1. ZJ VEHICLE SPEED CONTROL SYSTEM 8H - 1
VEHICLE SPEED CONTROL SYSTEM
CONTENTS
page page
DIAGNOSIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 SERVICE PROCEDURES ................... 9
GENERAL INFORMATION . . . . . . . . . . . . . . . . . . 1
GENERAL INFORMATION
The vehicle speed control system (Fig. 1) is an avail- control linkage to open or close the throttle valve in re-
able option on all Grand Cherokee models. The system is sponse to movement of the vacuum servo diaphragm.
electronically controlled and vacuum operated. Following
are general descriptions of the major components in the POWERTRAIN CONTROL MODULE
vehicle speed control system. Refer to Group 8W - Wiring The speed control electronic control circuitry is inte-
Diagrams for complete circuit descriptions and diagrams. grated into the Powertrain Control Module (PCM). The
PCM is located in the engine compartment on the right
SPEED CONTROL SERVO side of the dash panel. The PCM speed control functions
The speed control servo is mounted to a bracket on are monitored by the On-Board Diagnostics (OBD). All
the right side inner fender shield in the engine compart- OBD-sensed systems are monitored by the PCM. Each
ment. The servo unit consists of a solenoid valve body, a monitored circuit is assigned a Diagnostic Trouble Code
vacuum servo and the mounting bracket. The PCM con- (DTC). The PCM will store a DTC in electronic memory
trols the solenoid valve body. The solenoid valve body for any failure it detects. See Using On-Board Diagnos-
controls the application and release of vacuum to the di- tic System in this group for more information. The PCM
aphragm of the vacuum servo. The servo unit cannot be cannot be repaired and must be replaced if faulty.
repaired and is serviced only as a complete assembly.
VACUUM RESERVOIR
SPEED CONTROL SWITCH The vacuum reservoir is mounted under the battery
The speed control switch module is mounted to the tray. The reservoir contains a one-way check valve to
center of the steering wheel below the driver’s airbag trap engine vacuum in the reservoir. When engine vac-
module. The PCM monitors the state of the speed uum drops, as in climbing a grade while driving, the
reservoir supplies the vacuum needed to maintain
control switches. The individual switches are labeled:
proper speed control operation. The vacuum reservoir
OFF/ON, RESUME/ACCEL, SET/DECEL. Refer to
cannot be repaired and must be replaced if faulty.
the owner’s manual for more information on speed
control switch functions and setting procedures. The VEHICLE SPEED SENSOR
individual switches cannot be repaired. If one switch The Vehicle Speed Sensor (VSS) is a pulse generator
fails, the entire switch module must be replaced. mounted to an adapter near the transmission (two-wheel
drive) or transfer case (four-wheel drive) output shaft.
STOP LAMP SWITCH The sensor is driven through the adapter by a speedom-
Vehicles with the speed control option use a dual eter pinion gear. The VSS pulse signal to the speedome-
function stop lamp switch. The switch is mounted in ter/odometer is monitored by the PCM speed control
the same location as the conventional stop lamp circuitry to determine vehicle speed and to maintain
switch, on the brake pedal mounting bracket under speed control set speed. Refer to the appropriate Power-
the instrument panel. The PCM monitors the state of train Diagnostic Procedures manual for testing of this
the dual function stop lamp switch. Refer to Group 5 component. Refer to Group 14 - Fuel System for service
- Brakes for more information on stop lamp switch of this component.
service and adjustment procedures.
WARNING: THE USE OF VEHICLE SPEED CONTROL
SERVO CABLE IS NOT RECOMMENDED WHEN DRIVING CONDITIONS
The speed control servo cable is connected between DO NOT PERMIT MAINTAINING A CONSTANT SPEED,
the speed control vacuum servo diaphragm and the SUCH AS IN HEAVY TRAFFIC OR ON ROADS THAT
throttle control linkage. This cable causes the throttle ARE WINDING, ICY, SNOW COVERED, OR SLIPPERY.

