1994 JEEP CHEROKEE engine

IGNITION COIL
To perform a complete test of the ignition coil and
its circuitry, refer to the DRB scan tool. Also refer to
the appropriate Powertrain Diagnostics Procedures
manual. To test the coil only, refer to the following:
The ignition coil (Fig. 11) is designed to operate
without an external ballast resistor.
Inspect the ignition coil for arcing. Test the coil ac-
cording to coil tester manufacturer's instructions.
Test the coil primary and secondary resistance. Re-
place any coil that does not meet specifications. Refer
to the Ignition Coil Resistance chart.
If the ignition coil is being replaced, the secondary
spark plug cable must also be checked. Replace cable
if it has been burned or damaged.
Arcing at the tower will carbonize the cable nipple,
which if it is connected to a new ignition coil, will
cause the coil to fail.
If the secondary coil cable shows any signs of dam-
age, it should be replaced with a new cable and new
terminal. Carbon tracking on the old cable can cause
arcing and the failure of a new ignition coil.
IGNITION COIL RESISTANCE
Fig. 9 Firing OrderÐ4.0L 6 Cylinder Engine
Fig. 10 Rotor InspectionÐTypical
Fig. 11 Ignition CoilÐTypical
JIGNITION SYSTEMS 8D - 11

ENGINE COOLANT TEMPERATURE 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 installed in the thermostat housing
(Fig. 12).
(1) Disconnect wire harness connector from sensor
(Fig. 12).
(2) Test the resistance of the sensor with a high in-
put impedance (digital) volt-ohmmeter. The resis-
tance should be less than 1340 ohms at normal
engine operating idle temperature. For resistance
values, refer to the Sensor Resistance chart. Replace
the sensor if it is not within the range of resistance
specified in the chart.
(3) Test continuity of the wire harness. This is
done between Powertrain Control Module (PCM)
wire harness connector terminal-2 and the sensor
connector terminal. Also check continuity between
wire harness terminal-4 to the sensor connector ter-
minal. Repair the wire harness if an open circuit is
indicated.
IGNITION SECONDARY CIRCUIT DIAGNOSIS
CHECKING FOR SPARK
CAUTION: When disconnecting a high voltage cable
from a spark plug or from the distributor cap, twist
the rubber boot slightly (1/2 turn) to break it loose.
Grasp the boot (not the cable) and pull it off with a
steady, even force.
(1) Disconnect the ignition coil secondary cable
from center tower of the distributor cap. Hold the ca-
ble terminal approximately 12 mm (1/2 in.) from a
good engine ground (Fig. 13).WARNING: BE VERY CAREFUL WHEN THE ENGINE
IS CRANKING. DO NOT PUT YOUR HANDS NEAR
THE PULLEYS, BELTS OR THE FAN. DO NOT
WEAR LOOSE FITTING CLOTHING.
(2) Rotate (crank) the engine with the starter mo-
tor and observe the cable terminal for a steady arc. If
steady arcing does not occur, inspect the secondary
coil cable. Refer to Spark Plug Cables in this group.
Also inspect the distributor cap and rotor for cracks
or burn marks. Repair as necessary. If steady arcing
occurs, connect ignition coil cable to the distributor
cap.
(3) Remove a cable from one spark plug.
Fig. 12 Coolant Temperature SensorÐTypical
SENSOR RESISTANCE (OHMS)
Fig. 13 Checking for SparkÐTypical
8D - 12 IGNITION SYSTEMSJ

