Improper operation can be caused by any combination of mechanical, electrical and vacuum problems. In resolving any cruise system operating problem, always ensure there are no bare, broken, or disconnected wires or any pinched, damaged, or disconnected vacuum hoses. The servo and throttle linkage should operate freely and smoothly.
Since any problem in this system may be due to one or more causes, a quick visual inspection should always be made before beginning extended diagnosis. This can be done by eliminating an obvious vacuum, mechanical or electrical problem. Start the engine and use a vacuum gauge to confirm vacuum at the servo. Check the related fuse(s) and look for loose or corroded harness connectors. Inspect the brake and clutch switches for loose wires or hoses. Check that the switch plunger moves smoothly and is not stuck in the disconnect position. Inspect the throttle linkage from the servo for binding, breakage or excess slack.
If the speedometer is not operating correctly, vehicles using optic vehicle speed sensors will not provide the correct speed signal to the cruise control module.
- With the cruise control switch OFF , drive the car to highway speeds. Depress the SET switch button, then release. The system should remain inoperative.
- Set the cruise control system switch to ON .
- Engage the system, by momentarily depressing the SET button, then releasing it at approximately 50 mph, then remove your foot from the accelerator. The vehicle should maintain speed.
- Push the RESUME/ACCEL switch and hold. The vehicle should accelerate at a controlled rate. Release the slide to engage the system at a cruise speed of approximately 55 mph (88 kph). The system should now be set at this new speed, and the vehicle should maintain that speed.
- Depress the brake pedal about 1 / 2 in. (13mm), the indicator light, if equipped, should go out. This confirms the action of the electric brake release switch, and the vehicle should start to slow down. Allow the vehicle to slow to approximately 45 mph, using the brakes if desired.
- Momentarily push the RESUME/ACCEL switch, then release. The vehicle should accelerate at a controlled rate and resume previously set speed of about 55 mph (88 kph), and maintain that speed. The indicator light (if equipped) should come on when the slide switch is released.
- Depress the SET/COAST button and hold. The indicator light (if equipped) should go out. Coast to 50 mph. with the button pressed.
- Release the SET/COAST button to engage the system at 50 mph (81 kph). The vehicle should maintain the new speed.
- Check the tap-down feature. Depress the SET/COAST button for less than 1 / 2 second (tap). The vehicle speed should drop about 1 mph (2 kph). Tap the button a couple more times to check incremental speed decreases.
- Check the tap-up feature. Actuate the RESUME/ACCEL for less than 1 / 2 second (tap). The vehicle speed should increase about 1 mph (2 kph). Tap the switch a couple more times to check incremental speed increases.
- If all functional conditions are met during this test, the system is OK.
- The servo and throttle linkages should operate freely and smoothly. This linkage should be adjusted if necessary.
- Check the hose routing for pinches, leaks or restrictions.
- Check the vacuum hoses for proper routing, restrictions or leaks. Adjust or replace as required.
- Check the servo linkage for excess slack and adjust, if necessary.
- If no system problem is noted, replace the controller.
- Check all the hoses for vacuum leaks.
- Determine if the check valve to the vacuum accumulator is functional.
If all other functions of cruise control are operating correctly except the tap-up and/or tap-down, the controller is faulty.
See Figure 1
The use of tool J-34185, Cruise Control Quick Checker or its equivalent is recommended for vacuum operated systems. The checker connects to the harness in place of the module. It allows manual system testing and can considerably reduce diagnostic time. A test lamp may be required during some tests.
All systems rely on proper battery voltage to operate. Ensure the battery is correctly charged before beginning diagnostics. Do not allow battery to drain during extended testing. Turn system switch or ignition switch OFF when not actively testing the system. Involved testing and special tools are required for most testing. This would be best left to a professional shop with the necessary test equipment, as the cost involved for the tools is usually more than the cost of the repairs.