• Excessive engine rpm (indicates that the trans-
mission may be in a low gear)
• The speed 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)
• 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 RESUME switch when speed is
greater than 25 mph allows the vehicle to resume
control 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 ACCEL
switch. The new target speed is stored in the PCM
when the ACCEL switch is released. The PCM also
has a “tap-up9 feature in which target speed
increases by 2 mph for each momentary switch acti-
vation of the ACCEL switch. The PCM also provides
a means to decelerate to a new lower target speed
without disengaging speed control. Depress and hold
the COAST switch until the desired speed is reached,
then release the switch.
SPEED CONTROL SERVO-PCM OUTPUT
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 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 ser-
viced only as a complete assembly.
Power is supplied to the servo 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.
SPEED CONTROL SWITCHES
DESCRIPTION
There are two separate switch pods that operate
the speed control system and are located on the
steering wheel..
OPERATION
The speed control system has five separate resis-
tive switches that provide a single multiplexed
(MUX) voltage inputs to the PCM. The switch names
are: ON, OFF, SET, COAST, RESUME, ACCEL, TAP-
UP, COAST, and CANCEL. Based on conditions when
the buttons are pushed (and released), the five volt-
ages ranges provided to the PCM result in the follow-
ing functions: ON, OFF, SET, COAST, RESUME,
ACCEL, TAP-UP, COAST, and CANCEL. Refer to the
Speed Control Section for more information
Also the PCM receives an input from the brake
switch to sense whether the brake pedal has been
depressed. When the PCM receives the brake
depressed input, it turns off power to the speed con-
trol servo and disengages speed control.
The individual switches cannot be repaired. If one
switch fails, the entire switch module must be
replaced.
INTERACTIVE SPEED CONTROL
DESCRIPTION
Interactive means that communication between the
PCM and the TCM is taking place. Interactive speed
control avoids unnecessary shifting for smoother, qui-
eter operation and when downshifts are required,
makes the shifts smoother.
CLIMBING A GRADE
DESCRIPTION
When climbing a grade the interactive speed con-
trol tries to maintain the set speed by increasing the
throttle opening, while inability/delaying downshifts.
8H - 2 VEHICLE SPEED CONTROL SYSTEM LH
DESCRIPTION AND OPERATION (Continued)
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