Cruise Control: Description and Operation

When activated, the Speed Control System maintains vehicle road speed at a driver-selected rate. Using input information from the Vehicle Speed Sensor (VSS) and the Speed Control Amplifier varies the level of vacuum applied to the Speed Control Servo. The Speed Control Servo, in turn, operates the engine throttle linkage to speed up or slow down the vehicle.

Voltage is present at the Speed Control Switch Assembly ON contacts, at all times, through the HRN Fuse, the Horn Relay coil and the Clockspring/Slip Ring Assembly.

With the Ignition Switch in RUN, voltage is present at the Speed Control Amplifier through Fuse 17.

SPEED CONTROL AMPLIFIER INPUT

Speed Control Switch Assembly

The Speed Control Switch Assembly signals the Speed Control Amplifier to turn on the system, turn off the system, set and hold vehicle speed, accelerate or coast. With the Switch held ON, voltage is applied to the Amplifier through circuit 151 (LB/BK). In OFF, circuit 151 (LB/BK) is grounded. With the Switch held in SET/ACCEL, a 680 ohm resistor is placed in the ground circuit: in RESUME, a 2200 ohm resistor is placed in the ground circuit; in COAST, a 120 ohm resistor is used.

Stop Lamp Switch

With the brake pedal depressed, the Stop Lamp Switch contacts close and voltage is applied to the Speed Control Amplifier through circuit 511 (LG). This input causes the Amplifier to disengage the system.

Vehicle Speed Sensor (VSS)

The Vehicle Speed Sensor (VSS) is a small, permanent magnet generator that is driven by a transmission gear. Its output is proportional to vehicle road speed. With Sensor output indicating that vehicle road speed is above 30 MPH, the Speed Control Amplifier enables the Speed Control System. The Vehicle Speed Sensor (VSS) signal is also used to maintain vehicle speed when the System is operating.

Speed Control Servo

The Speed Control Servo contains a potentiometer that is connected to the throttle linkage. The Speed Control Amplifier applied a reference voltage to the potentiometer and monitors the voltage of the return signal. This feedback circuit provides information about throttle linkage position.

SPEED CONTROL AMPLIFIER OUTPUTS

Speed Control Servo

With the Speed Control System turned on and vehicle road speed above 30 MPH, the Speed Control Amplifier energizes the vacuum and vent valves in the Speed Control Servo. The vacuum valve passes vacuum to the Speed Control Servo, where it is used to move the throttle linkage; the vent valve controls the level of vacuum applied to the Servo.

The Vacuum Dump valve provides a mechanical means of disengaging the Speed Control System. With the brake pedal released, the Valve is closed, and the vacuum level in the system is determined by the vent valve. With the brake pedal depressed, the valve opens, disabling the vacuum circuit.

Speed Control Indicator

On vehicles equipped with the Digital Instrument Cluster, the center display contains a speed control indicator.

When the Speed Control System is engaged, the Speed Control Amplifier grounds circuit 203 (O/LB) to activate the indicator.