Whenever a speed control malfunction occurs, first verify that the speed control wire harness is properly connected to all connectors before starting repairs. A poor connection can cause a complete or intermittent malfunction and is also the only connection in the circuit that cannot be tested. For this reason, a loose connection may be misdiagnosed as a component malfunction.
Also, check all vacuum connections for tightness and cracked hoses. Road test the vehicle to verify speed control problems. The road test should include attention to the speedometer. Speedometer operation should be smooth and without flutter at all speeds. A flutter in the speedometer indicates a problem which might cause surging in the speed control system. The cause of any speedometer problems should be corrected before any more diagnosis is done.
If the road test verifies an inoperative system with correct speedometer operation, follow these steps:
- Inspect the cruise control fuse, and replace if it has blown.
- Check for a loose electrical or vacuum connection at the servo unit.
- Check for vacuum supply (refer to the tests below) and correct position of the vacuum check valve in the hose from the servo unit to the vacuum source. The word VAC on the valve must point toward the vacuum source.
- Remove corrosion from all electrical terminals in the speed control system.
- Verify that both ends of the speed control cable are securely attached. If either end is loose, the speed control system will be inoperative.
VACUUM SUPPLY TEST
- Disconnect the vacuum hose at the servo and install a vacuum gauge in the hose.
- Start the engine and observe at idle. The vacuum gauge should read at least 10 in. Hg (33.7 kPa). Shut off the engine; the vacuum should continue to hold to that specification.
- If the vacuum does not meet this specification, check the vacuum lines, check valve, vacuum reservoir and servo unit. Note that poor engine performance can cause problems with the speed control system.