Jeep Cherokee 2000 Service Manual - page 421
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.
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:
•
Vacuum
•
Vent
•
Dump
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 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
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.
SPEED CONTROL SOLENOID CIRCUITS
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 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 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:
•
Stepping on the brake pedal
•
Depressing the OFF switch
•
Depressing the CANCEL switch.
The speed control can be disengaged also by any of
the following conditions:
•
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)
•
Depressing the clutch pedal.
•
Excessive engine rpm (indicates that the trans-
mission may be in a low gear)
•
The VSS signal decreases at a rate of 10 mph
per second (indicates that the vehicle may have
decelerated at an extremely high rate)
•
If the actual speed is not within 20 mph of the
set speed
8H - 2
SPEED CONTROL SYSTEM
XJ
DESCRIPTION AND OPERATION (Continued)