Speed Control System

SYSTEM COMPONENTS
SERVO - This electro-mechanical device contains three solenoid valves that control vacuum in the diaphragm chamber, pulling the cable and thereby the throttle. The individual solenoid functions are:

Vacuum Valve: When energized, this valve allows vacuum into the diaphragm chamber. When not energized, the valve is spring-loaded to prevent vacuum from entering the diaphragm chamber.
Vent Valve: When energized, this valve prevents vacuum from bleeding off, thereby allowing speed control function. When not energized, the valve is spring-loaded to bleed off vacuum.
Safety Valve: This valve functions like the vent valve, but its vacuum bleed is greater.

Power for all the solenoids is supplied through the ON/OFF and brake switches with one wire. Both the vacuum and vent solenoids are grounded by the engine controller to operate speed control. The safety valve is always grounded.

SPEED CONTROL SWITCH - Minivans and trucks are equipped with a stalk switch. All other vehicles with speed control have the switch assembly mounted in the steering wheel. Both switches are wired in a similar manner; the primary difference is that the steering wheel switch is wired through a clockspring assembly in the upper end of the steering column while the stalk switch is not.

NOTE: Special precautions and procedures must be followed when servicing clocksprings on vehicles with air bags.

BRAKE SWITCH - Vehicles equipped with speed control have a brake switch that controls three separate circuits. One circuit controls the power to the brake lights; the other two circuits relate to the speed control system as follows:

SERVO VALVE POWER CIRCUIT: Power for the servo comes from the ON/OFF switch and goes through the brake switch to the servo. The switch circuit is normally closed when the brake is not depressed. Either depressing the brake or turning the ON/OFF switch off will open the circuit and cut all power to the servo.

BRAKE SWITCH SENSE: A wire from the ON/OFF switch is connected to cavity number 29 of the engine controller 60-way connector. This wire is what the controller uses to indicate the ON/OFF status.

ENGINE CONTROLLER - The engine controller controls the speed control system using information from several inputs. Throttle actuation when speed control is engaged is controlled by alternately grounding (at the engine controller) the vacuum and vent solenoids to add or subtract vacuum from the servo.

A most important sensor signal used by the engine controller for speed control is the vehicle distance sensor input. The controller determines vehicle speed using input from this sensor and is very sensitive to speed sensor problems.