The cruise control system a vacuum-based system and is designed to work at speeds above 25 mph. To activate the system, the cruise control dash mounted main switch must be in the on position. The main switch toggles between on and off by simply pressing it. When in the on position, an indicator light in the switch will light.
The cruise control system works a mechanical linkage to the throttle by way of a vacuum motor which is inside a server. This is a diaphragm moved by vacuum applied to one side. A solenoid driven valve connects the vacuum motor to a vacuum tank. Another solenoid vents the vacuum. The cruise control module controls the servo and the throttle by pulsing these solenoid valves on and off.
One input to the cruise control module is the vehicle speed, which is sent to the Powertrain Control Module (PCM) by the Vehicle Speed Sensor (VSS)
To diagnose the system, first note the symptom.
If the system does not engage at all:
If the system loses speed:
If the vehicle loses speed before the system engages: