* Vacuum Leaks: This is the most common culprit. Even a tiny leak in the vacuum lines, the intake manifold, or the booster can disrupt the cruise control's operation. A small leak might be sufficient when the system is under load (going uphill, for example). Carefully inspect *all* vacuum lines, connections, and the intake manifold for cracks, holes, or loose clamps. Consider using a vacuum gauge to test the system's vacuum at idle and while the cruise is engaged.
* Switch Issues: The cruise control switch on your steering wheel may be faulty. A failing switch can intermittently interrupt the electrical signal needed for the system to function.
* Speed Sensor Problems: The vehicle speed sensor (VSS) provides the crucial speed information for the cruise control to maintain speed. A failing or dirty VSS can cause erratic or intermittent cruise control function.
* Brake Switch: A malfunctioning brake switch could be triggering the cruise control to disengage unintentionally. This is a common cause as the brake switch is directly involved in disabling the cruise control.
* Throttle Position Sensor (TPS): The TPS tells the cruise control the engine's throttle position. A faulty TPS can lead to inconsistent or disengaged cruise control.
* Wiring Issues: There could be a break or short in the cruise control wiring harness. This is difficult to diagnose without a wiring diagram and a multimeter to check continuity and voltage.
* Cruise Control Module (CCM): While you replaced the servo, the cruise control module itself might have an internal fault. This is less likely if the other components are functioning correctly but is possible.
Troubleshooting Steps:
1. Thorough Vacuum Check: This should be your first step. Use a vacuum gauge and carefully inspect every inch of vacuum line.
2. Inspect the Wiring: Look for any visible damage to the wiring harness related to the cruise control.
3. Check the Brake and Speed Sensors: Ensure they are functioning correctly. You may need a multimeter to test their output.
4. Test the Cruise Control Switch: Try replacing the switch if other checks fail.
5. Inspect the Throttle Position Sensor: Test or replace if necessary.
6. Consult a Wiring Diagram: A wiring diagram specific to your Chevy G20 year and model will be invaluable in troubleshooting electrical issues.
If you're not comfortable performing these checks yourself, a qualified mechanic will have the tools and experience to diagnose the problem effectively. Providing the year and model of your G20 will help find more specific information related to your vehicle.
