exj 8h, JEEP Manuals, Electronic Service Manual (jeep cherokee xj do 2000)
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XJ
SPEED CONTROL SYSTEM
8H - 1
SPEED CONTROL SYSTEM
TABLE OF CONTENTS
page
page
DESCRIPTION AND OPERATION
VEHICLE SPEED SENSOR (SPEED CONTROL
DIAGNOSIS AND TESTING
REMOVAL AND INSTALLATION
SPECIFICATIONS
DESCRIPTION AND OPERATION
²
Depressing the clutch pedal (if equipped).
NOTE: Depressing the OFF switch or turning off
the ignition switch will erase the set speed stored
in the PCM.
SPEED CONTROL SYSTEM
DESCRIPTION
The speed control system is electronically con-
trolled and vacuum operated. Electronic control of
the speed control system is integrated into the Pow-
ertrain Control Module (PCM). The controls consist
of
For added safety, the speed control system is pro-
grammed to disengage for any of the following condi-
tions:
²
An indication of Park or Neutral
two
steering
wheel
mounted
switches.
The
A rapid increase rpm (indicates that the clutch
has been disengaged)
²
²
switches
are
labeled:
ON/OFF,
RES/ACCEL,
SET,
COAST, and CANCEL.
The system is designed to operate at speeds above
30 mph (50 km/h).
Excessive engine rpm (indicates that the trans-
mission may be in a low gear)
²
The speed signal increases at a rate of 10 mph
per second (indicates that the coefficient of friction
between the road surface and tires is extremely low)
²
WARNING: THE USE OF SPEED CONTROL IS NOT
RECOMMENDED WHEN DRIVING CONDITIONS DO
NOT PERMIT MAINTAINING A CONSTANT SPEED,
SUCH AS IN HEAVY TRAFFIC OR ON ROADS THAT
ARE
The speed signal decreases at a rate of 10 mph
per
second
(indicates
that
the
vehicle
may
have
decelerated at an extremely high rate)
Once the speed control has been disengaged,
depressing the RES/ACCEL switch (when speed is
greater than 30 mph) restores the vehicle to the tar-
get speed that was stored in the PCM.
While the speed control is engaged, the driver can
increase the vehicle speed by depressing the RES/AC-
CEL switch. The new target speed is stored in the
PCM when the RES/ACCEL is released. The PCM
also has a “tap-up
WINDING,
ICY,
SNOW
COVERED,
OR
SLIP-
PERY.
OPERATION
When speed control is selected by depressing the
ON switch, the PCM allows a set speed to be stored
in PCM RAM for speed control. To store a set speed,
depress the SET switch while the vehicle is moving
at a speed between 35 and 85 mph. In order for the
speed control to engage, the brakes cannot be
applied, nor can the gear selector be indicating the
transmission is in Park or Neutral.
The speed control can be disengaged manually by:
²
feature in which vehicle speed
increases at a rate of approximately 2 mph for each
momentary
9
switch
activation
of
the
RES/ACCEL
switch.
Stepping on the brake pedal
²
Depressing the OFF switch
²
Depressing the CANCEL switch.
8H - 2
SPEED CONTROL SYSTEM
XJ
DESCRIPTION AND OPERATION (Continued)
SPEED CONTROL SOLENOID CIRCUITS
A “tap down” feature is used to decelerate without
disengaging the speed control system. To decelerate
from an existing recorded target speed, momentarily
depress the COAST switch. For each switch activa-
tion, speed will be lowered approximately 1 mph.
OPERATION
When all of the speed control parameters are met,
and the SET button is pressed, the PCM actuates the
vent solenoid and “duty-cycles” the vacuum solenoid
to open the throttle and bring the vehicle up to tar-
get speed. When the vehicle is at target speed, it will
actuate the vent solenoid with the vacuum solenoid
de-activated to maintain the vehicle at target speed.
When the vehicle is above target speed, the PCM will
“duty-cycle” the vent solenoid with the vacuum sole-
noid still de-activated to close the throttle to return
to target speed.
SPEED CONTROL SERVO
DESCRIPTION
The servo unit consists of a solenoid valve body,
and a vacuum chamber. The solenoid valve body con-
tains three solenoids:
²
Vacuum
²
Vent
Dump
The vacuum chamber contains a diaphragm with a
cable attached to control the throttle linkage.
