1995 JEEP CHEROKEE fuel

The gauge needle moves as the movable permanent
magnet aligns itself to the changing magnetic fields
created around it by the electromagnets.
COOLANT TEMPERATURE GAUGE
The coolant temperature gauge gives an indication
of engine coolant temperature. The coolant tempera-
ture sending unit is a thermistor that changes elec-
trical resistance with changes in engine coolant
temperature. High sending unit resistance causes
low coolant temperature readings. Low resistance
causes high coolant temperature readings. Sending
unit resistance values are shown in a chart in Spec-
ifications.
FUEL GAUGE
The fuel gauge gives an indication of the level of
fuel in the fuel tank. The fuel gauge sending unit has
a float attached to a swing-arm in the fuel tank. The
float moves up or down within the fuel tank as fuel
level changes. As the float moves, an electrical con-
tact on the swing-arm wipes across a resistor coil,
which changes sending unit resistance. High sending
unit resistance causes high fuel level readings. Low
resistance causes low fuel level readings. Sending
unit resistance values are shown in a chart in Spec-
ifications.
OIL PRESSURE GAUGE
The oil pressure gauge gives an indication of en-
gine oil pressure. The combination oil pressure send-
ing unit contains a flexible diaphragm. The
diaphragm moves in response to changes in engine
oil pressure. As the diaphragm moves, sending unit
resistance increases or decreases. High resistance on
the gauge side of the sending unit causes high oil
pressure readings. Low resistance causes low oil
pressure readings. Sending unit resistance values are
shown in a chart in Specifications.
SPEEDOMETER/ODOMETER
The speedometer/odometer give an indication of ve-
hicle speed and travel distance. The speedometer re-
ceives a vehicle speed pulse signal from the Vehicle
Speed Sensor (VSS). An electronic integrated circuit
contained within the speedometer reads and analyzes
the pulse signal. It then adjusts the ground path re-
sistance of one electromagnet in the gauge to control
needle movement. It also sends signals to an electric
stepper motor to control movement of the odometer
number rolls. Frequency values for the pulse signal
are shown in a chart in Specifications.
The VSS is mounted to an adapter near the trans-
fer case output shaft. The sensor is driven through
the adapter by a speedometer pinion gear. The
adapter and pinion vary with transmission, axle ratio
and tire size. Refer to Group 21 - Transmission and
Transfer Case for more information.
TACHOMETER
The tachometer gives an indication of engine speed
in Revolutions-Per-Minute (RPM). With the engine
running, the tachometer receives an engine speed
pulse signal from the Powertrain Control Module
(PCM). An electronic integrated circuit contained
within the tachometer reads and analyzes the pulse
signal. It then adjusts the ground path resistance of
one electromagnet in the gauge to control needle
movement. Frequency values for the pulse signal are
shown in a chart in Specifications.
TRIP ODOMETER
The trip odometer is driven by the same electronic
integrated circuit as the speedometer/odometer. How-
ever, by depressing the trip odometer reset knob on
the face of the speedometer, the trip odometer can be
reset to zero. The trip odometer is serviced only as a
part of the speedometer/odometer gauge assembly.
VOLTMETER
The voltmeter is connected in parallel with the bat-
tery. With the ignition switch ON, the voltmeter in-
dicates battery or generator output voltage,
whichever is greater.
INDICATOR LAMPS
All indicator lamps, except the four-wheel drive in-
dicator, are located in the main cluster tell-tale area
above the steering column opening. Each of the
lamps is served by the main cluster printed circuit
and cluster connector. The four-wheel drive indicator
lamp is located in the gauge package cluster and is
served by the gauge package printed circuit and clus-
ter connector.
Up to eleven indicator lamps can be found in the
tell-tale area of the main cluster. These lamps are ar-
ranged in two rows, with six lamps in the upper row
and five lamps in the lower row.
ANTI-LOCK BRAKE SYSTEM LAMP
The Anti-Lock Brake System (ABS) lamp is
switched to ground by the ABS module. The module
lights the lamp when the ignition switch is turned to
the START position as a bulb test. The lamp will
stay on for 3 to 5 seconds after vehicle start-up to in-
dicate a system self-test is in process. If the lamp re-
mains on after start-up, or comes on and stays on
while driving, it may indicate that the ABS module
has detected a system malfunction or that the system
has become inoperative. Refer to Group 5 - Brakes
for more information.
