1988 JEEP CHEROKEE Service Repair Manual

ignition timing for the particular cylinder is either advanced or
retarded as necessary by the ECU according to sensor inputs.
Fig. 9: View of Engine Speed Sensor, Showing Trigger Notches & TDC
Position
Courtesy of Chrysler Motors.
SYNC PULSE SIGNAL GENERATOR (STATOR)

The sync pulse signal generator is located in the distributor
and works in conjunction with the engine speed sensor to provide the
ECU with input to establish and maintain correct injector firing
order. See Fig. 10. A pulse ring mounted to the distributor shaft
references the position of pistons one and six as it rotates through
the sync pulse signal generator's magnetic field.
Fig. 10: Location of Sync Pulse Signal Generator & Pulse Ring
Courtesy of Chrysler Motors.
The pulse ring rotates through the sync pulse signal
generator for l80
. When the leading edge of the pulse enters the sync

pulse signal generator, the magnetic field becomes weaker. This
indicates the position of piston number one to the ECU. When the
trailing edge of the pulse ring leaves the sync pulse signal
generator, the magnetic field becomes stronger. This indicates the
position of piston number six.
The sync pulse signal input and engine speed sensor input
allow the ECU to establish the necessary reference point to
synchronize the fuel injection.
Fig. 11: Sync Pulse Signal Generator Operation
Courtesy of Chrysler Motors.
KNOCK SENSOR
The knock sensor is located on the lower left side of the
engine block just above the oil pan. See Fig. 12. The knock sensor
provides an input to the ECU, indicating detonation (knock) during
engine operation. When knock occurs, the ECU retards the ignition
advance to eliminate the knock at the appropriate cylinder(s).

Fig. 12: Location of Knock Sensor
Courtesy of Chrysler Motors.
IGNITION SYSTEM PRECAUTIONS
When disconnecting wire from spark plug or distributor cap,
twist rubber boot slightly to loosen. Grasp boot (not wire) and pull
with steady, even force.
When separating control unit connectors, pull with firm,
straight force. Do not pry apart with screwdriver. When connecting,
press together firmly to overcome hydraulic pressure of silicone
grease.

If connector locking tabs weaken or break, press together
firmly and bind with electrical tape or a harness tie strap to assure
good connection.
COMPONENT LOCATIONS
COMPONENT LOCATIONS








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Component Location
Coolant Temperature
Sensor (CTS) .................... On left side of engine
Engine Speed Sensor .... Attached to flywheel cover housing
Ignition Control Module ......... Mounted to ignition coil
Knock Sensor ................ On lower left side of engine
Manifold Absolute Pressure
(MAP) Sensor ................ Under the hood, on firewall
Manifold Air Temperature
(MAT) sensor .......................... On intake manifold
Sync Pulse Signal Generator ........... Inside distributor









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TESTING
BASIC DIAGNOSTIC PROCEDURES
This section contains information for determining individual
system component performance. Diagnosis of ECU/Engine Control System
is performed using the DRB-II Diagnostic Tester. See the appropriate
Computerized Engine Control System article in this section.
COOLANT TEMPERATURE SENSOR (CTS) TEST
1) Disconnect the CTS wire harness connector from the CTS.
See Fig. 13 .
Fig. 13: Location of Coolant Temperature Sensor (CTS)
Courtesy of Chrysler Motors.
2) Using a high input impedance (digital) volt-ohmmeter, tes\
t
the resistance of the sensor. Resistance should be less than l000 ohms

with a warm engine. See the COOLANT TEMPERATURE SENSOR (CTS)
RESISTANCE VALUES table below.
COOLANT TEMPERATURE SENSOR (CTS) RESISTANCE VALUES








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Degrees F  Degrees C  Resistance (Ohms) 
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-40  -40  100,700 
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0  -18  25,000 
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20  -7  13,500 
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40  4  7,500 
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70  20  3,400 
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100  38  1,600 
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160  70  450 
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212  100  185 










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3) If any resistance is not within the range as specified in
the table, replace the coolant temperature sensor.
4) Test the resistance of the wire harness between the ECU
wire harness connector terminal "D-3" and the sensor connector
terminal "C-10".
5) Repair any open circuits.

