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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.
» Part 1: What Is CC?
» Part 2: CC Parts
Part 3: What Goes Wrong
» Part 4: Adaptive CC 

What is cruise control?

The purpose of a cruise control system is to accurately maintain the driver's desired set speed, without intervention from the driver, by actuating the throttle-accelerator pedal linkage. A modern automotive cruise control is a control loop that takes over control of the throttle, which is normally controlled by the driver with the gas pedal, and holds the vehicle speed at a set value.

The driver can set the cruise control with the cruise switches, usually ON, OFF, RESUME, SET/ACCEL and COAST, that are located in the steering wheel or on the windshield wiper or turn signal stalk. On most cars the cruise control can accelerate or decelerate the car by 1 mph with the tap of the SET/ACCEL button. Hit the button five times to go 5 mph faster.

  1. The on and off buttons really don't do too much. Some cruise controls don't have these buttons; instead, they turn off when the driver hits the brakes, and turn on when the driver hits the set button.
  2. The set/accel button tells the car to maintain the speed you are currently driving. If you hit the set button at 45 mph, the car will maintain your speed at 45 mph. Holding down the set/accel button will make the car accelerate. On most cars, tapping it once will make the car go 1 mph faster.
  3. If you recently disengaged the cruise control by hitting the brake or clutch pedal, hitting the resume button will command the car to accelerate back to the most recent speed setting.
  4. Holding down the coast button will cause the car to decelerate, just as if you took your foot completely off the gas. On most cars, tapping the coast button once will cause the car to slow down by 1 mph.
  5. The brake pedal and clutch pedal each have a switch that disengages the cruise control as soon as the pedal is pressed. So you can disengage the cruise control with a light tap on the brake or clutch.

At speeds below 30 mph, the control unit will prevent application of cruise control functions and above 30 mph the driver can choose to turn it on or not. For reasons of safety, cruise control should not be used on wet or icy roads, heavy traffic or on roads with sharp bends.

Cruise control systems are designed to turn off immediately with a slight touch of the brake or clutch pedal. Most cruise controls will cut out if you accidentally shift from drive to neutral.

Cruise control has been around for a long time. Over the years the way they control speed has been improved with better electronics. And as a consequence, have become more difficult to troubleshoot. Most car manufacturers have special testers that hook up between the cruise control module and harness to pinpoint a specific problem.

The cruise control system controls the speed of your car the same way you do, by adjusting the throttle position. But cruise control actuates the throttle valve by a cable connected to an actuator, instead of by pressing a pedal. The throttle valve controls the power and speed of the engine by limiting how much air the engine takes in. Many cars use actuators powered by engine vacuum to open and close the throttle. These systems use a small, electronically controlled valve to regulate the vacuum in a diaphragm. This works in a similar way to the brake booster, which provides power to your brake system.

» Part 1       »
Part 2       » Part 3       » Part 4 

Additional Information provided courtesy of and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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