BRAKE WARNING LAMP
The brake warning lamp warns the driver that the
parking brake is applied or that the pressures in the
two halves of the split brake hydraulic system are
unequal. With the ignition switch turned ON, battery
JINSTRUMENT PANEL AND GAUGESÐYJ 8E - 25

voltage is supplied to one side of the indicator bulb. A
ground path for the bulb is provided by 3 switches.
The bulb will light when:
²the brake warning switch is closed (indicating un-
equal brake system hydraulic pressures possibly due
to brake fluid leakage)
²the ignition switch is in the START position (bulb
test)
²the parking brake switch is closed (parking brake
is applied).
Refer to Group 5 - Brakes for more information.
FOUR-WHEEL DRIVE INDICATOR LAMP
This lamp lights when the transfer case is engaged
in the 4H or 4L position. Voltage is supplied to one
side of the indicator bulb. A switch on the front axle
disconnect housing is connected to the other side of
the indicator bulb. When the switch is closed, a path
to ground is provided and the indicator bulb lights.
HEADLAMP HIGH BEAM INDICATOR LAMP
The high beam indicator lamp is controlled by the
headlamp dimmer switch. One side of the indicator
bulb is grounded at all times. The other side of the
bulb receives battery feed through the contacts of the
dimmer switch when the turn signal switch lever is
actuated to turn the headlamp high beams on. Refer
to Group 8L - Lamps for more information.
MALFUNCTION INDICATOR LAMP
The CHECK ENGINE or Malfunction Indicator
Lamp (MIL) lights each time the ignition switch is
turned ON, and stays on for 3 seconds as a bulb test.
If the Powertrain Control Module (PCM) receives an
incorrect signal or no signal from certain fuel or
emission system related circuits or components, the
lamp is turned on. This will indicate that the PCM
has recorded a Diagnostic Trouble Code (DTC) in
electronic memory for a circuit or component mal-
function. Refer to Group 14 - Fuel System for more
information.
SEAT BELT REMINDER LAMP
The seat belt reminder lamp lights for 4 to 8 sec-
onds after the ignition switch is turned to the ON po-sition. A timer in the chime/buzzer module controls
ignition-switched battery feed to the lamp. Refer to
Group 8U - Chime/Buzzer Warning Systems for more
information.
TURN SIGNAL INDICATOR LAMPS
The left and right turn signal indicator lamps are
controlled by the turn signal and hazard warning
switches. One side of the bulb for each lamp is
grounded at all times. The other side of the bulb re-
ceives battery feed through the contacts of the turn
signal switch, when the turn signal lever or hazard
warning button are actuated. Refer to Group 8J -
Turn Signal and Hazard Warning Systems for more
information.
UPSHIFT INDICATOR LAMP
Vehicles equipped with manual transmissions have
an optional upshift indicator lamp. Ground feed for
the lamp is switched by the Powertrain Control Mod-
ule (PCM). The lamp lights to indicate when the
driver should shift to the next highest gear for best
fuel economy. The PCM will turn the lamp off after 3
to 5 seconds if the upshift is not performed. The lamp
will remain off until the vehicle stops accelerating
and is brought back to the range of lamp operation,
or until the transmission is shifted into another gear.
The indicator lamp is normally on when the igni-
tion switch is turned ON and is turned off when the
engine is started. The lamp will be turned on during
vehicle operation according to engine speed and load.
CLUSTER ILLUMINATION LAMPS
All cluster illumination lamps receive battery feed
from the instrument lamps fuse in the fuseblock
module through the panel dimmer switch. When the
park or headlamps are on, the cluster illumination
lamps light. Illumination brightness can be adjusted
by rotating the panel dimmer thumb-wheel, which is
next to the headlamp switch.
8E - 26 INSTRUMENT PANEL AND GAUGESÐYJJ

DIAGNOSIS
GAUGES
If an individual gauge is inoperative, see the diag-
nostic procedure under the heading for that gauge. If
more than one gauge in the main cluster or gauge
package cluster is inoperative, perform the following:
(1) Check fuse 9 in the fuseblock module. If OK, go
to next step. If not OK, replace fuse.