²
SPEED CONTROL SWITCHES
DESCRIPTION
There are two separate switch pods that operate
the speed control system. The steering-wheel-
mounted switches use multiplexed circuits to provide
inputs to the PCM for ON, OFF, RESUME, ACCEL-
ERATE, SET, DECEL and CANCEL modes. 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
OPERATION
The Powertrain Control Module (PCM) controls the
solenoid valve body. The solenoid valve body controls
the application and release of vacuum to the dia-
phragm of the vacuum servo. The servo unit cannot
be repaired and is serviced only as a complete assem-
bly.
Power
is
supplied
to
the
servo’s
by
the
PCM
through
the
brake
switch.
The
PCM
controls
the
fails,
the
entire
switch
module
must
be
ground path for the vacuum and vent solenoids.
The dump solenoid is energized anytime it receives
power. If power to the dump solenoid is interrupted,
the solenoid dumps vacuum in the servo. This pro-
vides a safety backup to the vent and vacuum sole-
noids.
The vacuum and vent solenoids must be grounded
at the PCM to operate. When the PCM grounds the
vacuum servo solenoid, the solenoid allows vacuum
to enter the servo and pull open the throttle plate
using the cable. When the PCM breaks the ground,
the solenoid closes and no more vacuum is allowed to
enter the servo. The PCM also operates the vent sole-
noid via ground. The vent solenoid opens and closes a
passage to bleed or hold vacuum in the servo as
required.
The PCM duty cycles the vacuum and vent sole-
noids to maintain the set speed, or to accelerate and
decelerate the vehicle. To increase throttle opening,
the PCM grounds the vacuum and vent solenoids. To
decrease throttle opening, the PCM removes the
grounds from the vacuum and vent solenoids. When
the brake is released, if vehicle speed exceeds 30
mph to resume, 35 mph to set, and the RES/ACCEL
switch has been depressed, ground for the vent and
vacuum circuits is restored.
replaced.
OPERATION
When speed control is selected by depressing the
ON, OFF switch, the PCM allows a set speed to be
stored in its RAM for speed control. To store a set
speed, depress the SET switch while the vehicle is
moving at a speed between approximately 35 and 85
mph. In order for the speed control to engage, the
brakes cannot be applied, nor can the gear selector
be indicating the transmission is in Park or Neutral.
The speed control can be disengaged manually by:
²
Stepping on the brake pedal
²
Depressing the OFF switch
Depressing the CANCEL switch.
The speed control can be disengaged also by any of
the following conditions:
²
²
An indication of Park or Neutral
The VSS signal increases at a rate of 10 mph
per second (indicates that the co-efficient of friction
between the road surface and tires is extremely low)
²
²
Depressing the clutch pedal.
Excessive engine rpm (indicates that the trans-
mission may be in a low gear)
²
²
The VSS signal decreases at a rate of 10 mph
per
second
(indicates
that
the
vehicle
may
have
decelerated at an extremely high rate)
²
If the actual speed is not within 20 mph of the
set speed
XJ
SPEED CONTROL SYSTEM
8H - 3
DESCRIPTION AND OPERATION (Continued)
The
previous
disengagement
conditions
are
pro-
Refer to the Brake section for more information on
brake
grammed for added safety.
Once the speed control has been disengaged,
depressing the ACCEL switch restores the vehicle to
the target speed that was stored in the PCM’s RAM.
lamp
switch
service
and
adjustment
proce-
dures.
The brake switch is equipped with three sets of
contacts, one normally open and the other two nor-
mally closed (brakes disengaged). The PCM sends a
12 volt signal to one of the normally closed contacts
in the brake switch, which is returned to the PCM as
a brake switch state signal. With the contacts closed,
the 12 volt signal is pulled to ground causing the sig-
nal to go low. The low voltage signal, monitored by
the PCM, indicates that the brakes are not applied.
When the brakes are applied, the contacts open,
causing the PCM’s output brake signal to go high,
disengaging the speed control, cutting off PCM power
to the speed control solenoids.
The second set of normally closed contacts supplies
12 volts from the PCM any time speed control is
turned on. Through the brake switch, current is
routed to the speed control servo solenoids. The
speed control solenoids (vacuum, vent and dump) are
provided this current any time the speed control is
ON and the brakes are disengaged.
When the driver applies the brakes, the contacts
open and current is interrupted to the solenoids. The
normally open contacts are fed battery voltage. When
the brakes are applied, battery voltage is supplied to
the brake lamps.
NOTE: Depressing the OFF switch will erase the
set speed stored in the PCM’s RAM.
