Dodge Ram Truck 1500-2500-3500. Manual — part 573

SERVO

DESCRIPTION

A speed control servo is not used with any 5.7L V-8 engine, or with the 5.9L diesel engine.

The speed control servo is attached to the bottom of the battery tray.

The servo unit consists of a solenoid valve body, and a vacuum chamber. The solenoid valve body contains three
solenoids:

•

Vacuum

•

Vent

•

Dump

The vacuum chamber contains a diaphragm with a cable attached to control the throttle linkage.

OPERATION

A speed control servo is not used with any 5.7L V-8 engine, or with the 5.9L diesel engine.

The following information applies only to vehicles equipped with a mechanical servo.

When/if a servo is used on gasoline powered vehicles, 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 sole-
noid 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 open-
ing, 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.

8P - 6

SPEED CONTROL

DR/DH

REMOVAL

The speed control servo assembly is attached to the
bottom of the battery tray (1).

1. Disconnect negative battery cable at battery (both

cables at both batteries if diesel).

2. To gain access to servo, remove plastic wheel-

house splash shield over left-front wheel.

3. Disconnect vacuum line (7) at servo.

4. Disconnect electrical connector (4) at servo.

5. Remove 3 servo mounting screws (5). Depending

on engine application, different sets of mounting
lugs (2) are used to support servo to battery tray.
While removing, note proper lugs.

6. Disconnect servo cable at throttle body. Refer to

Servo Cable Removal/Installation.

7. Remove 2 mounting nuts (1) holding servo cable

sleeve to bracket.

8. Pull speed control cable sleeve and servo away

from servo mounting bracket to expose cable
retaining clip (3) and remove clip. Note: The servo
mounting bracket displayed is a typical bracket and
may/may not be applicable to this model vehicle.

9. Remove servo (2) 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 2 servo-to-mounting bracket nuts and tighten. Refer to torque specifications.

5. Position servo assembly to correct mounting lugs on battery tray and install 3 screws. Tighten 3 screws. Refer to

torque specifications.

6. Connect vacuum line at servo.

7. Connect electrical connector at servo.

8. Connect servo cable to throttle body. Refer to servo Cable Removal/Installation.

9. Install left-front wheel-well liner.

10. Connect negative battery cable to battery (connect both cables if diesel).

11. Before starting engine, operate accelerator pedal to check for any binding.

DR/DH

SPEED CONTROL

8P - 7

SWITCH

DESCRIPTION

Two separate switch pods operate the speed control system. The steering-wheel-mounted switches use multiplexed
circuits to provide inputs to the PCM (to the ECM for diesel) 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 set-
ting procedures.

The individual switches cannot be repaired. If one switch fails, the entire switch module must be replaced.

Depending on engine control computer (JTEC having a 3– plug connector or NGC having a 4– plug con-
nector), 2 types of switches are used. Both types of switches are internally and externally different. The
switch used with the NGC system has an attached pigtail lead. The switch used with the JTEC system does
not have an attached pigtail lead.

OPERATION

When speed control is selected by depressing the ON, OFF switch, the PCM (ECM for diesel) 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 (auto. trans.)

•

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 (manual trans.).

•

Excessive engine rpm (indicates that the transmission 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

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 (ECM for diesel).

NOTE: Depressing the OFF switch will erase the set speed stored in the PCM’s/ECM’s RAM.

If, while the speed control is engaged, the driver wishes to increase vehicle speed, the PCM (ECM for diesel) 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/ECM’s RAM when the ACCEL switch is released. The
PCM/ECM also has a

9

tap-up

9

feature in which vehicle speed increases at a rate of approximately 2 mph for each

momentary switch activation of the ACCEL switch.

The PCM/ECM also provides a means to decelerate without disengaging speed control. To decelerate from an exist-
ing 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/ECM’s ON, OFF input, and the battery voltage to
the brake switch.

8P - 8

SPEED CONTROL

DR/DH

REMOVAL

Depending on engine control computer (JTEC hav-
ing a 3–plug connector or 5.7L V-8 NGC having a
4–plug connector), 2 types of switches are used.
Both types of switches are internally and exter-
nally different. The switches used with the NGC
system have attached pigtail leads. The switch
used with the JTEC system does not have an
attached pigtail lead.

1. Remove switch mounting screw (2).,

2. Pull switch (2) from steering wheel.

3. Unplug electrical connector from switch , or, switch

pigtail wire harness from steering wheel wire har-
ness (4) and remove switch.

INSTALLATION

1. Plug electrical connector into switch , or connect pigtail wire harness to steering wheel wire harness. Be sure

wires are not pinched.

2. Position switch to steering wheel.

3. Install switch mounting screw and tighten. Refer to torque specifications.

DR/DH

SPEED CONTROL

8P - 9