(2) Check for battery voltage at fuse 9 with igni-
tion switch in ON position. If OK, go to next step. If
not OK, repair open in circuit from ignition switch
and/or refer to Group 8D - Ignition Systems for test-
ing of ignition switch.
(3) Turn ignition switch to OFF. Disconnect battery
negative cable. Remove the affected instrument clus-
ter bezel and cluster assembly. Disconnect the cluster
connector.
(4) Connect battery negative cable. Turn ignition
switch to ON. If problem is in main cluster, check for
battery voltage at cavity 3 and cavity 19 of main
cluster connector. If problem is in gauge package
cluster, check for battery voltage at cavity 2 and cav-
ity 12 of gauge package cluster connector. If OK, go
to next step. If not OK, repair open in circuit from
fuse 9 as required.
(5) Turn ignition switch to OFF. Disconnect battery
negative cable. Probe cavities 14 and 20 of main clus-
ter connector, or cavities 1 and 13 of gauge package
cluster connector. Check for continuity to a good
ground. There should be continuity. If OK, replace
the cluster printed circuit. If not OK, repair open in
circuit as required.
COOLANT TEMPERATURE GAUGE
The diagnosis found here addresses an inoperative
gauge condition. If the problem being diagnosed is re-
lated to gauge accuracy, be certain to confirm that
problem is with gauge and not with cooling system
performance. Actual engine coolant temperature
should be checked with a test gauge or thermometer
and compared to gauge readings before you proceed
with gauge diagnosis. Refer to Group 7 - Cooling Sys-
tem for more information.
(1) Turn ignition switch to ON. Disconnect coolant
temperature sending unit connector. Sending unit
(Fig. 1) is located near left rear corner of the cylinder
head. The gauge needle should move to low end of
gauge scale. If OK, go to next step. If not OK, go to
step 3.
(2) Install a jumper wire from sending unit wiring
to ground. The gauge needle should move to high end
of gauge scale. If OK, replace sending unit. If not
OK, remove jumper wire and go to next step.(3) Turn ignition switch to OFF. Disconnect battery
negative cable. Remove center instrument cluster be-
zel and gauge package cluster assembly. Disconnect
cluster connector.
(4) Probe cavity 11 of cluster connector. Check for
continuity to a good ground. There should be no con-
tinuity. If OK, go to next step. If not OK, repair short
in circuit as required.
(5) Still probing cavity 11 of cluster connector,
check for continuity to sending unit wiring connector.
There should be continuity. If OK, replace gauge. If
not OK, repair open in circuit as required.
FUEL GAUGE
The diagnosis found here addresses an inoperative
gauge condition. If the problem being diagnosed is re-
lated to gauge accuracy, be certain to confirm that
problem is with gauge and not with fuel tank. In-
spect fuel tank for signs of damage or distortion that
could affect sending unit performance before you pro-
ceed with gauge diagnosis. Refer to Group 14 - Fuel
System for more information.
(1) Turn ignition switch to ON. Disconnect fuel
gauge sending unit connector. Connector is located
near the left front upper corner of fuel tank. The
gauge needle should move to high end of gauge scale.
If OK, go to next step. If not OK, go to step 4.
(2) Connect a jumper wire between terminals 1
and 2 in the body half of the fuel gauge sending unit
connector (Fig. 2). The gauge needle should move to
low end of gauge scale. If OK, refer to Group 14 -
Fuel System for procedure to replace sending unit. If
not OK, remove jumper wire and go to next step.
(3) Turn ignition switch to OFF. Disconnect battery
negative cable. Check for continuity between termi-
nal 1 in the body half of fuel gauge sending unit con-
nector and a good ground. There should be
Fig. 1 Coolant Temperature Sending Unit - Typical
JINSTRUMENT PANEL AND GAUGESÐYJ 8E - 27

continuity. If OK, go to next step. If not OK, repair
circuit to ground as required.
(4) Remove center instrument cluster bezel and
gauge package cluster assembly. Disconnect cluster
connector.
(5) Probe cavity 6 of cluster connector. Check for
continuity to a good ground. There should be no con-
tinuity. If OK, go to next step. If not OK, repair short
circuit as required.
(6) Still probing cavity 6 of cluster connector, check
for continuity to cavity 2 of sending unit wiring body
half connector. There should be continuity. If OK, re-
place gauge. If not OK, repair open circuit as re-
quired.
