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J
IGNITION SYSTEMS
8D - 1
IGNITION SYSTEMS
CONTENTS
page
page
COMPONENT IDENTIFICATION/SYSTEM
OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . 1
COMPONENT REMOVAL/INSTALLATION . . . . . 20
DIAGNOSTICS/SERVICE PROCEDURES . . . . . . . 8
IGNITION SWITCH . . . . . . . . . . . . . . . . . . . . . . 30
SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . 33
COMPONENT IDENTIFICATION/SYSTEM OPERATION
INDEX
page
Automatic Shut Down (ASD) Relay
Camshaft Position Sensor . . . . . . .
Crankshaft Position Sensor . . . . . .
Distributors . . . . . . . . . . . . . . . . . .
Engine Coolant Temperature Sensor
General Information . . . . . . . . . . . .
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1
1
2
3
4
1
Ignition Coil . . . . . . . . . . . . . . . . . . . . . . .
Intake Manifold Air Temperature Sensor .
Manifold Absolute Pressure (MAP) Sensor
Powertrain Control Module (PCM) . . . . . .
Throttle Position Sensor . . . . . . . . . . . . .
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4
5
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6
6
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
•
Ignition distributor (contains rotor and camshaft
position sensor)
•
Powertrain Control Module (PCM)
•
Crankshaft Position Sensor
AUTOMATIC SHUT DOWN (ASD) RELAY
The automatic shut down (ASD) relay is located in
the Power Distribution Center (PDC) near the bat-
tery (Fig. 1 or 2). As one of its functions, it will sup-
ply battery voltage to the ignition coil. The ground
circuit for the ASD relay is controlled by the Power-
train Control Module (PCM). The PCM regulates
ASD relay operation by switching the ground circuit
on-and-off.
CAMSHAFT POSITION SENSOR
The camshaft position sensor is located in the igni-
tion distributor (Figs. 3 or 4) on all engines.
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
8D - 2
IGNITION SYSTEMS
J
Fig. 1 PDC—XJ Models
Fig. 3 Camshaft Position Sensor—Typical
Fig. 2 PDC—YJ Models
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.
Fig. 4 Distributor—Typical
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).
J
IGNITION SYSTEMS
8D - 3
Fig. 5 Crankshaft Position Sensor—2.5L
Engine—Typical
Fig. 7 Crankshaft Position Sensor—4.0L Engine—YJ
models With Automatic Transmission
to 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.
Fig. 6 Crankshaft Position Sensor—4.0L 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.
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-
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 input
8D - 4
IGNITION SYSTEMS
J
Fig. 8 Sensor Operation—2.5L Engine
Fig. 10 Sensor Operation—4.0L Engine—YJ Models
With Automatic Transmission
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.
Fig. 9 Sensor Operation—4.0L Engine—All Except
YJ Models With Automatic Transmission
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.
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.
J
IGNITION SYSTEMS
8D - 5
manifold 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).
Fig. 11 Ignition Coil—Typical
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).
Fig. 13 Air Temperature Sensor Location—4.0L
Engine
Fig. 14 Air Temperature Sensor Location—2.5L
Engine
Fig. 12 Coolant Temperature Sensor—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.
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
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 intake
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

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