System Description

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). For diagnosis of the entire speed control system, refer to the appropriate Powertrain Management/Computers and Control Systems/Testing and Inspection/Diagnostic Charts Procedures and the DRB scan tool.

SPEED CONTROL SERVO
The speed control servo is located in the engine compartment, mounted to a bracket on the right inner fender. 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 SWITCHES
Speed control switch modules are mounted on the steering wheel to the left and right side of the driver's airbag module. Within the two switch modules, five momentary contact switches, supporting seven 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.

A speed control indicator lamp, located on the instrument panel cluster is energized by the PCM via the CCD Bus. This occurs when speed control system power has been turned ON, and the engine is running.

The two switch modules are labeled: ON/OFF, SET, RESUME/ACCEL, CANCEL and COAST. The individual switches cannot be repaired. If one individual switch fails, the 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 Brake Switch for more information on stop lamp switch service and adjustment procedures.

STOP LAMP SWITCH CIRCUIT OPERATION
Circuit L50 from the stop lamp switch provides the brake switch input to the Controller, Anti-Lock Brakes (CAB). When the brake pedal is depressed, the stop lamp switch closes to supply battery voltage from circuit L16 to circuit L50. Circuit L50 connects to the CAB. Circuit L16 originates at fuse 9 in the junction block. Circuit A250 from fuse 11 in the Power Distribution Center (PDC) supplies power to junction block fuse 9.

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 (PCM)
The speed control electronic control circuitry is integrated into the Powertrain Control Module (PCM). The PCM is located in the engine compartment behind the coolant recovery tank. 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. See On-Board Diagnostic Test For Speed Control System for more information. The PCM cannot be repaired and must be replaced if faulty.

VACUUM RESERVOIR
The vacuum reservoir is mounted below the battery tray. 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 is monitored by the PCM speed control circuitry to determine vehicle speed and to maintain speed control set speed. Refer to the appropriate Powertrain Management/Computers and Control Systems/Diagnosis and Testing/ Procedures/Diagnostic Charts for diagnosis and testing of this component.

SYSTEM CIRCUIT OPERATION
The Powertrain Control Module (PCM) operates the vehicle speed control system. The vehicle speed control switches are located in the steering wheel.

Circuit V32 from cavity C11 of the PCM connects to circuit V30 through the stop lamp switch. Circuit V30 powers the vehicle speed control servo.

Circuit K95 from PCM cavity C32 connects to the vehicle speed control switches. The switches are wired in parallel and each contains a separate resistor. The voltage level present on circuit K95 (at PCM cavity C32) depends on which speed control switch is selected. Circuit K4 from PCM cavity A4 supplies ground for the speed control switches.
- When the ON/OFF switch is open, the voltage level on circuit K95 at PCM cavity C32 has a nominal value of 5.0 Volts with a range from 4.8 to 5.0 Volts.
- When the ON/OFF switch closes, the voltage level on circuit K95 at PCM cavity C32 has nominal value of 1.51 Volts with a range from 1.31 to 1.61 Volts.
- When the SET switch closes, the voltage level on circuit K95 at PCM cavity C32 has nominal value of 3.8 Volts with a range from 3.6 to 3.9 Volts.
- When the RESUME/ACCEL switch closes, the voltage level on circuit K95 at PCM cavity C32 has nominal value of 4.4 Volts with a range from 4.2 to 4.5 Volts.
- When the COAST switch closes, the voltage level on circuit K95 at PCM cavity C32 has nominal value of 2.92 Volts with a range from 2.72 to 3.02 Volts.
- When the CANCEL switch closes, the voltage level on circuit K95 at PCM cavity C32 has is 0.1 Volts or less.

The PCM controls the vent and vacuum functions of the vehicle speed control servo on circuits V35 and V36. Depending on the signal it receives from vehicle speed control switches, the PCM either applies vacuum to or vents vacuum from the servo. Circuit V36 from cavity C4 of the PCM sends the vacuum signal to the servo. Circuit V35 from cavity C5 sends the vent signal.

Circuit L53 provides the stop lamp switch sense input to the PCM at cavity C24. The stop lamp switch connects circuit L53 to ground on circuit Z1. When the brake pedal is depressed, the stop lamp switch opens and disconnects circuits L53 and Z1, and circuits V32 and V30. When the stop lamp switch disconnects circuits V32 and V30, power is removed from the speed control servo.