OIL PRESSURE GAUGE
The diagnosis found here addresses an inoperative
gauge condition. If the problem being diagnosed is re-
lated to gauge accuracy, be certain to confirm that
problem is with gauge and not with engine oiling sys-
tem performance. Actual engine oil pressure should
be checked with a test gauge and compared to gauge
readings before you proceed with gauge diagnosis.
Refer to Group 9 - Engines for more information.
(1) Turn ignition switch to ON. Disconnect oil pres-
sure sending unit connector. The sending unit (Fig. 3)
is located on right side of engine block. On 2.5L en-
gine, it is just forward of ignition distributor and just
to the rear of generator mounting bracket. On 4.0L
engine, it is just to the rear of ignition distributor
and above oil filter adapter. The gauge needle should
move to high end of gauge scale. If OK, go to next
step. If not OK, go to step 3.
(2) Install a jumper wire from sending unit wiring
to ground. The gauge needle should move to low end
of gauge scale. If OK, replace sending unit. If not
OK, remove jumper wire and go to next step.(3) Turn ignition switch to OFF. Disconnect battery
negative cable. Remove center instrument cluster be-
zel and gauge package cluster assembly. Disconnect
cluster connector.
(4) Probe cavity 9 of cluster connector. Check for
continuity to a good ground. There should be no con-
tinuity. If OK, go to next step. If not OK, repair short
circuit as required.
(5) Still probing cavity 9 of cluster connector, check
for continuity to sending unit wire connector. There
should be continuity. If OK, replace gauge. If not OK,
repair open circuit as required.
SPEEDOMETER/ODOMETER
The diagnosis found here addresses an inoperative
gauge condition. If the problem being diagnosed is re-
lated to gauge accuracy, be certain to confirm that
problem is with gauge and not with incorrect speed-
ometer pinion, axle ratio or tire size. Refer to Group
21 - Transmission and Transfer Case for more infor-
mation.
(1) Perform vehicle speed sensor test as described
in the appropriate Powertrain Diagnostic Procedures
manual. If OK, go to next step. If not OK, replace ve-
hicle speed sensor.
(2) Disconnect battery negative cable. Unplug vehi-
cle speed sensor, PCM, and daytime running lamp
module connectors. Remove left instrument cluster
bezel and main cluster assembly. Disconnect cluster
connector.
(3) Probe cavity 13 of cluster connector. Check for
continuity to a good ground. There should be no con-
tinuity. If OK, go to next step. If not OK, repair short
circuit as required.
(4) Still probing cavity 13 of cluster connector,
check for continuity to cavity 1 of vehicle speed sen-
sor connector (Fig. 4). There should be continuity. If
OK, replace speedometer/odometer. If not OK, repair
open circuit as required.
Fig. 2 Fuel Gauge Sending Unit ConnectorFig. 3 Oil Pressure Sending Unit - Typical
8E - 28 INSTRUMENT PANEL AND GAUGESÐYJJ

(9) Turn ignition switch to START position and
hold there. Check for continuity between terminal for
circuit G11 of ignition switch and a good ground.
There should be continuity. If not OK, replace igni-
tion switch.
(10) Unplug brake warning switch connector.
Check for continuity between parking brake switch
connector and a good ground. There should be no
continuity. If OK, go to next step. If not OK, repair
short circuit as required.
(11) Check for continuity between parking brake
switch connector and brake warning switch connector
cavity for circuit G11. There should be continuity. If
OK, replace parking brake switch. If not OK, repair
open circuit to brake warning switch as required.
FOUR-WHEEL DRIVE INDICATOR LAMP
(1) Apply parking brake, start engine, vehicle in
4WD Lock or 4WD.
(2) Unplug switch and touch harness side of wire
to ground. Lamp should light. If OK, check switch
operation, replace if bad. If bulb is OK, repair open
to indicator.
HEADLAMP HIGH BEAM INDICATOR LAMP
(1) Check that headlamp high beams are func-
tional. If OK, go to next step. If not OK, refer to
Group 8L - Lamps for diagnosis of headlamp system.
(2) Disconnect battery negative cable. Remove left
instrument cluster bezel and main cluster assembly.
