Diagnose and Repair Cruise Control
This article covers the diagnosis and repair of cruise control systems. Understanding the theory of operation can be helpful before commencing an investigation into problems. Another important piece of information needs to be verified before you get started, you need to know if the speed control is mechanical and vacuum controlled or an electronic version. There is an article here on autorepair.answers.com that provides a good explanation of how cruise control works and provides specific information about identifying the different kinds of systems deployed on older automobiles.
Types of Cruise Problems
Repairing cruise control begins with understanding the complaint of what’s wrong. Diagnosis of a system that doesn’t work at all is far different than if the system worked intermittently or slowly lost the set speed. Other more complicated problems like failure to resume or a system that doesn’t disengage when the brake pedal is tapped will take you in two completely separate directions when investigating the issues logically.
Cruise Will Not Set
When you have a situation where the cruise turns on, but won’t set diagnosis should begin in two main areas. First, the set control is usually a single spring-loaded button that activates the servo to hold the throttle open in its current position. This simple spring-loaded switch can fail just like any other. Testing can be performed with a voltmeter, but a wiring diagram will be necessary to identify which wire voltage should be present on when the set button is pushed. The second thing to check is the brake switch operation. First verify if the brake lights come on when the pedal is depressed and how soon they are activated. Many standard brake switches can be adjusted. Therefore, you want to verify the lights come on with a slight depression of the pedal. If the brake lights don’t work, it’s a strong sign this is responsible for the cruise not setting, but Keep in mind some cruise systems have a separate brake switch for the speed control and the brake lights.
Less Likely No Set Issues
If the cruise turns on, but it won’t set there are other things that can cause this condition besides the two main ones listed above. If the system uses manual linkage to pull on the throttle this may have become disconnected. And if the vehicle has a manual transmission, there will be a separate clutch pedal switch that performs the same action as a brake switch. Testing differs because this type of switch only controls cruise control disengagement. Another possible cause, but not common is problems with the vehicle speed sensor (VSS). Most automobiles use the main VSS mounted in the transmission and used by the power train control module (PCM) as a cruise input signal. The instrument cluster also uses VSS data to provide a miles per hour read out. When this sensor fails, it not only sets a check engine light code the driver often complains the speedometer on the dash doesn’t work. Mechanics can also scan the automobile and pull up the vehicle speed to see if the sensor is working.
Cruise Will Not Turn On
Most cruise systems will have some type of notification when it’s turned on. Some will have tiny amber lights next to the on switch, others can have an indicator light on the instrument cluster to notify the system is turned on and ready to use. In many cases, these dash board notifications are cryptic symbols. You can always refer to your owner’s manual when in doubt about the location of the power indicator light.
When the switch is activated and will not power up, it’s time to go back to the basics like with any other electrically powered system. A cruise control will have a fuse in the main panel. This can be a separate cruise fuse or one that shares power with other accessories. Sometimes it’s easier just to check all of the fuses to make sure none have burned open. If no fuse problems are found it’s possible the switch has failed. Power making up to the switch can be verified with a voltmeter. After locating which wire is power out of the switch this can also be verified.
Problems with Vacuum Cruise Control
Since these systems are on older vehicles, many times, problems are with deteriorated vacuum lines or leaking servo diaphragms. Often these leaks begin small which can generate driver complaints of a cruise control system that slowly loses speed over time. In this situation if deteriorated rubber hoses are found it is best to replace all of them. One that is often overlooked is the vacuum vent hose that goes to the brake pedal mounted disengage switch.
When it comes to testing the vacuum operated servo that connects to the throttle linkage this is best done with a handheld vacuum pump tester. This can be directly connected to the servo input port and the handle can be pumped to move the diaphragm and linkage. The tester is capable of holding pressures, so it can be verified whether the diaphragm has been compromised or not. If replacement of the vacuum servo is found to be necessary it’s not recommended to use junkyard parts that may be in the same exact condition as the one that failed.
In some cases people with do-it-yourself skills and initiative can diagnose and repair cruise control. In other situations especially on newer automobiles, it might be necessary for professional intervention. When attempting to do-it-yourself fully understanding the symptoms and beginning diagnosis in the right place is key to a successful repair. As an example, you wouldn’t waste your time checking the fuses if the power light turns on when the switch is activated. You also wouldn’t check or replace the set switch if the complaint was the cruise slowly loses miles per hour after being set.