MANIFOLD AIR TEMPERATURE (MAT) SENSOR TEST
1) Disconnect the MAT wire harness connector from the MAT
sensor. See Fig. 14*.
Fig. 14: Location of Manifold Air Temperature (MAT) Sensor
Courtesy of Chrysler Motors.
2) Using a high input impedance (digital) volt-ohmmeter, tes\
t
the resistance of the sensor. Resistance should be less than l000 ohms

with a warm engine. See the MANIFOLD AIR/FUEL TEMPERATURE (MAT) SENSOR\
RESISTANCE VALUES table below.
MANIFOLD AIR/FUEL TEMPERATURE (MAT)
SENSOR RESISTANCE VALUES








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Degrees F  Degrees C  Resistance (Ohms) 
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-40  -40  100,700 
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0  -18  25,000 
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20  -7  13,500 
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40  4  7,500 
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70  20  3,400 
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100  38  1,600 
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160  70  450 
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212  100  185 










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3) If any resistance is not within the range as specified in
the table, replace the MAT sensor.
4) Test the resistance of the wire harness between the ECU
wire harness connector terminal "D-3" and the sensor connector
terminal "C-8".
5) Repair as necessary any circuits displaying a value
greater than one ohm.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR TEST
1) Inspect the MAP sensor vacuum hose connections at the
throttle body and the MAP sensor. See Fig. 15. Repair vacuum hose or
connections as necessary.
Fig. 15: Manifold Absolute Pressure (MAP) Sensor Connectors
Courtesy of Chrysler Motors.
2) With the ignition switch ON and engine OFF, test the MAP
sensor output voltage at the MAP sensor connector terminal "B". See

Fig. 15. With a hot idle condition, the voltage reading should drop to
between 0.5-1.5 volts.
3) Test ECU terminal "C-6" for the same voltage as described
in step 2) to verify the wiring harness. Repair as necessary.
4) Test the MAP sensor supply voltage at the sensor connector
terminal "C" with the ignition ON. Voltage reading should be between
4.5-5.5 volts. Check that the voltage reading at terminal "C-14" is
also 4.5-5.5 volts.
5) Repair or replace the wire harness as necessary.
6) Test the MAP sensor ground circuit at the sensor connector
terminal "A" and ECU connector terminal "D-3". Repair the wire harness
as necessary.
7) Using an ohmmeter, test the MAP sensor ground circuit at
the ECU connector between terminal "D-3" and terminal "B-ll". If an
open circuit is indicated, check for a defective sensor ground
connection. The MAP sensor ground is located on the right side of the
engine block.
8) If, after performing step 7), the ground connection is
verified to be good, check for a short to 12 volts at terminal "D-3".
If a short is found, repair the short and then replace the ECU.
KNOCK SENSOR TEST
1) Connect Diagnostic Tester M.S. 1700 to the vehicle.
2) Go to "State Display" mode.
3) Start the engine and let idle.
4) Observe knock sensor value.
5) Using the tip of a screwdriver, lightly tap the cylinder
block near the sensor while observing the knock sensor value.
6) The knock sensor value should increase when the cylinder
block is tapped.
7) If the knock sensor value does not increase while tapping
on the cylinder block near the knock sensor, check for proper
connections to the knock sensor. If connections are good, replace
the knock sensor. Refer to REMOVAL & INSTALLATION in this article.
ENGINE SPEED SENSOR TEST
1) Disconnect the engine speed sensor connector from the
ignition control module.
2) place an ohmmeter between terminals "A" and "B" (marked on\
the connector). On a hot engine, the resistance reading should be
between 125-275 ohms.
3) Replace sensor if readings are not within specification.
Fig. 16: View of Engine Speed Sensor Connectors (At Sensor)
Courtesy of Chrysler Motors.