Unplug cluster connector. Connect battery negative
cable. Turn headlamps on and select high beam.
Check for battery voltage at cavity 17 of cluster con-
nector. If OK, replace indicator bulb. If not OK, re-
pair circuit to headlamp dimmer switch as required.
MALFUNCTION INDICATOR LAMP
The diagnosis found here addresses an inoperative
lamp condition. If the lamp comes on and stays on
with engine running, refer to Group 14 - Fuel System
for diagnosis. If no fuel or emission system problem
is found, the following procedure will help locate a
short or open in the lamp circuit.
(1) Disconnect battery negative cable. Unplug PCM
connector. Install a jumper wire from cavity 32 of
PCM connector (Fig. 5) to a good ground. Connect
battery negative cable. Turn ignition switch to ON.
Lamp should light. Remove jumper wire and lamp
should go OFF. If OK, refer to Powertrain Diagnostic
Procedures to check PCM. If not OK, go to next step.
(2) Turn ignition switch to OFF. Disconnect battery
negative cable. Remove left instrument cluster bezel
and main cluster assembly. Install a jumper wire
from cavity 2 of cluster connector to a good ground.
Connect battery negative cable. Turn ignition switch
to ON. Lamp should light. If OK, go to next step. If
not OK, replace bulb.(3) Turn ignition switch to OFF. Disconnect battery
negative cable. Unplug cluster connector. Check for
continuity between cavity 2 of cluster connector and
a good ground. There should be no continuity. If OK,
go to next step. If not OK, repair short circuit to
PCM as required.
(4) Check continuity between cavity 2 of cluster
connector and cavity 32 of PCM connector. There
should be continuity. If not OK, repair open circuit to
PCM as required.
SEAT BELT REMINDER LAMP
(1) Refer to Group 8U - Chime/Buzzer Warning
Systems to check chime/buzzer module operation. If
OK, go to next step. If not OK, replace chime/buzzer
module.
(2) Disconnect battery negative cable. Remove left
instrument cluster bezel and main cluster assembly.
Unplug cluster connector. Check for continuity be-
tween cavity 20 of cluster connector and a good
ground. There should be continuity. If OK, plug clus-
ter connector back into cluster and go to next step. If
not OK, repair open circuit to ground as required.
(3) Connect battery negative cable. Install a
jumper wire between a 12-volt battery feed and cav-
ity 16 of cluster connector. Lamp should light. If OK,
go to next step. If not OK, replace bulb.
(4) Disconnect battery negative cable. Unplug
chime/buzzer module from fuseblock module. Unplug
cluster connector. Check for continuity between cav-
ity 16 of cluster connector and a good ground. There
should be no continuity. If OK, go to next step. If not
OK, repair short circuit to chime/buzzer module as
required.
(5) Check continuity between cavity 16 of cluster
connector and cavity for terminal A3 of chime/buzzer
module (Fig. 7) in fuseblock module. There should be
continuity. If not OK, repair open circuit to chime/
buzzer module as required.
Fig. 7 Chime/Buzzer Module Receptacle
JINSTRUMENT PANEL AND GAUGESÐYJ 8E - 33

(5) The coil ground terminal (85) is connected to
the electromagnet in the relay. Check as follows:
(a) On YJ models, it is grounded at all times.
There should be continuity to ground at cavity for
relay terminal 85 at all times. If not, repair circuit
to ground as required.
(b) On XJ models, it is energized when the head-
lamp switch is on. There should be battery voltage
at cavity for relay terminal 85 with the headlamp
switch on. If not, repair circuit to headlamp switch
as required.
ANTENNA
The following four tests are used to diagnose the
antenna with an ohmmeter:
²mast to ground test (Test 1)
²tip-of-mast to tip-of-conductor test (Test 2)
²body ground to battery ground test (Test 3)
²body ground to coaxial shield test (Test 4).
Ohmmeter test lead connections for each test are
shown in Figure 3.
TEST 1
Test 1 determines if the antenna mast is insulated
from the base. Proceed as follows:
(1) Disconnect antenna cable lead from radio chas-
sis and isolate.
(2) Connect one ohmmeter lead to tip of antenna
mast and the other lead to the antenna base. Check
for continuity.
(3) There should be no continuity. If continuity is
found, replace defective or damaged antenna base
and cable assembly.
