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Cruise Control: What Is It?
| Most cars have cruise control systems. Here is how they work and some things that can go wrong with them. |
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System Components
Vehicle's Speed Sensor:
The Vehicle Speed Sensor (VSS) is mounted to the transmission and provides a low voltage Alternating Current (AC) signal to the Cruise Control Module (CCM). The CCM converts the AC signal to a pulse width modulated Direct Current (DC) signal, which is sent to the cruise control module at a rate of 4000 pulses per mile.
Cruise Control Module:
The cruise control module has to do three things. First it remembers the speed you set. It stores this set speed until you change it or turn off the ignition. Next it takes the speed signal from the vehicle speed sensor and compares it to the set speed. Lastly it sends pulse signals to the actuator. The actuator will move the throttle linkage to bring the vehicle up to the set speed and then modulate vacuum to maintain that speed.
Actuator:
The actuator is what actually moves the throttle linkage. It is most often vacuum operated although some actuators are electrically controlled with small, stepper type motors. The actuator moves the linkage as directed by the cruise control module until the set speed has been achieved. It then maintains this speed by controlling the amount of vacuum. It actually modulates the vacuum as the pulses from the control module direct.
Brake Switch:
The cruise control release switch and stop lamp switch are used to disengage the cruise control system. A cruise control release switch and a stop lamp switch, mounted on the brake pedal bracket disengage the system electrically when the brake pedal is pressed. This is accomplished by interrupting the flow of current to the cruise control module. The cruise speed of the vehicle at brake actuation will be stored in the cruise control module memory.
Clutch Switch:
In addition to the brake switch, a vehicle with a manual transmission has a switch very similar to the brake switch and disengages the cruise control system when the clutch pedal is depressed.
Throttle Linkage:
The actual mechanical connection between the cruise control actuator and the engine throttle.
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