System Operation

INTRODUCTION
The vehicle speed control system is electronically controlled and vacuum operated. The system is designed to operate between approximately 35 and 85 mph (56 and 137 km/h). Following are general descriptions of the major components in the speed control system. For diagnosis of the entire speed control system, refer to the appropriate Powertrain Diagnostic Procedures service and the DRB scan tool in Powertrain Management/Computers and Control Systems.

SYSTEM OPERATION
Speed Control Servo
The speed control servo is mounted to a bracket on the passenger side headlamp closure panel in the engine compartment. The servo unit consists of a solenoid valve body, a vacuum servo and the mounting bracket. 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 serviced only as a complete assembly

Speed Control Switch
The speed control switch module is mounted to the center of the steering wheel below the driver's airbag module. Three momentary contact switches, supporting six different speed control functions are used. The outputs from these switches are filtered into one input. The Powertrain Control Module (PCM) determines which output has been applied through resistive multiplexing. The input circuit voltage is measured by the PCM to determine which switch function has been selected.

An indicator lamp (LED) located on the switch module is energized by the PCM. This occurs when the speed control system has been engaged.

The three switches are labeled: OFF/ON, RESUME/ACCEL, SET/DECEL.The individual switches cannot be repaired. If one individual switch fails, or the indicator lamp fails, the entire switch module must be replaced.

Stop Lamp Switch
Vehicles equipped with the speed control option use a dual function stop lamp switch. The switch is mounted in the same location as the conventional stop lamp switch, on the brake pedal mounting bracket under the instrument panel. The PCM monitors the state of the dual function stop lamp switch. Refer to Lighting and Horns/Brake Light Switch for more information on stop lamp switch service and adjustment procedures.

Servo Cable
The speed control servo cable is connected between the speed control vacuum servo diaphragm and the throttle body control linkage. This cable causes the throttle control linkage to open or close the throttle valve in response to movement of the vacuum servo diaphragm.

Powertrain Control Module
The speed control electronic control circuitry is integrated into the Powertrain Control Module (PCM). The PCM is located in the engine compartment on the dash panel near the windshield wiper motor. The PCM speed control functions are monitored by the On-Board Diagnostics (OBD). All OBD-sensed systems are monitored by the PCM. Each monitored circuit is assigned a Diagnostic Trouble Code (DTC). The PCM will store a DTC in electronic memory for certain failures it detects. The PCM cannot be repaired and must be replaced if faulty.

Vacuum Reservoir
The vacuum reservoir is mounted near the speed control servo on the right cowl side panel in the engine compartment. The reservoir contains a one-way check valve to trap engine vacuum in the reservoir. When engine vacuum drops, as in climbing a grade while driving, the reservoir supplies the vacuum needed to maintain proper speed control operation. The vacuum reservoir cannot be repaired and must be replaced if faulty.

Vehicle Speed Sensor
The Vehicle Speed Sensor (VSS) is a pulse generator mounted to an adapter near the transmission output shaft. The sensor is driven through the adapter by a speedometer pinion gear. The VSS pulse signal to the speedometer/odometer is monitored by the PCM speed control circuitry to determine vehicle speed and to maintain speed control set speed. Refer to the appropriate Powertrain Diagnostic Procedures in Powertrain Management/Computers and Control Systems for diagnosis and testing of this component.