TEST 2
Test 2 checks the antenna for an open circuit as fol-
lows:
(1) Disconnect the antenna cable lead from the ra-
dio chassis.
(2) Connect one ohmmeter test lead to tip of an-
tenna mast. Connect remaining lead to tip of an-
tenna cable lead (the part inserted into the radio).
(3) Continuity should exist (ohmmeter should only
register a fraction of an ohm). High or infinite resis-
tance indicates damage to the base and cable assem-
bly. Replace if required.
TEST 3
Test 3 checks condition of the vehicle body ground
connection as follows:
(1) Connect one ohmmeter test lead to the vehicle
fender and the other lead to the battery negative
post.
(2) Resistance should be less than one ohm.
(3) If resistance is more than one ohm, check the
braided ground strap connected to the engine and ve-
hicle body for being loose, corroded, or damaged. Re-
pair as necessary.
TEST 4
Test 4 checks condition of the ground between the
antenna base and vehicle body as follows:
(1) Connect one ohmmeter test lead to the fender
and the other lead to the crimp on the coaxial an-
tenna cable shield.
(2) Resistance should be less then one ohm.
(3) If resistance is more then one ohm:
(a) On YJ models, replace the antenna base at-
taching screws with new cadmium plated screws.
(b) On XJ models, clean and/or tighten antenna
base to fender mounting hardware.
RADIO FREQUENCY INTERFERENCE
Inspect ground connections at:
²blower motor
²electric fuel pump
²generator
²ignition module
²wiper motor
²antenna coaxial ground
²radio ground
²body-to-engine ground strap (braided).
Clean, tighten or repair as required.
Also inspect the following secondary ignition sys-
tem components:
²spark plug wire routing and condition
²distributor cap and rotor
²ignition coil
²spark plugs.
Reroute spark plug wires or replace components as
required.
Fig. 3 Antenna Tests
JAUDIO SYSTEMS 8F - 5

VEHICLE SPEED CONTROL SYSTEM
CONTENTS
page page
DIAGNOSIS............................. 2
GENERAL INFORMATION.................. 1SERVICE PROCEDURES................... 9
GENERAL INFORMATION
The vehicle speed control system (Fig. 1) is an
available option on all XJ (Cherokee) models. The
system is electronically controlled and vacuum oper-
ated. Following are general descriptions of the major
components in the vehicle speed control system. Re-
fer to Group 8W - Wiring Diagrams for complete cir-
cuit descriptions and diagrams.
SPEED CONTROL SERVO
The speed control servo is mounted to a bracket on
the right side inner fender shield in the engine com-
partment. The servo unit consists of a solenoid valve
body, a vacuum servo and the mounting bracket. The
PCM controls the solenoid valve body. The solenoid
valve body controls the application and release of
vacuum to the diaphragm of the vacuum servo. The
servo unit cannot be repaired and is serviced only as
a complete assembly.
SPEED CONTROL SWITCH
The speed control switch module is mounted to the
center of the steering wheel below the driver's airbag
module. The PCM monitors the state of the speed
control switches. The individual switches are labeled:
OFF/ON, RESUME/ACCEL, SET/COAST. Refer to
the owner's manual for more information on speed
control switch functions and setting procedures. The
individual switches cannot be repaired. If one switch
fails, the entire switch module must be replaced.
STOP LAMP SWITCH
Vehicles with the speed control option use a dual
function stop lamp switch. The switch is mounted in
the same location as the conventional stop lamp
switch, on the brake pedal mounting bracket under
the instrument panel. The PCM monitors the state of
the dual function stop lamp switch. Refer to Group 5
- Brakes for more information on stop lamp switch
service and adjustment procedures.
SERVO CABLE
The speed control servo cable is connected betweenthe speed control vacuum servo diaphragm and the
throttle control linkage. This cable causes the throt-
tle control linkage to open or close the throttle valve
in response to movement of the vacuum servo dia-
phragm.
POWERTRAIN CONTROL MODULE
The speed control electronic control circuitry is in-
tegrated into the Powertrain Control Module (PCM).