2. 8H - 2 VEHICLE SPEED CONTROL SYSTEM ZJ
Fig. 1 Vehicle Speed Control System

3. ZJ VEHICLE SPEED CONTROL SYSTEM 8H - 3
DIAGNOSIS
Before beginning diagnosis, perform a vehicle road A DTC does not identify which component in a cir-
test to verify reports of speed control system mal- cuit is bad. Thus, a DTC should be treated as a
function. The road test should include attention to symptom, not as the cause for the problem. In some
the speedometer. Speedometer operation should be cases, because of the design of the diagnostic test
smooth and without flutter at all speeds. procedure, a DTC can be the reason for another DTC
Flutter in the speedometer indicates a problem to be set. Therefore, it is important that the test pro-
which might cause surging in the speed control sys- cedures be followed in sequence, to understand what
tem. The cause of any speedometer problems should caused a DTC to be set.
be corrected before proceeding. Refer to Group 8E - See Speed Control Diagnostic Trouble Code chart
Instrument Panel and Gauges for speedometer diag- for DTC’s which apply to the speed control system.
nosis. Refer to the Powertrain Diagnostic Procedures man-
If a road test verifies a system problem and the ual to diagnose an on-board diagnostic system trou-
speedometer operates properly, check for: ble code.
(1) Loose or corroded electrical connections at the
servo. Corrosion should be removed from electrical RETRIEVING DIAGNOSTIC TROUBLE CODES
terminals and a light coating of Mopar MultiPurpose To start this function, cycle the ignition switch ON-
Grease, or equivalent, applied. OFF-ON-OFF-ON within 5 seconds. This will cause
(2) Correct installation of the vacuum check valve any DTC stored in the PCM memory to be displayed.
in the hose from servo to vacuum source. The word The malfunction indicator (Check Engine) lamp will
VAC on the valve must point toward the vacuum display a DTC by flashing on and off. There is a
source. short pause between flashes and a longer pause be-
(3) Loose or leaking vacuum hoses or connections. tween digits. All DTC’s displayed are two-digit num-
(4) Secure attachment of both ends of the speed bers, with a four-second pause between codes.
control servo cable. An example of a DTC is as follows:
(1) Lamp on for 2 seconds, then turns off.
CAUTION: When test probing for voltage or conti- (2) Lamp flashes 1 time pauses and then flashes 5
nuity at electrical connectors, care must be taken times.
not to damage connector, terminals, or seals. If (3) Lamp pauses for 4 seconds, flashes 3 times,
these components are damaged, intermittent or pauses, then flashes 4 times.
complete system failure may occur.
The two DTC’s are 15 and 34. Any number of
DTC’s can be displayed, as long as they are in mem-
USING ON-BOARD DIAGNOSTIC SYSTEM ory. The lamp will flash until all stored DTC’s are
The Powertrain Control Module (PCM) monitors displayed (55 = end of test).
critical input and output circuits of the speed control If a DTC 15 is observed, see diagnosis for Vehicle
system, making sure they are operational. A Diagnos- Speed Sensor in this group. If a DTC 34 is observed,
tic Trouble Code (DTC) is assigned to each input and see diagnosis for Speed Control Servo and Power-
output circuit monitored by the On-Board Diagnostic train Control Module in this group. Correct any prob-
(OBD) system. Some circuits are checked continu- lems found in your diagnosis, then recheck for DTC
ously and some are checked only under certain con- after corrections are completed.
ditions.
If the OBD system senses that a monitored circuit VEHICLE SPEED SENSOR
is bad, it will put a DTC into electronic memory. The For diagnosis of the VSS, refer to the appropriate
DTC will stay in electronic memory as long as the Powertrain Diagnostic Procedures manual.
circuit continues to be bad. The PCM is programmed
to clear the memory after 50 engine starts, if the SPEED CONTROL SWITCH
problem does not occur again.
WARNING: ON VEHICLES EQUIPPED WITH AN AIR-
DIAGNOSTIC TROUBLE CODES BAG, REFER TO GROUP 8M - RESTRAINT SYS-
Diagnostic Trouble Codes (DTC) are two-digit num- TEMS BEFORE ATTEMPTING STEERING WHEEL
bers flashed on the malfunction indicator (Check En- COMPONENT DIAGNOSIS OR SERVICE. FAILURE
gine) lamp that identify which circuit is bad. A DTC TO TAKE PROPER PRECAUTIONS COULD RESULT
description can also be read using the DRB scan tool.
IN ACCIDENTAL AIRBAG DEPLOYMENT AND POS-
Refer to Group 14 - Fuel Systems for more informa-
SIBLE PERSONAL INJURY.
tion.

