Cruise Control: What Is It?

Cruise Control: What it is and how it works.

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.
  1. 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.
  2. 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 whit 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 they 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.

    Copyright © 2001 - 2003 Vincent T. Ciulla All Rights Reserved

    Cruise Control: What it is and how it works.

    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.

    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.

    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.

    Copyright © 2001 - 2003 Vincent T. Ciulla All Rights Reserved

    Cruise Control: What it is and how it works.

    Adaptive Cruise Control

    There is a new type of cruise coming onto the market called adaptive cruise control. Two companies, TRW and Delphi Automotive Systems are developing a more advanced cruise control that can automatically adjust a car's speed to maintain a safe following distance. This new technology uses forward-looking radar, installed behind the grill of a vehicle, to detect the speed and distance of the vehicle ahead of it.

    Mercedes-Benz became the first car manufacturer to install TRW's adaptive cruise control, called Auto-cruise, adding the device to its European S-Class Saloons. BMW followed Mercedes-Benz later by adding the system to some of its European models.

    Delphi Automotive Systems has developed a similar adaptive cruise control system, already available on the 2000 Jaguar XKR in Europe.

    Adaptive cruise control is similar to conventional cruise control in that it maintains the vehicle's pre-set speed. However, unlike conventional cruise control, this new system can automatically adjust speed in order to maintain a proper distance between vehicles in the same lane. This is achieved through a radar headway sensor, digital signal processor and longitudinal controller. If the lead vehicle slows down, or if another object is detected, the system sends a signal to the engine or braking system to decelerate.

    Then, when the road is clear, the system will re-accelerate the vehicle back to the set speed.

    The Auto-cruise radar system has a forward-looking range of up to 500 feet, and operates at vehicle speeds ranging from 18.6 miles per hour to 111 mph. Delphi's system can also detect objects as far away as 500 feet, and operate at speeds as low as 20 mph.

    Adaptive cruise control is just a preview of the technology being developed by both companies. These systems will be enhanced to include collision-warning capabilities that will warn drivers through visual and/or audio signals that a collision is imminent, and that braking or evasive steering is needed.

    Copyright © 2001 - 2003 Vincent T. Ciulla All Rights Reserved