
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
For an operational description, diagnosis and re-
moval/installation procedures, refer to Group 14,
Fuel System.
INTAKE MANIFOLD AIR TEMPERATURE SENSOR
For an operational description, diagnosis or remov-
al/ installation procedures, refer to Group 14, Fuel
Systems.
Fig. 13 PCM Location—YJ Models
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 PCM diagnostics, refer to the appropriate Pow-
ertrain Diagnostic Procedures service manual for op-
eration of the DRB scan tool.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
For an operational description, diagnosis and re-
moval/installation procedures, refer to Group 14,
Fuel System.
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. 12). On YJ models, the PCM is located in the
engine compartment behind the windshield washer
fluid reservoir (Fig. 13).
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.
THROTTLE POSITION SENSOR
For an operational description, diagnosis and re-
moval/installation procedures, refer to Group 14,
Fuel System.
OXYGEN (O2S) SENSOR
For an operational description, diagnosis and re-
moval/installation procedures, refer to Group 14,
Fuel System.

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