4. 8H - 4 VEHICLE SPEED CONTROL SYSTEM ZJ
DIAGNOSIS CHART 1

5. ZJ VEHICLE SPEED CONTROL SYSTEM 8H - 5
DIAGNOSIS CHART 2

6. 8H - 6 VEHICLE SPEED CONTROL SYSTEM ZJ
SPEED CONTROL DIAGNOSTIC TROUBLE CODE
(1) Disconnect battery negative cable. Remove
speed control switch from steering wheel.
(2) Check speed control switch continuity as shown
in chart (Fig. 2). If OK, reinstall switch. If not OK,
replace switch.
STOP LAMP SWITCH
(1) Remove the stop lamp switch from the mount-
ing bracket. Refer to Group 5 - Brakes.
(2) Disconnect switch from wiring harness.
(3) Using an ohmmeter, check switch continuity
(Fig. 3) as follows:
(a) With switch plunger released, there should be
continuity between pin 5 and pin 6.
(b) With switch plunger depressed, there should
be continuity between pin 1 and pin 2.
(c) With switch plunger still depressed, there
should be continuity between pin 3 and pin 4.
(4) If the switch fails the above continuity tests, it
is faulty. Replace switch. If switch is OK, reinstall
and check adjustment. Refer to Group 5 - Brakes for
correct installation and adjustment procedures.
VACUUM SUPPLY TEST
(1) Disconnect vacuum hose at the servo and in-
stall a vacuum gauge in the hose.
(2) Start engine and observe gauge at idle. Vacuum
gauge should read at least ten inches of mercury.
(3) If vacuum does not meet this requirement, Fig. 2 Speed Control Switch Continuity
check for vacuum leaks or poor engine performance. (2) Turn ignition switch to the ON position. Check
for battery voltage at fuse 20. If OK, go to next step.
SPEED CONTROL SERVO If not OK, repair open circuit to ignition switch as re-
(1) Check fuse 20 in the fuseblock module. If OK, quired.
go to next step. If not OK, replace fuse.

7. ZJ VEHICLE SPEED CONTROL SYSTEM 8H - 7
Fig. 3 Stop Lamp Switch
(3) Connect the negative lead of a voltmeter to a
good chassis ground near the servo. Unplug the Fig. 5 Servo Harness Connector
4-way connector going to the servo (Fig. 4). Push the
speed control switch to the ON position. Check for POWERTRAIN CONTROL MODULE
battery voltage at servo harness connector cavity for (1) Disconnect 60-way connector from the PCM, lo-
pin 2 (Fig. 5). If OK, go to next step. If not OK, see cated on the right side dash panel in the engine com-
diagnosis for Stop Lamp Switch. partment (Fig. 6).
Fig. 4 Speed Control Servo Fig. 6 Powertrain Control Module
(4) Connect a jumper wire between servo harness (2) Connect negative lead of voltmeter to a good
connector cavity for pin 2 and pin 2 of the servo. body ground near the module.
Check for battery voltage at pins 1, 3 and 4 of the (3) For the following tests, the ignition switch
servo. If OK, go to next step. If not OK, replace the must be in the ON position. See Fig. 7 for controller
servo. terminal locations. Touch the positive lead of the
(5) Turn ignition switch to OFF position. Check for voltmeter to the terminal in cavity number 33. With
continuity between servo harness connector cavity for the speed control switch in the OFF position, the
pin 1 and a good ground. There should be continuity. voltmeter should read zero volts. With the speed con-
If not OK, repair open circuit to ground as required.

8. 8H - 8 VEHICLE SPEED CONTROL SYSTEM ZJ
trol switch in the ON position, the voltmeter should long as the switch is held. If not, see diagnosis for
read battery voltage. If not, repair the main harness Speed Control Switch. If the switch is not at fault,
as necessary. then check the main harness and repair as necessary.
(6) Touch the positive lead of the voltmeter to the
terminal in cavity number 50. The voltmeter should
read zero volts with the speed control switch in ei-
ther the OFF or ON position. With switch in either
RESUME or SET position, the voltmeter should read
battery voltage. If not, see diagnosis for Speed Con-
trol Switch. If the switch is not at fault, then check
the main harness and repair as necessary.
(7) Touch the positive lead of the voltmeter to the
terminal in cavity number 49. The voltmeter should
Fig. 7 PCM 60-Way Connector - Terminal End read zero volts with the switch in the OFF position.
(4) Touch the positive lead of the voltmeter to the With the switch in the ON position, the voltmeter
terminal in cavity number 53. As in step (3), the volt- should read battery voltage. The voltmeter will con-
meter should read zero volts with the switch in the tinue to read battery voltage when either the SET or
OFF position, and battery voltage with the switch in RESUME switch is pressed. If not, see diagnosis for
the ON position. Speed Control Switch. If the switch is not at fault,
(5) Touch the positive lead of the voltmeter to the then check the main harness and repair as necessary.
terminal in cavity number 48. With the speed control (8) Turn ignition switch OFF. Using an ohmmeter,
switch in the OFF position, the voltmeter should connect one lead to a good body ground and touch the
read zero volts. With the switch in the ON position, other lead to the terminal in cavity number 29. With
the voltmeter should read battery voltage. Pressing the brake pedal released, the meter should show con-
the SET/DECEL button should cause the voltmeter tinuity. When the pedal is depressed, the meter
to change from battery voltage to zero volts for as should show an open circuit.