If, while the speed control is engaged, the driver
wishes to increase vehicle speed, the PCM is pro-
grammed for an acceleration feature. With the
ACCEL switch held closed, the vehicle accelerates
slowly to the desired speed. The new target speed is
stored in the PCM’s RAM when the ACCEL switch is
released. The PCM also has a “tap-up” feature in
which vehicle speed increases at a rate of approxi-
mately 2 mph for each momentary switch activation
of the ACCEL switch.
The PCM also provides a means to decelerate with-
out disengaging speed control. To decelerate from an
existing recorded target speed, depress and hold the
COAST switch until the desired speed is reached.
Then release the switch. The ON, OFF switch oper-
ates two components: the PCM’s ON, OFF input, and
the battery voltage to the brake switch, which powers
the speed control servo.
Multiplexing
The PCM sends out 5 volts through a fixed resistor
and monitors the voltage change between the fixed
resistor and the switches. If none of the switches are
depressed, the PCM will measure 5 volts at the sen-
sor point (open circuit). If a switch with no resistor is
closed, the PCM will measure 0 volts (grounded cir-
cuit). Now, if a resistor is added to a switch, then the
PCM will measure some voltage proportional to the
size of the resistor. By adding a different resistor to
each switch, the PCM will see a different voltage
depending on which switch is pushed.
Another resistor has been added to the ’at rest cir-
cuit’ causing the PCM to never see 5 volts. This was
done for diagnostic purposes. If the switch circuit
should open (bad connection), then the PCM will see
the 5 volts and know the circuit is bad. The PCM will
then set an open circuit fault.
SERVO CABLE
DESCRIPTION
The speed control servo cable is connected between
the speed control vacuum servo diaphragm and the
throttle body control linkage.
OPERATION
This cable causes the throttle control linkage to
open or close the throttle valve in response to move-
ment of the vacuum servo diaphragm.
VACUUM RESERVOIR
DESCRIPTION
The vacuum reservoir is a plastic storage tank con-
nected to an engine vacuum source by vacuum lines.
BRAKE LAMP SWITCH
OPERATION
The vacuum reservoir is used to supply the vac-
uum needed to maintain proper speed control opera-
tion when engine vacuum drops, such as in climbing
a grade while driving. A one-way check valve is used
in the vacuum line between the reservoir and the
vacuum source. This check valve is used to trap
engine vacuum in the reservoir. On certain vehicle
applications, this reservoir is shared with the heat-
DESCRIPTION
The switch is mounted on the brake pedal mount-
ing bracket under the instrument panel.
OPERATION
Vehicles equipped with the speed control option use
a dual function brake lamp switch. The PCM moni-
tors the state of the dual function brake lamp switch.
8H - 4
SPEED CONTROL SYSTEM
XJ
DESCRIPTION AND OPERATION (Continued)
ing/air-conditioning system. The vacuum reservoir
cannot be repaired and must be replaced if faulty.
Smooth operation of throttle linkage and throttle
body air valve.
²
²
Failed speed control servo. Do the servo vacuum
VEHICLE SPEED SENSOR (SPEED CONTROL
OPERATION)
test.
CAUTION: When test probing for voltage or conti-
nuity at electrical connectors, care must be taken
not
DESCRIPTION
The Vehicle Speed Sensor (VSS) is mounted to an
adapter near the transmission output shaft. The sen-
sor is driven through the adapter by a speedometer
pinion gear.
to
damage
connector,
terminals
or
seals.
If
these
components
are
damaged,
intermittent
or
complete system failure may occur.
VACUUM SUPPLY TEST
(1) Disconnect vacuum hose at speed control servo
and install a vacuum gauge into the disconnected
hose.
(2) Start engine and observe gauge at idle. Vac-
uum gauge should read at least ten inches of mer-
cury.
(3) If vacuum is less than ten inches of mercury,
determine source of leak. Check vacuum line to
engine for leaks. Also check actual engine intake
manifold vacuum. If manifold vacuum does not meet
this requirement, check for poor engine performance
and repair as necessary.
(4) If vacuum line to engine is not leaking, check
for leak at vacuum reservoir. To locate and gain
access to reservoir, refer to Vacuum Reservoir Remov-
al/Installation in this group. Disconnect vacuum line
at reservoir and connect a hand-operated vacuum
pump to reservoir fitting. Apply vacuum. Reservoir
vacuum should not bleed off. If vacuum is being lost,
replace reservoir.
(5) Verify operation of one-way check valve and
check it for leaks.
(a) Locate one-way check valve. The valve is
located in vacuum line between vacuum reservoir
and
OPERATION
The VSS is a pulse generator. The VSS pulse sig-
nal to the speedometer/odometer is monitored by the
PCM speed control circuitry to determine vehicle
speed and to maintain speed control set speed.
