Cruise control is an automatic speed control mechanism. It relieves the car driver of having to maintain speed manually, by pushing the accelerator pedal. Once the control is activated, besides maneuvering duties, the driver need not worry about acceleration control. This mechanism is very useful while driving on long routes and highways, that stretch for hundreds of miles. Besides the ease in driving, this mechanism increases fuel efficiency of the car. By setting the pace to lesser than the speed limit, you can ensure that you do not land up with speeding tickets.
The control over speed is achieved through control over throttle valve of the engine, which decides the volume of gasoline consumed at a time. The cruise control settings are within reach of the driver and usually have 'on/off', 'resume', 'set', 'accelerate', and 'coast' buttons. Once the driver has manually accelerated the car above a certain minimum speed, the control mechanism can be activated, when you set a speed limit. This leaves the accelerator on autopilot and you need not push it down anymore.
Even if you push on the pedal and accelerate above the set value, the speed will lower to the set value, once you release the accelerator pedal. The mechanism stops working in event of braking or tapping of the clutch. The control over speed is achieved by pulling of the throttle cable, by a vacuum-operated servomechanism. The module (CCM) monitors the overall functioning, through a feedback and relay mechanism.
Cruise control mechanism, being a mechanical contraption, is vulnerable to certain problems due to failure of components or failure of coordination between them. Here are the most common problems that you could face.
Vehicle Speed Sensor Failure
Vehicle speed sensor (VSS) is the eye of the control module, that monitors the speed of the vehicle, by sending alternating current signals, whose frequency is directly proportional to speed. When the vehicle speed sensor fails, it can no longer receive signal information about vehicle speed, rendering it blind and useless. VSS failure is one of the most common problems that you could face. How can you know that cruise control is not working, as a result of VSS failure? Look at the speedometer, as it is dependent on the VSS too. If the control and speedometer stop working simultaneously, you are facing VSS failure. In such a case, the only way out is to repair or replace the VSS.
Cruise Control Module Problems
The control module is the brain of the mechanism. It processes the signals that it receives from the VSS, compares it with set speed value and prods the actuator to accordingly control the throttle, through a vacuum mechanism. A blown fuse and broken wires can be the bane of the CCM mechanism, leading to functional failure and consequently, complete speed control failure. In such a case, repairing or replacing the CCM is the only way to restore functionality.
Actuator Vacuum Diaphragm Leak
The actuator is the device that actually controls the throttle, through a vacuum mechanism. If the vacuum diaphragm inside the actuator, develops a leak, the control mechanism will fail. This is also one of the most common problems. The only way out is fixing the leak and restoring vacuum inside the actuator.
Throttle Linkage Breakdown
Throttle linkage is the connector between the actuator and the throttle. These linkages are usually metal chains. If they fail or break down, cruise control will stop working. Replacing the linkage will solve the problem.
Using this 'Auto Pilot' mechanism for speed control, comes with its share of risks. These intelligent transport systems need to be used carefully and are most useful in long drives. Diagnosing and repairing this equipment, requires a lot of specialized machinery, to which, you may not have any access. Unless you possess the requisite expertise and tools, it is a job, best left to car repair professionals.