9. ZJ VEHICLE SPEED CONTROL SYSTEM 8H - 9
SERVICE PROCEDURES
SPEED CONTROL SERVO REMOVE/INSTALL (2) Remove 2 screws from back side of steering
wheel (Fig. 8).
4.0L ENGINES (3) Rock switch away from horn pad while lifting
(1) Disconnect vacuum hose at servo. switch out of steering wheel.
(2) Unplug electrical connector at servo. (4) Disconnect 4-way electrical connector from
(3) Remove 2 nuts holding servo cable sleeve. clockspring.
(4) Pull speed control cable sleeve away from servo
to expose cable retaining clip.
(5) Remove clip attaching cable to servo.
(6) Remove servo from mounting bracket.
(7) Reverse removal procedures to install. Block
throttle to full open position to align hole in cable
connector with hole in servo pin and install retaining
clip. Tighten servo mounting nuts to 8.5 N⅐m (75 in.
lbs.).
CAUTION: The cable sleeve must be installed on
the OUTSIDE face of the bracket to avoid possible
binding of the cable.
5.2L ENGINES
(1) Disconnect vacuum hose at servo. Fig. 8 Speed Control Switch Remove
(2) Unplug electrical connector at servo.
(3) Remove 2 nuts from servo mounting bracket. (5) Reverse removal procedures to install. When
(4) Remove and discard push nuts on servo studs. placing switch in steering wheel, slide the forward
(5) Remove servo from mounting bracket. edge of switch under horn pad. Line up locating pins
(6) Pull speed control cable sleeve away from servo on switch with holes in steering wheel frame. Attach
to expose cable retaining clip. switch to wheel with 2 screws starting with the
(7) Remove clip attaching cable to servo. screw at the left end of the switch.
(8) Reverse removal procedures to install. Block
throttle to full open position to align hole in cable
SERVO CABLE REMOVE/INSTALL
connector with hole in servo pin and install retaining
CAUTION: Use finger pressure only to remove the
clip. Install new push nuts on servo studs. Tighten
speed control cable connector at the bellcrank. Pli-
servo mounting nuts to 8.5 N⅐m (75 in. lbs.).
ers or screwdriver can break the connector requir-
CAUTION: The cable sleeve must be installed BE- ing complete cable replacement.
TWEEN the servo and bracket to avoid possible
(1) Using finger pressure only, remove speed con-
binding of the cable.
trol cable connector at bell crank by pushing connec-
tor off the bellcrank (Fig. 9 or 10). DO NOT try to
SPEED CONTROL SWITCH REMOVE/INSTALL pull connector off perpendicular to the bellcrank.
(2) Squeeze tabs on speed control cable and push
WARNING: BEFORE BEGINNING ANY AIR BAG out of locking plate (Fig. 11).
SYSTEM COMPONENT REMOVAL OR INSTALLA- (3) Pull cable out of locking plate.
TION, REMOVE AND ISOLATE THE NEGATIVE (-) (4) Remove cable from servo as described in Speed
CABLE FROM THE BATTERY. THIS IS THE ONLY Control Servo Remove/Install procedure.
SURE WAY TO DISABLE THE AIR BAG SYSTEM. (5) Reverse the removal procedures to install.
THEN WAIT TWO MINUTES FOR SYSTEM CAPACI-
TOR TO DISCHARGE BEFORE FURTHER SYSTEM VACUUM RESERVOIR REMOVE/INSTALL
SERVICE. FAILURE TO DO THIS COULD RESULT IN (1) Disconnect both battery cables, negative cable
ACCIDENTAL AIR BAG DEPLOYMENT AND POSSI- first.
BLE INJURY. (2) Remove battery holddowns.
(3) Remove battery from battery tray.
(1) Disconnect battery negative cable. (4) Remove 5 screws securing battery tray.

10. 8H - 10 VEHICLE SPEED CONTROL SYSTEM ZJ
Fig. 11 Speed Control Cable Locking Plate - Typical
Fig. 9 Remove Bellcrank Connector - 4.0L Engines
Fig. 12 Vacuum Reservoir
(7) Reverse removal procedures to install. Tighten
hardware as follows:
• battery tray mounting screws to 10 N⅐m (90 in.
lbs.)
• battery holddown bolts to 10 N⅐m (90 in. lbs.)
Fig. 10 Remove Bellcrank Connector - 5.2L Engines • battery cable clamp bolts to 8.5 N⅐m (75 in. lbs.).
(5) Pull up battery tray and remove vacuum line
from reservoir (Fig. 12).
(6) Remove 2 screws holding reservoir to battery
tray.