Cruise Control Servo

There are two main components of the cruise control system: the controller and the servo.

The controller integrates all the inputs and tells the servo how far to actuate the throttle plates. Normally, there is no way you can service this item, so if you trace a problem to the controller, you'll have to buy a new one. A bad controller probably will require a service manual for specific diagnostics. You'll also need a scan tool to access any computer trouble codes to boot.

The servo does the work of moving the throttle blades to speed up the vehicle. Conventional systems are actuated by manifold vacuum. Check the linkage from the servo to the throttle to ensure it's properly hooked up and not binding or sticky. Inside the servo is a diaphragm that moves to pull on the linkage. Some older systems, used on cars or trucks that don't have throttle-position sensors, may have a rod that moves in and out of a magnetic coil to tell the controller the throttle position. There are also two electrically operated solenoid valves. One valve admits vacuum to the diaphragm chamber to add more throttle. The other bleeds air back into the chamber to reduce throttle. Normally, they will never be open at the same time, so if one is sticky or leaking, cruise control operation will be erratic at best. Check for leakage with a handheld vacuum pump. A leaky valve may benefit from a quick shot of silicone spray