Section 130 (Speed Control): Servo: Operation

WARNING: This page is about a different car, the 2004 Dodge Pickup and 2004 Dodge Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.

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

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. When/if a servo is used on certain diesel powered vehicles, the Engine Control Module (ECM) 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/ECM through the brake switch. The PCM/ECM 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/ECM to operate. When the PCM/ECM 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/ECM breaks the ground, the solenoid closes and no more vacuum is allowed to enter the servo. The PCM/ECM 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/ECM 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/ECM grounds the vacuum and vent solenoids. To decrease throttle opening, the PCM/ECM 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.