The most common problem with cruise control systems is failure to engage and failure to maintain a set speed. Depending on the year and model of the vehicle, special tools such as a volt meter and vacuum tester may be needed for diagnosing the problem. Consult your service manual before beginning of this procedure for anything special to servicing your vehicle.
Diagnose the problem if the cruise control does not activate. Check the cruise control fuse. This step is often overlooked and diagnosis focuses on what seems to be more likely problems. Locate the fuse box under the driver's console. It has a plastic cover that can be removed . Check the inside diagram to determine which fuse you need to check. Probe both sides of the fuse. If one or neither lights the test light, the fuse is defected and replace. Check the cruise control operation after changing the fuse. If it works, no further repairs are necessary. If not, continue on with these procedures.
Do a visual check of all wiring relating to the cruise control system. Use your inspection mirror to see places you can't see otherwise. Look for melted insulation, broken wires, burned wires and twisted wires without a wire connector. Repair any wire damage with the splicing and wire connecting tool and the right size wire connector for the wire you are repairing. Use a wiring diagram and a volt meter to locate wires going to the controller and servo terminals. Your service manual will tell you what the voltage should be and whether the servo and controller are operating at the correct voltage. Wiring and wire connectors can be purchased at your local parts store or dealer.
Visually inspect the brake (and clutch if so equipped). If these are out of adjustment, the voltage cannot get to the components down stream with enough power to operate the cruise control. These can be adjusted to the right setting by using your service manual for the correct position.
For vacuum-operated systems, you will need to perform different steps than those used for an electronic set-up. Start the engine. Using your vacuum gauge, check for vacuum at the vacuum line to the servo. Vacuum should read 10 inches of mercury at idle speed. If you can't locate these parts, use your service manual to locate them.
Visually inspect for vacuum leaks in the release hose and valve. Replace the hose if leaking. Use the digital volt ohm meter to check the resistance of the servo solenoid windings. Check your service manual for the correct readings. Check for a vacuum leak at the servo. If the results of these checks are satisfactory, the controller is usually defective. Buy a new part at your local parts store or dealer.
If the cruise control system fails to maintain the set speed, check the throttle linkage adjustment. Use your service manual for the correct procedure for adjusting the linkage. If the linkage is properly adjusted, check for vacuum leaks at the servo. Turn the engine of if turned on, and use jumper wires to energize the servo solenoid valves. Apply vacuum using the hand held vacuum pump. If the servo fails to hold vacuum, replace it with a new part. This part can be purchased at your local part store or dealer. If both tests are good, this means that the cruise controller is inoperative and must be replaced. This part can also be found at your local parts store or dealer.