(4) Using insulated pliers, hold the cable terminal
approximately 12 mm (1/2 in.) from the engine cylin-
der head or block while rotating the engine with the
starter motor. Observe the spark plug cable terminal
for an arc. If steady arcing occurs, it can be expected
that the ignition secondary system is operating cor-
rectly. If steady arcing occurs at the spark plug ca-
bles, but the engine will not start, connect the DRB
scan tool. Refer to the Powertrain Diagnostic Proce-
dures service manual for DRB operation.
FAILURE TO START TEST
To prevent unnecessary diagnostic time and wrong
test results, the previous Checking For Spark test
should be performed prior to this test.
WARNING: SET PARKING BRAKE OR BLOCK THE
DRIVE WHEELS BEFORE PROCEEDING WITH THIS
TEST.
(1) Unplug the ignition coil harness connector at
the coil (Fig. 14).
(2) Connect a set of small jumper wires (18 gauge
or smaller) between the ignition coil and coil electri-
cal connector (Fig. 15).
(3) Determine that sufficient battery voltage (12.4
volts) is present for the starting and ignition sys-
tems.
(4) Crank the engine for 5 seconds while monitor-
ing the voltage at the coil positive terminal:
²If the voltage remains near zero during the entire
period of cranking, refer to On-Board Diagnostics in
Group 14, Fuel Systems. Check the powertrain con-
trol module (PCM) and auto shut down relay.
²If voltage is at near battery voltage and drops to
zero after 1-2 seconds of cranking, check the cam-
shaft position sensor-to-PCM circuit. Refer to On-
Board Diagnostics in Group 14, Fuel Systems.
²If voltage remains at near battery voltage during
the entire 5 seconds, turn the key off. Remove the60-way connector (Fig. 16) from the PCM. Check 60-
way connector for any spread terminals.
(5) Remove test lead from the coil positive termi-
nal. Connect an 18 gauge jumper wire between the
battery positive terminal and the coil positive termi-
nal.
(6) Make the special jumper shown in Figure 17.
Using the jumper,momentarilyground terminal-19
of the 60-way connector. A spark should be generated
at the coil cable when the ground is removed.
(7) If spark is generated, replace the powertrain
control module (PCM).
(8) If spark is not seen, use the special jumper to
ground the coil negative terminal directly.
(9) If spark is produced, repair wiring harness for
an open condition.
Fig. 14 Coil Harness ConnectorÐTypical
Fig. 15 Coil TerminalsÐTypical
Fig. 16 PCM 60-Way Connector
Fig. 17 Special Jumper Ground-to-Coil Negative
Terminal
JIGNITION SYSTEMS 8D - 13

(10) If spark is not produced, replace the ignition
coil.
IGNITION TIMING
Base (initial) ignition timing is NOT adjustable
on any of the 2.5L 4 cylinder or 4.0L 6 cylinder
engines. Do not attempt to adjust ignition timing
by rotating the distributor.
Do not attempt to modify the distributor hous-
ing to get distributor rotation. Distributor posi-
tion will have no effect on ignition timing.
All ignition timing functions are controlled by the
powertrain control module (PCM). Refer to On-Board
Diagnostics in the Multi-Port Fuel InjectionÐGen-
eral Diagnosis section of Group 14, Fuel Systems for
more information. Also refer to the appropriate Pow-
ertrain Diagnostics Procedures service manual for
operation of the DRB Scan Tool.
INTAKE MANIFOLD AIR TEMPERATURE 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:
(1) Disconnect the wire harness connector from the
sensor (Figs. 18 or 19).
(2) Test the resistance of the sensor with a input
impedance (digital) volt-ohmmeter. Do not remove
the sensor from the engine for testing. For resistance
values, refer to the Sensor Resistance chart. Replace
the sensor if it is not within the range of resistance
specified in the chart.
(3) Test the resistance of the wire harness. This is
done between the Powertrain Control Module (PCM)
wire harness connector terminal-21 and the sensor
connector terminal. Also check continuity between
terminal-4 to the sensor connector terminal. Repair
the wire harness as necessary if the resistance is
greater than 1 ohm.
MANIFOLD ABSOLUTE PRESSURE (MAP) 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 MAP sensor is located on the cowl panel near
the rear of the engine cylinder head (valve) cover
(Fig. 20).
(1) Inspect the sensor vacuum hose connections at
the throttle body and sensor (Fig. 20). Repair as nec-
essary.
CAUTION: When testing the sensor, be sure that
the harness wires are not damaged by the test
meter probes.Fig. 18 Air Temperature SensorÐ2.5L Engine
Fig. 19 Air Temperature SensorÐ4.0L Engine
SENSOR RESISTANCE (OHMS)
8D - 14 IGNITION SYSTEMSJ

