Cruise Control: What it is and how it works.
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.
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.
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.
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.
The actual mechanical connection between the cruise control actuator and the engine throttle.
What Can Go Wrong
Since each manufacturer has slightly different cruise control systems, specific troubleshooting procedures will vary. Most vehicle shop manuals have a multi-page diagnostic flow chart that the dealer mechanics use to solve failures. If there isn't an obvious problem like a broken wire, a blown fuse or a leaking vacuum line then the problem most likely lies in the brains of the unit or in the switch that sets the speed and contains the other functions of resume and accelerate.
But since each system has the same basic parts, different systems will share the same basic problems. As always, you should consult the repair manual for you particular vehicle before trying to troubleshoot your cruise control system.
If the vehicle speed sensor fails, the cruise control module will not get a speed signal. If this is the case the speedometer will usually stop working as well.
The cruise control module can go bad and either not understand the signal coming in from the vehicle speed sensor or is unable to send the signal to the actuator. In addition blown fuses or broken wires can prevent the cruise control module from working properly as well.
The vacuum diaphragm inside the actuator can develop a leak and prevent it from operating properly. If it is a large vacuum leak the actuator will probably not set at all. Leaking or broken vacuum lines are usually the major cause of this problem.
Lastly, the linkage itself may break or become disconnected. Some cruise control systems use a bead type chain, like the pull chain on an old light socket, and they just plain break.
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