The cruise-control system operates via a mechanical linkage that actuates the throttle according to input from a vacuum motor. Solenoid-operated valves regulate the linkage which, in turn, open or close the throttle according to the dictates of the speed sensor and module. One valve opens to increase vacuum in the vacuum motor while the other valve opens to dump the vacuum to close the vacuum motor. The speed module (included in the instrument cluster on 84-89 cars) controls the vacuum motor and throttle by pulsing the solenoid valves on and off.
The C4s cruise-control system can be intimidating if considered as only a part of the myriad of hoses, wires, and electronics that comprise the car. When viewed as a smaller system of components with an understanding of what they do, the fear factor is greatly reduced. Well be following Chris Petris as he troubleshoots an 86 Corvettes cruise-control system. The major difference between this car and the 90-and-up Corvettes is that the module for the cruise control will no longer be part of the instrument cluster on the later Corvettes. Obviously, if your 84-89 Corvettes instrument-cluster speed indicator doesnt work, dont expect the cruise to work. The good news for owners of 90-and-newer Corvettes is that if your module goes bad, you dont have to replace the entire instrument cluster. Chris says that the modules are rarely the problem, though.
Follow along as we go looking for trouble in a C4s cruise-control system.