(2) Test the sensor output voltage at the sensor
connector between terminals A and B as marked on
the sensor body (Fig. 21). This is done with the igni-
tion switch ON and the engine OFF. Output voltage
should be 4-to-5 volts.The voltage should drop to
1.5-to-2.1 volts with a hot, neutral idle speed con-
dition.
(3) Test Powertrain Control Module (PCM) termi-
nal-1 for the same voltage described above to verify
the wire harness condition. Repair as necessary.
(4) Test sensor supply voltage at sensor connector
between terminals A and C with the ignition ON.
The voltage should be approximately 5 volts (60.5V).
Five volts (60.5V) should also be at terminal-6 of the
corresponding Powertrain Control Module (PCM)
wire harness connector. Repair or replace the wire
harness as necessary.(5) Test the sensor ground circuit at sensor connec-
tor terminal-A and PCM connector terminal-4. Re-
pair the wire harness if necessary.
(6) Test the sensor ground circuit at the PCM con-
nector between terminal-4 and terminal-11 with an
ohmmeter. If the ohmmeter indicates an open circuit,
inspect for a defective sensor ground connection. Re-
fer to Group 8W, Wiring for location of ground con-
nection. If the ground connection is good, replace the
PCM. If terminal-4 has a short circuit to 12 volts,
correct this condition before replacing the PCM.
POWERTRAIN CONTROL MODULE (PCM)
The PCM (formerly called the SBEC or engine con-
troller) is located in the engine compartment behind
the windshield washer fluid tank on YJ models (Fig.
22). It is located in the engine compartment next to
the air cleaner on XJ models (Fig. 23).
The ignition system is controlled by the PCM.
For removal and installation of this component, re-
fer to the Component Removal/Installation section of
this group.
Fig. 20 MAP SensorÐTypical
Fig. 21 MAP Sensor TestÐTypical
Fig. 22 PCM LocationÐYJ Models
Fig. 23 PCM LocationÐXJ Models
JIGNITION SYSTEMS 8D - 15

For diagnostics, refer to the appropriate Powertrain
Diagnostic Procedures service manual for operation
of the DRB scan tool.
SPARK PLUGS
For spark plug removal, cleaning, gap adjustment
and installation, refer to the Component Removal/In-
stallation section of this group.
Faulty carbon and/or gas fouled plugs generally
cause hard starting, but they will clean up at higher
engine speeds. Faulty plugs can be identified in a
number of ways: poor fuel economy, power loss, de-
crease in engine speed, hard starting and, in general,
poor engine performance.
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. An iso-
lated plug displaying an abnormal condition indi-
cates that a problem exists in the corresponding
cylinder. Replace spark plugs at the intervals recom-
mended in the maintenance chart in Group 0, Lubri-
cation and Maintenance.
Spark plugs that have low mileage may be cleaned
and reused if not otherwise defective. Refer to the
following Spark Plug Condition section of this group.
CONDITION
NORMAL OPERATING
The few deposits present on the spark plug will
probably be light tan or slightly gray in color. This is
evident with most grades of commercial gasoline
(Fig. 24). There will not be evidence of electrode
burning. Gap growth will not average more than ap-
proximately 0.025 mm (.001 in) per 1600 km (1000
miles) of operation. Spark plugs that have normal
wear can usually be cleaned, have the electrodes
filed, have the gap set and then be installed.Some fuel refiners in several areas of the United
States have introduced a manganese additive (MMT)
for unleaded fuel. During combustion, fuel with
MMT causes the entire tip of the spark plug to be
coated with a rust colored deposit. This rust color can
be misdiagnosed as being caused by coolant in the
combustion chamber. Spark plug performance is not
affected by MMT deposits.
COLD FOULING/CARBON FOULING
Cold fouling is sometimes referred to as carbon
fouling. The deposits that cause cold fouling are ba-
sically carbon (Fig. 24). A dry, black deposit on one
or two plugs in a set may be caused by sticking
valves or defective spark plug cables. Cold (carbon)
fouling of the entire set of spark plugs may be caused
by a clogged air filter or repeated short operating
times (short trips).
ELECTRODE GAP BRIDGING
Electrode gap bridging may be traced to loose de-
posits in the combustion chamber. These deposits ac-
cumulate on the spark plugs during continuous stop-
and-go driving. When the engine is suddenly
subjected to a high torque load, deposits partially liq-
uefy and bridge the gap between electrodes (Fig. 25).
This short circuits the electrodes. Spark plugs with
electrode gap bridging can be cleaned using standard
procedures.
SCAVENGER DEPOSITS
Fuel scavenger deposits may be either white or yel-
low (Fig. 26). They may appear to be harmful, but
this is a normal condition caused by chemical addi-
tives in certain fuels. These additives are designed to
change the chemical nature of deposits and decrease
spark plug misfire tendencies. Notice that accumula-
tion on the ground electrode and shell area may be
heavy, but the deposits are easily removed. Spark
Fig. 24 Normal Operation and Cold (Carbon) Fouling
Fig. 25 Electrode Gap Bridging
8D - 16 IGNITION SYSTEMSJ

