Solutions for Cruise Control System Problems
Believe it or not cruise control systems have been around for the last hundred years. They really didn’t start to gain traction until the gas crunch of the early 70s. It was then that people realized a steady throttle position could increase fuel economy and alleviate stress on long trips. Although major technology advances have occurred the basics remain the same. In order for the system to work properly it needs an on switch, accurate way to measure current speed, a method to hold the throttle open to maintain it and a brake switch to disengage it when slowing down. In this article we will discuss the basic operation and talk more about the modern systems. Also covered will be the different types on the road today and some common problems and solutions for each.
Cruise Control Operation
Automatic speed control was developed to allow motorists to maintain a constant speed without having to apply pressure on the gas pedal. Some have minimum speed requirements of around 25 mph and on a manual transmission, you might have to be in third gear or higher, depending on the year, make and model. A properly operating system will automatically compensate for changing loads such as hills and gusting headwinds.Operation begins at the cruise switch that is often mounted in a convenient location, either on the steering wheel or combination lever that might include the turn signal and wipers. This allows the driver to turn on the system and set functions without having to take their eyes off the road. The cruise switch functions include on/off, a set button and a resume function that allows you to reengage the cruise at the last set speed. This is handy when you tap the brakes in heavy traffic.
Vacuum Cruise Control Systems
Although modern vehicles will use some type of electronic speed control, cars from the 70s through the early 90s, used a mechanical system controlled by engine vacuum. Since plenty of these vehicles are still found on the road today the vacuum operated speed control is still considered common by mechanics. As an example of typical operation these older vacuum systems would sense the speed using a transducer that was mounted in between the speedometer cable and the transmission. A vacuum operated servo is connected to the throttle by mechanical linkage and some even used an adjustable chain. Throttle position is controlled by the servo receiving a specific amount of vacuum determined by input from the speed sensing transducer. Changes in this vacuum would change throttle position. When the brake switch is activated vacuum will be vented to the atmosphere and the servo releases the throttle to the closed position.
Problems with Vacuum Cruise
Overall, the systems were quite reliable in the prime of their life cycle. As they age, problems with the rubber hoses and vacuum operated servo which is made of a rubber diaphragm are likely. When the diaphragm begins to leak it is unable to hold the speed. The speedometer cable and transducer system that report on the current mph is also capable of failing. The speed cable is packed with grease, which can deteriorate. This condition can also be accompanied with inaccurate or fluctuating speedometer needle and squeaky noise generated from the cable itself.
Electronic Cruise Control Systems
As electronics advanced manufacturers took advantage of the new technology and stopped using the mechanical components associated with a vacuum system. Now it’s more common to see an integrated or separate speed control module that senses current miles per hour from the transmission mounted vehicle speed sensor and controls the output of a stepper motor that connects to the throttle plates with mechanical linkage.Early versions of the system had separate components, but now automobiles with fly by wire systems no longer need a separate stepper motor to control the throttle. Some vehicle manufacturers have also done away with a separate cruise module and now use the power train control unit to sense speed and control throttle position.
Common Problems with Electronic Cruise
Electronic systems are even more reliable than the mechanical vacuum ones. This does not mean they’re trouble-free for the life of the automobile. They rely heavily on switches and sensors which are capable of individual failure. The cruise control will not set if there is a problem with the on/off switch, brake switch or the vehicle speed is not being reported correctly.The indicator lights showing the system is turned on, or the cruise has been set can help perform some basic diagnosis. If on is selected and the power light does not illuminate it’s time to check the fuse and power relays. If these test okay it could be a problem with the power switch itself. Electronic cruise that turns on, but will not set is more complicated to diagnose. Professional mechanics will scan the computer for check engine light codes and pull up the data stream from the speed sensor to see if the output matches actual miles per hour. If the speed sensor checks out the mechanic will move on to the brake switch and test the operation. Sometimes the cruise function is separate from the brake lights therefore testing of the switch can be vehicle specific.
Identifying what kind of speed control system you have is a good place to start diagnosis. Telltale signs of vacuum type system are a large rubber and plastic diaphragm called a servo mounted near the throttle linkage in the engine compartment. Another sign is a brake switch with a vacuum line going to it. Electronic cruise started out with separate components and then began to integrate them into self contained assemblies. The newer a car is the more likely diagnosis and repair will have to be performed by a factory trained technician.