The PCM is located in the engine compartment on
the left side inner fender shield. The PCM speed con-
trol functions are monitored by the On-Board Diag-
nostics (OBD). All OBD-sensed systems are
monitored by the PCM. Each monitored circuit is as-
signed a Diagnostic Trouble Code (DTC). The PCM
will store a DTC in electronic memory for any failure
it detects. See Using On-Board Diagnostic System in
this group for more information. The PCM cannot be
repaired and must be replaced if faulty.
VACUUM RESERVOIR
The vacuum reservoir is mounted behind the left
end of the front bumper bar. The reservoir contains a
one-way check valve to trap engine vacuum in the
reservoir. When engine vacuum drops, as in climbing
a grade while driving, the reservoir supplies the vac-
uum needed to maintain proper speed control opera-
tion. The vacuum reservoir cannot be repaired and
must be replaced if faulty.
VEHICLE SPEED SENSOR
The Vehicle Speed Sensor (VSS) is a pulse genera-
tor mounted to an adapter near the transmission
(two-wheel drive) or transfer case (four-wheel drive)
output shaft. The sensor is driven through the
adapter by a speedometer pinion gear. The VSS pulse
signal to the speedometer/odometer is monitored by
the PCM speed control circuitry to determine vehicle
speed and to maintain speed control set speed. Refer
to the appropriate Powertrain Diagnostic Procedures
manual for testing of this component. Refer to Group
14 - Fuel System for service of this component.
JVEHICLE SPEED CONTROL SYSTEM 8H - 1

USING ON-BOARD DIAGNOSTIC SYSTEM
The Powertrain Control Module (PCM) monitors
critical input and output circuits of the speed control
system, making sure they are operational. A Diagnos-
tic Trouble Code (DTC) is assigned to each input and
output circuit monitored by the OBD system. Some
circuits are checked continuously and some are
checked only under certain conditions.
If the OBD system senses that a monitored circuit
is bad, it will put a DTC into electronic memory. The
DTC will stay in electronic memory as long as the
circuit continues to be bad. The PCM is programmed
to clear the memory after 50 engine starts, if the
problem does not occur again.
DIAGNOSTIC TROUBLE CODES
Diagnostic Trouble Codes (DTC) are two-digit num-
bers flashed on the malfunction indicator (Check En-
gine) lamp that identify which circuit is bad. A DTC
description can also be read using the DRB scan tool.
Refer to Group 14 - Fuel Systems for more informa-
tion.
A DTC does not identify which component in a cir-
cuit is bad. Thus, a DTC should be treated as a
symptom, not as the cause for the problem. In some
cases, because of the design of the diagnostic test
procedure, a DTC can be the reason for another DTC
to be set. Therefore, it is important that the test pro-
cedures be followed in sequence, to understand what
caused a DTC to be set.
See Speed Control Diagnostic Trouble Code chart
for DTC's which apply to the speed control system.Refer to the Powertrain Diagnostic Procedures man-
ual to diagnose an on-board diagnostic system trou-
ble code.
RETRIEVING DIAGNOSTIC TROUBLE CODES
To start this function, cycle the ignition switch ON-
OFF-ON-OFF-ON within 5 seconds. This will cause
any DTC stored in the PCM memory to be displayed.
The malfunction indicator (Check Engine) lamp will
display a DTC by flashing on and off. There is a
short pause between flashes and a longer pause be-
tween digits. All DTC's displayed are two-digit num-
bers, with a four-second pause between codes.
An example of a DTC is as follows:
(1) Lamp on for 2 seconds, then turns off.
(2) Lamp flashes 1 time pauses and then flashes 5
times.
(3) Lamp pauses for 4 seconds, flashes 3 times,
pauses, then flashes 4 times.
The two DTC's are 15 and 34. Any number of
DTC's can be displayed, as long as they are in mem-
ory. The lamp will flash until all stored DTC's are
displayed (55 = end of test).
If a DTC 15 is observed, see diagnosis for Vehicle
Speed Sensor in this group. If a DTC 34 is observed,
see diagnosis for Speed Control Servo and Power-
train Control Module in this group. Correct any prob-
lems found in your diagnosis, then recheck for DTC
after corrections are completed.
VEHICLE SPEED SENSOR
For diagnosis of the VSS, refer to the appropriate
Powertrain Diagnostic Procedures manual.
SPEED CONTROL DIAGNOSTIC TROUBLE CODE
JVEHICLE SPEED CONTROL SYSTEM 8H - 5