1. Fuses and Relays:
* Check the fuse: Consult your owner's manual to locate the cruise control fuse and check if it's blown. Replace it if necessary with one of the same rating.
* Inspect the relay (if applicable): Some Jeeps use a relay for the cruise control system. Locate the relay (again, consult your manual), and visually inspect it for damage. You might even try swapping it with a similar relay (if possible and safe) to see if that solves the problem. A clicking sound from the relay when you engage cruise control could indicate a faulty relay.
2. Vacuum System (Most Likely Culprit in Older Jeeps):
* Vacuum Leaks: Many older vehicles, including some 1999 Jeeps, rely on a vacuum system to operate the cruise control. Leaks in the vacuum lines can prevent the system from functioning correctly. Inspect all vacuum lines connected to the cruise control system for cracks, loose connections, or damage. Pay close attention to the area near the brake booster and intake manifold.
* Vacuum Reservoir/Pump: Ensure the vacuum reservoir (if equipped) is functioning correctly and holding sufficient vacuum.
3. Brake and Clutch Switches:
* Brake Switch: The brake switch is a safety mechanism. If it's faulty or misaligned, it can prevent the cruise control from engaging or disengage it unexpectedly. Test the brake switch's continuity using a multimeter (or have a mechanic do this).
* Clutch Switch (Manual Transmission): Similar to the brake switch, a faulty clutch switch on manual transmission Jeeps will disable cruise control. Test it for continuity.
4. Speed Sensor:
* Vehicle Speed Sensor (VSS): The cruise control relies on the VSS to know the vehicle's speed. A malfunctioning VSS will prevent the cruise control from operating. This often requires a scan tool to diagnose.
5. Cruise Control Module/Actuator:
* Cruise Control Module (CCM): This is the "brain" of the cruise control system. A faulty CCM will require replacement. Diagnosis usually needs a scan tool.
* Actuator: This is the mechanism that controls the throttle. A faulty actuator will require replacement.
6. Wiring:
* Inspect Wiring: Look for any damaged, frayed, or disconnected wires in the cruise control system. Pay close attention to the wiring harness near the steering column and under the dash.
Troubleshooting Steps:
1. Start with the easiest: Check fuses and visually inspect vacuum lines first.
2. Use a multimeter: If you're comfortable with basic electrical testing, a multimeter can help diagnose faulty switches and sensors.
3. Consult a Repair Manual: A repair manual specific to your 1999 Jeep model will provide detailed diagrams and troubleshooting procedures.
4. OBD-II Scanner: While not always necessary for cruise control issues, an OBD-II scanner can sometimes identify related trouble codes that might point to the problem.
5. Professional Mechanic: If you're not comfortable troubleshooting electrical systems or vacuum lines, take your Jeep to a qualified mechanic. They have the tools and expertise to diagnose and repair the problem efficiently.
Remember safety first! Always disconnect the negative battery terminal before working on electrical components.
