Jeep Grand Cherokee WJ. Manual - part 94

rail containing three independent coils. Although cyl-
inder firing order is the same as 4.0L engines of pre-
vious years, spark plug firing is not. The 3 coils dual-
fire the spark plugs on cylinders 1–6, 2–5 and/or 3–4.
When one cylinder is being fired (on compression
stroke), the spark to the opposite cylinder is being
wasted (on exhaust stroke). The one-piece coil bolts
directly to the cylinder head. Rubber boots seal the
secondary terminal ends of the coils to the top of all

6 spark plugs. One electrical connector (located at
the rear end of the coil rail) is used for all three coils.

The 4.7L V-8 engine uses 8 dedicated and individ-

ually fired coil for each spark plug. Each coil is
mounted directly to the top of each spark plug. A sep-
arate electrical connector is used for each coil.

Because of coil design, spark plug cables (second-

ary cables) are not used on either engine. A distrib-
utor is not used with either the 4.0L or 4.7L
engines.

The ignition system is controlled by the powertrain

control module (PCM) on all engines.

The ignition system consists of:
• Spark Plugs

• Ignition Coil(s)

• Powertrain Control Module (PCM)

• Crankshaft Position Sensor

• Camshaft Position Sensor

• The MAP, TPS, IAC and ECT also have an effect

on the control of the ignition system.

IGNITION SYSTEM

8D - 1

SPARK PLUGS

DESCRIPTION

Both the 4.0L 6–cylinder and the 4.7L V-8 engine

use resistor type spark plugs. 4.7L V-8 engines are
equipped with “fired in suppressor seal” type spark
plugs using a copper core ground electrode.

Because of the use of an aluminum cylinder head

on the 4.7L engine, spark plug torque is very critical.

To prevent possible pre-ignition and/or mechanical

engine damage, the correct type/heat range/number
spark plug must be used.

OPERATION

Plugs on both engines have resistance values rang-

ing from 6,000 to 20,000 ohms (when checked with at
least a 1000 volt spark plug tester). Do not use an
ohmmeter to check the resistance values of the
spark plugs. Inaccurate readings will result.
Remove the spark plugs and examine them for
burned electrodes and fouled, cracked or broken por-
celain insulators. Keep plugs arranged in the order
in which they were removed from the engine. A sin-
gle plug displaying an abnormal condition indicates
that a problem exists in the corresponding cylinder.
Replace spark plugs at the intervals recommended in
Group O, Lubrication and Maintenance.

Spark plugs that have low mileage may be cleaned

and reused if not otherwise defective, carbon or oil
fouled. Also refer to Spark Plug Conditions.

CAUTION: Never use a motorized wire wheel brush
to clean the spark plugs. Metallic deposits will
remain on the spark plug insulator and will cause
plug misfire.

IGNITION COIL—4.0L ENGINE

DESCRIPTION

A one-piece coil rail assembly containing three

individual coils is used on the 4.0L 6–cylinder engine
(Fig. 1). The coil rail must be replaced as one assem-
bly. The bottom of the coil is equipped with 6 individ-
ual rubber boots (Fig. 1) to seal the 6 spark plugs to
the coil. Inside each rubber boot is a spring. The
spring is used for a mechanical contact between the
coil and the top of the spark plug. These rubber boots
and springs are a permanent part of the coil and are
not serviced separately.

(1) The coil is bolted directly to the cylinder head

(Fig. 2). One electrical connector (located at rear of
coil) is used for all three coils.

Fig. 1 Ignition Coil Assembly—4.0L 6–Cylinder

Engine

1 – CYL. #6
2 – CYL. #5
3 – CYL. #4
4 – CYL. #3
5 – CYL. #2
6 – CYL. #1
7 – COILS (3)
8 – MOUNTING BOLTS (4)
9 – BOLT BASES (4)
10 – RUBBER BOOTS (6)

Fig. 2 Coil Location—4.0L Engine

1 – COIL RAIL
2 – COIL MOUNTING BOLTS (4)
3 – COIL
4 – COIL ELECTRICAL CONNECTION

8D - 2

IGNITION SYSTEM

DESCRIPTION AND OPERATION (Continued)

OPERATION

Although cylinder firing order is the same as 4.0L

Jeep engines of previous years, spark plug firing is
not. The 3 coils dual-fire the spark plugs on cylinders
1–6, 2–5 and/or 3–4. When one cylinder is being fired
(on compression stroke), the spark to the opposite
cylinder is being wasted (on exhaust stroke).

Battery voltage is supplied to the three ignition

coils 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.

Because of coil design, spark plug cables (second-

ary cables) are not used. The cables are integral
within the coil rail.

IGNITION COIL—4.7L ENGINE

DESCRIPTION

The 4.7L V–8 engine uses 8 dedicated, and individ-

ually fired coil (Fig. 3) for each spark plug. Each coil
is mounted directly to the top of each spark plug
(Fig. 4).

OPERATION

Battery voltage is supplied to the 8 ignition coils

from the ASD relay. The Powertrain Control Module
(PCM) opens and closes each ignition coil ground cir-
cuit at a determined time 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.

Because of coil design, spark plug cables (second-

ary cables) are not used.

Fig. 3 Ignition Coil—4.7L Engine

1 – O-RING
2 – IGNITION COIL
3 – ELECTRICAL CONNECTOR

Fig. 4 Ignition Coil Location—4.7L Engine

1 – IGNITION COIL
2 – COIL ELECTRICAL CONNECTOR
3 – COIL MOUNTING STUD/NUT

IGNITION SYSTEM

8D - 3

DESCRIPTION AND OPERATION (Continued)

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