Jeep Cherokee (XJ): Description and operation
DESCRIPTION The speed control system is electronically controlled
and vacuum operated. Electronic control of
the speed control system is integrated into the Powertrain
Control Module (PCM). The controls consist
of two steering wheel mounted switches. The
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).
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 WINDING, ICY, SNOW COVERED, OR SLIPPERY. 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: NOTE: Depressing the OFF switch or turning off
the ignition switch will erase the set speed stored
in the PCM.
For added safety, the speed control system is programmed
to disengage for any of the following conditions: Once the speed control has been disengaged,
depressing the RES/ACCEL switch (when speed is
greater than 30 mph) restores the vehicle to the target
speed that was stored in the PCM.
While the speed control is engaged, the driver can
increase the vehicle speed by depressing the RES/ACCEL
switch. The new target speed is stored in the
PCM when the RES/ACCEL is released. The PCM
also has a "tap-up9 feature in which vehicle speed
increases at a rate of approximately 2 mph for each
momentary switch activation of the RES/ACCEL
switch. 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 activation,
speed will be lowered approximately 1 mph. DESCRIPTION The servo unit consists of a solenoid valve body,
and a vacuum chamber. The solenoid valve body contains
three solenoids: The vacuum chamber contains a diaphragm with a
cable attached to control the throttle linkage. OPERATION The Powertrain Control Module (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.
Power is supplied to the servo's by the PCM
through the brake switch. The PCM controls the
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 provides
a safety backup to the vent and vacuum solenoids.
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 solenoid
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 solenoids
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. 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 target
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 solenoid
still de-activated to close the throttle to return
to target speed. DESCRIPTION There are two separate switch pods that operate
the speed control system. The steering-wheelmounted
switches use multiplexed circuits to provide
inputs to the PCM for ON, OFF, RESUME, ACCELERATE,
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 fails, the entire switch module must be
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: The speed control can be disengaged also by any of
the following conditions: The previous disengagement conditions are programmed
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.
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 programmed
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 approximately
2 mph for each momentary switch activation
of the ACCEL switch.
The PCM also provides a means to decelerate without
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 operates
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 sensor
point (open circuit). If a switch with no resistor is
closed, the PCM will measure 0 volts (grounded circuit).
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 circuit'
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. DESCRIPTION The switch is mounted on the brake pedal mounting
bracket under the instrument panel. OPERATION Vehicles equipped with the speed control option use
a dual function brake lamp switch. The PCM monitors
the state of the dual function brake lamp switch.
Refer to the Brake section for more information on
brake lamp switch service and adjustment procedures.
The brake switch is equipped with three sets of
contacts, one normally open and the other two normally
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 signal
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. 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 movement
of the vacuum servo diaphragm. DESCRIPTION The vacuum reservoir is a plastic storage tank connected
to an engine vacuum source by vacuum lines. OPERATION The vacuum reservoir is used to supply the vacuum
needed to maintain proper speed control operation
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 heating/air-conditioning system.
The vacuum reservoir
cannot be repaired and must be replaced if faulty. DESCRIPTION The Vehicle Speed Sensor (VSS) is mounted to an
adapter near the transmission output shaft. The sensor
is driven through the adapter by a speedometer
pinion gear. OPERATION The VSS is a pulse generator. 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.Speed control system
Speed control servo
Speed control solenoid circuits
Speed control switches
Brake lamp switch
Servo cable
Vacuum reservoir
Vehicle speed sensor (speed control
operation)
Other materials:
Blower motor. Blower motor relay. Blower motor resistor
Blower motor
WARNING: ON VEHICLES EQUIPPED WITH AIRBAGS,
REFER TO GROUP 8M - PASSIVE RESTRAINT
SYSTEMS BEFORE ATTEMPTING ANY STEERING
WHEEL, STEERING COLUMN, OR INSTRUMENT
PANEL COMPONENT DIAGNOSIS OR SERVICE. FAILURE
TO TAKE THE PROPER PRECAUTIONS COULD
RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT
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