DIAGNOSIS AND TESTING
ROAD TEST
Perform a vehicle road test to verify reports of
speed control system malfunction. The road test
should include attention to the speedometer. Speed-
ometer operation should be smooth and without flut-
ter at all speeds.
Flutter in the speedometer indicates a problem
which might cause surging in the speed control sys-
tem. The cause of any speedometer problems should
be corrected before proceeding. Refer to Group 8E,
Instrument Panel and Gauges for speedometer diag-
nosis.
If a road test verifies a system problem and the
speedometer operates properly, check for:
²
A Diagnostic Trouble Code (DTC). If a DTC
exists, conduct tests per the Powertrain Diagnostic
Procedures service manual.
²
engine
vacuum
source.
Disconnect
vacuum
hoses (lines) at each end of valve.
(b) Connect a hand-operated vacuum pump to
reservoir end of check valve. Apply vacuum. Vac-
uum should not bleed off. If vacuum is being lost,
replace one-way check valve.
(c) Connect a hand-operated vacuum pump to
vacuum source end of check valve. Apply vacuum.
Vacuum should flow through valve. If vacuum is
not flowing, replace one-way check valve. Seal the
fitting at opposite end of valve with a finger and
apply vacuum. If vacuum will not hold, diaphragm
within check valve has ruptured. Replace valve.
A misadjusted brake (stop) lamp switch. This
could also cause an intermittent problem.
²
Loose, damaged or corroded electrical connec-
tions at the servo. Corrosion should be removed from
electrical
terminals
and
a
light
coating
of
Mopar
MultiPurpose Grease, or equivalent, applied.
²
Leaking vacuum reservoir.
²
Loose or leaking vacuum hoses or connections.
²
Defective one-way vacuum check valve.
Secure attachment of both ends of the speed con-
trol servo cable.
²
XJ
SPEED CONTROL SYSTEM
8H - 5
REMOVAL AND INSTALLATION
SPEED CONTROL SERVO
REMOVAL
(1) Disconnect negative battery cable at battery.
(2) Disconnect vacuum line at servo (Fig. 1).
(3) Disconnect electrical connector at servo.
(4) Disconnect servo cable at throttle body. Refer to
Servo Cable Removal/Installation.
(5) Remove 2 mounting nuts holding servo cable
sleeve to bracket (Fig. 2).
(6) Pull speed control cable sleeve and servo away
from servo mounting bracket to expose cable retain-
ing clip (Fig. 2) and remove clip. Note: The servo
mounting bracket displayed in (Fig. 2) is a typical
bracket and may/may not be applicable to this model
vehicle.
Fig. 2 Servo Cable Clip Remove/Install—Typical
1 – SERVO MOUNTING NUTS (2)
2 – SERVO
3 – CABLE RETAINING CLIP
4 – SERVO CABLE AND SLEEVE
INSTALLATION
(1) Position servo to mounting bracket.
(2) Align hole in cable connector with hole in servo
pin. Install cable-to-servo retaining clip.
(3) Insert servo mounting studs through holes in
servo mounting bracket.
(4) Install servo mounting nuts and tighten to 8.5
N·m (75 in. lbs.).
(5) Connect vacuum line at servo.
(6) Connect electrical connector at servo.
(7) Connect servo cable to throttle body. Refer to
Servo Cable Removal/Installation.
(8) Connect negative battery cable to battery.
(9) Before
starting
engine,
operate
accelerator
pedal to check for any binding.
Fig. 1 Speed Control Servo Location
1 – R. F. FENDER
2 – SPEED CONTROL SERVO
3 – ELECTRICAL CONNECTOR
4 – MOUNTING BRACKET
5 – VACUUM LINE
SPEED CONTROL SWITCH
WARNING: BEFORE ATTEMPTING TO DIAGNOSE,
REMOVE OR INSTALL ANY AIRBAG SYSTEM OR
RELATED STEERING WHEEL AND STEERING COL-
UMN COMPONENTS YOU MUST FIRST DISCON-
NECT AND ISOLATE THE BATTERY NEGATIVE
(GROUND) CABLE. WAIT 2 MINUTES FOR SYSTEM
CAPACITOR TO DISCHARGE BEFORE FURTHER
SYSTEM SERVICE. FAILURE TO DO SO COULD
RESULT IN ACCIDENTAL DEPLOYMENT AND POS-
SIBLE PERSONAL INJURY.
(7) Remove servo from mounting bracket. While
removing, note orientation of servo to bracket.
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