Dodge Durango (DN). Manual - part 291

SPEED CONTROL SYSTEM

TABLE OF CONTENTS

page

DESCRIPTION AND OPERATION

SPEED CONTROL SYSTEM . . . . . . . . . . . . . . . . . 1
SPEED CONTROL SERVO. . . . . . . . . . . . . . . . . . . 2
SPEED CONTROL SOLENOID CIRCUITS . . . . . . . 2
SPEED CONTROL SWITCHES . . . . . . . . . . . . . . . 2
BRAKE LAMP SWITCH. . . . . . . . . . . . . . . . . . . . . 3
SERVO CABLE . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
VACUUM RESERVOIR . . . . . . . . . . . . . . . . . . . . . 3
VEHICLE SPEED INPUT . . . . . . . . . . . . . . . . . . . . 3

DIAGNOSIS AND TESTING

ROAD TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

VACUUM SUPPLY TEST . . . . . . . . . . . . . . . . . . . . 4

REMOVAL AND INSTALLATION

SPEED CONTROL SERVO. . . . . . . . . . . . . . . . . . . 4
SPEED CONTROL SWITCHES . . . . . . . . . . . . . . . 5
SERVO CABLE (EXCEPT 4.7L ENGINE) . . . . . . . . 6
SERVO CABLE—4.7L V-8 ENGINE. . . . . . . . . . . . 6
VACUUM RESERVOIR . . . . . . . . . . . . . . . . . . . . . 7

SPECIFICATIONS

TORQUE CHART . . . . . . . . . . . . . . . . . . . . . . . . . . 9

DESCRIPTION AND OPERATION

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

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 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:
• Stepping on the brake pedal

• Depressing the OFF switch

• Depressing the CANCEL switch.

• 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.

For added safety, the speed control system is pro-

grammed to disengage for any of the following condi-
tions:

• An indication of Park or Neutral

• A rapid increase rpm (indicates that the clutch

has been disengaged)

• 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 coefficient 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 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

9 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

SPEED CONTROL SYSTEM

8H - 1

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

DESCRIPTION AND OPERATION (Continued)

The previous disengagement conditions are pro-

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.

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.

BRAKE LAMP SWITCH

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.
Refer to the Brake section for more information on
brake 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.

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.

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-
ing/air-conditioning system. The vacuum reservoir
cannot be repaired and must be replaced if faulty.

VEHICLE SPEED INPUT

OPERATION

The Vehicle Speed Sensor (VSS) is no longer used

for any Dodge Truck.

Vehicle speed and distance covered are measured

by the Rear Wheel Speed Sensor. The sensor is
mounted to the rear axle. A signal is sent from this
sensor to the Controller Antilock Brake (CAB) com-
puter. A signal is then sent from the CAB to the Pow-
ertrain Control Module (PCM) to determine vehicle
speed and distance covered. The PCM will then
determine strategies for speed control system opera-
tion.

SPEED CONTROL SYSTEM

8H - 3

DESCRIPTION AND OPERATION (Continued)

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:

• A Diagnostic Trouble Code (DTC). If a DTC

exists, conduct tests per the Powertrain Diagnostic
Procedures service manual.

• A misadjusted brake (stop) lamp switch. This

could also cause an intermittent problem.

• 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.

• Leaking vacuum reservoir.

• Loose or leaking vacuum hoses or connections.

• Defective one-way vacuum check valve.

• Secure attachment of both ends of the speed con-

trol servo cable.

• Smooth operation of throttle linkage and throttle

body air valve.

• Failed speed control servo. Do the servo vacuum

test.

CAUTION: When test probing for voltage or conti-
nuity at electrical connectors, care must be taken
not 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 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.

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.

REMOVAL AND INSTALLATION

SPEED CONTROL SERVO

REMOVAL

(1) Disconnect negative battery cable at battery.
(2) Disconnect vacuum hose (line) at servo (Fig. 1).
(3) Disconnect electrical connector at servo.
(4) Disconnect servo cable at throttle body. Refer to

Servo Cable Removal/Installation in this group.

(5) Remove 2 mounting nuts holding servo cable

sleeve to bracket (Fig. 1) or (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.

(7) Remove servo from mounting bracket. While

removing, note orientation of servo to bracket.

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.

8H - 4

SPEED CONTROL SYSTEM

Content .. 289 290 291 292 ..