plugs with scavenger deposits can be considered nor-
mal in condition and can be cleaned using standard
procedures.
CHIPPED ELECTRODE INSULATOR
A chipped electrode insulator usually results from
bending the center electrode while adjusting the
spark plug electrode gap. Under certain conditions,
severe detonation can also separate the insulator
from the center electrode (Fig. 27). Spark plugs with
this condition must be replaced.
PREIGNITION DAMAGE
Preignition damage is usually caused by excessive
combustion chamber temperature. The center elec-
trode dissolves first and the ground electrode dis-
solves somewhat latter (Fig. 28). Insulators appear
relatively deposit free. Determine if the spark plug
has the correct heat range rating for the engine. De-
termine if ignition timing is over advanced, or if
other operating conditions are causing engine over-
heating. (The heat range rating refers to the operat-
ing temperature of a particular type spark plug.
Spark plugs are designed to operate within specifictemperature ranges. This depends upon the thickness
and length of the center electrodes porcelain insula-
tor.)
SPARK PLUG OVERHEATING
Overheating is indicated by a white or gray center
electrode insulator that also appears blistered (Fig.
29). The increase in electrode gap will be consider-
ably in excess of 0.001 inch per 1000 miles of opera-
tion. This suggests that a plug with a cooler heat
range rating should be used. Over advanced ignition
timing, detonation and cooling system malfunctions
can also cause spark plug overheating.
SPARK PLUG SECONDARY CABLES
TESTING
Spark plug cables are sometimes referred to as sec-
ondary ignition cables or secondary wires. The cables
transfer electrical current from the distributor to in-
dividual spark plugs at each cylinder. The spark plug
cables are of nonmetallic construction and have a
built in resistance. The cables provide suppression of
radio frequency emissions from the ignition system.
Fig. 26 Scavenger Deposits
Fig. 27 Chipped Electrode Insulator
Fig. 28 Preignition Damage
Fig. 29 Spark Plug Overheating
JIGNITION SYSTEMS 8D - 17

Check the high-tension cable connections for good
contact at the ignition coil, distributor cap towers
and spark plugs. Terminals should be fully seated.
The terminals and spark plug covers should be in
good condition. Terminals should fit tightly to the ig-
nition coil, distributor cap and spark plugs. The
spark plug cover (boot) of the cable should fit tight
around the spark plug insulator. Loose cable connec-
tions can cause corrosion and increase resistance, re-
sulting in shorter cable service life.
Clean the high tension cables with a cloth moist-
ened with a nonflammable solvent and wipe dry.
Check for brittle or cracked insulation.
When testing secondary cables for damage with an
oscilloscope, follow the instructions of the equipment
manufacturer.
If an oscilloscope is not available, spark plug cables
may be tested as follows:
CAUTION: Do not leave any one spark plug cable
disconnected for longer than necessary during test-
ing. This may cause possible heat damage to the
catalytic converter. Total test time must not exceed
ten minutes.
With the engine not running, connect one end of a
test probe to a good ground. Start the engine and run
the other end of the test probe along the entire
length of all spark plug cables. If cables are cracked
or punctured, there will be a noticeable spark jump
from the damaged area to the test probe. The cable
running from the ignition coil to the distributor cap
can be checked in the same manner. Cracked, dam-
aged or faulty cables should be replaced with resis-
tance type cable. This can be identified by the words
ELECTRONIC SUPPRESSION printed on the cable
jacket.
Use an ohmmeter to test for open circuits, exces-
sive resistance or loose terminals. Remove the dis-
tributor cap from the distributor.Do not remove
cables from cap.Remove cable from spark plug.
Connect ohmmeter to spark plug terminal end of ca-
ble and to corresponding electrode in distributor cap.
Resistance should be 250 to 1000 Ohms per inch of
cable. If not, remove cable from distributor cap tower
and connect ohmmeter to the terminal ends of cable.
If resistance is not within specifications as found in
the Spark Plug Cable Resistance chart, replace the
cable. Test all spark plug cables in this manner.To test ignition coil-to-distributor cap cable, do not
remove the cable from the cap. Connect ohmmeter to
rotor button (center contact) of distributor cap and
terminal at ignition coil end of cable. If resistance is
not within specifications as found in the Spark Plug
Cable Resistance chart, remove the cable from the
distributor cap. Connect the ohmmeter to the termi-
nal ends of the cable. If resistance is not within spec-
ifications as found in the Spark Plug Cable
Resistance chart, replace the cable. Inspect the igni-
tion coil tower for cracks, burns or corrosion.
For removal and installation of spark plug cables,
refer to Spark Plug Secondary Cables in the Compo-
nent Removal/Installation section.
THROTTLE 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 throttle position sensor can be tested with a
digital voltmeter. The center terminal of the sensor
connector is the output terminal (Figs. 30 or 31).
SPARK PLUG CABLE RESISTANCE
Fig. 30 SensorÐ2.5L Engine
Fig. 31 SensorÐ4.0L Engine
8D - 18 IGNITION SYSTEMSJ