1988 JEEP CHEROKEE sensor

position. The engine speed sensor senses TDC, BDC, and engine speed by
detecting the flywheel teeth as they pass by the sensor during engine
operation. The engine speed sensor is non-adjustable.
The flywheel has three trigger notches, 120
apart. See
Fig. 7 . There are 20 small teeth between each trigger notch. Each
large trigger notch is located 12 small teeth before each Top Dead
Center (TDC) position of the corresponding pistons.
Fig. 7: Location of Engine Speed Sensor
Courtesy of Chrysler Motors.
When a small tooth and notch pass the magnet core in the
sensor, the concentration, followed by the collapse of the magnetic

flux induces a small voltage spike to the sensor pickup coil winding.
These small voltage spikes enable the ECU to count the teeth as they
pass the sensor. When a large trigger tooth and notch pass the
magnetic core in the sensor, the increased concentration, and then
collapse of the magnetic flux induces a higher voltage spike into the
sensor pickup coil winding. See Fig. 8.
Fig. 8: Engine Speed Sensor Operation
Courtesy of Chrysler Motors.
The higher voltage spike is an indication to the ECU that a
piston will reach its TDC position, 12 teeth later. See Fig. 9. The

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