Cruise control systems have been with us for years. They work very effectively to control the speed of the vehicle, but when they fail they can be a bear to diagnose and fix.
Cruise control systems are comprised of electronic and mechanical subsystems. This is how they work.
We all know that the things that control the speed of the car are the gas pedal and the brakes. And the brain that normally controls the speed of the car is the brain of the driver. The driver senses the speed by looking at the speedometer and then adjusting the pressure on the gas pedal or the brakes to compensate for variations in the desired speed. The cruise control system does the same thing with one exception. It only controls the gas pedal - it doesn't even know there are brakes in the car!!
The vehicle's speed sensor which is mounted on the output shaft of the transmission (the thing that drives the wheels) sends electrical pulses to the computer, pulses which are generated by a magnet spinning past a sensor coil. When the vehicle's speed increases the frequency of the pulses increases. For any given speed of the vehicle there is a corresponding pulse frequency. It is this pulse frequency which the cruise control tries to maintain as a constant. You think of it as the vehicle's speed.
The brains of the control box of the cruise control has three functions. First, it stores the speed of the vehicle when you press the "set" button while traveling at the desired speed. It keeps this value in its memory until you turn the ignition off. Second, it receives the pulses from the transmission sensor and compares the frequency of those pulses to the frequency value stored in its memory - the set point. Third, it sends pulses to a vacuum controlled diaphragm connected to the accelerator linkage. The pulses it sends regulates the amount of vacuum the diaphragm receives. The more pulses, the more vacuum and the more vacuum the more force on the accelerator linkage. The system continues to add vacuum force until the set point speed is reached. At that point the system modulates the amount of vacuum the diaphragm receives in an effort to maintain the number of pulses coming from the speed sensor as close to the stored value as possible.
OK, so this "brain" works just fine in controlling the speed of the vehicle until something goes wrong. What can go wrong?
First, the VSS, the thing that sends pulses to the brain might fail. Normally the speedometer also fails so that's pretty easy to diagnose.
Next, the power to the brain can be interrupted. A blown fuse or a corroded connector can prevent the brain from working correctly or at all.
Next, the brains can lose its ability to function. A faulty component can prevent the brain from doing its thing. The brain is a pretty sophisticated box that contains a lot of electronic components including a microprocessor. Normally when the brains fail you need to replace the box..
The vacuum diaphragm can develop a leak. If that happens then the cruise control might set and hold the speed for some time however if the leak is larger than the supply line and modulator can add vacuum to the system the system will slowly lose control and the vehicle will slow down. This can also happen if the vacuum line to the diaphragm is cracked or loose.
Finally, the linkage that connects the diaphragm to the accelerator linkage can fail. Some aftermarket cruise control systems use a short length of what looks like fat key chain - bead chain. I have seen several units fail when the chain simply breaks.
Diagnosis of a failed system can be a complex process. 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. Most cruise control switches are on the directional signal stem, a multifunction switch assembly with fine wires that break due to the constant motion of the wires as you use the directionals in your daily travels.
If you do an initial diagnosis and can't find the problem then go to the library in your town and get the shop manual. That is the only way you stand a chance of diagnosing and fixing the problem.