Speed Control Is Inoperative

1. Apply brake pedal and check if brake lights work. If brake lights work, proceed to step 2. If brake lights do not work, check stop lamp circuit and stop lamps.
2. On models with manual transmission, check to ensure proper operation of clutch deactivator switch. If switch operation is satisfactory, proceed to step 3. If switch operation is not satisfactory, service or replace as necessary.
3. On all models, check for proper operation of actuator lever and throttle linkage. If satisfactory, proceed to step 4. If not satisfactory, adjust or service as necessary.
4. Ensure that all system vacuum hoses are tightly connected and are free of cuts, cracks and kinks.
5. Disconnect hose between check valve and servo, at servo end. Apply 18 inches of vacuum to open end of hose. If vacuum can be pumped and held, proceed to step 6 or 7. If vacuum cannot be pumped or held, replace check valve.
6. On Thunderbird SC models, disconnect hose between aspirator check valves and manifold check valve at manifold check valve end. Apply 18 inches of vacuum to open end of hose. If vacuum can be pumped and held, proceed to step 7. If vacuum cannot be pumped or held, replace aspirator check valves.
7. On all models, check vacuum dump valve. If valve operation is satisfactory, proceed to step 8. If valve operation is not satisfactory, adjust or replace as necessary.
8. Perform tests on control switches and circuits. If satisfactory, proceed to step 9. If not satisfactory, repair or replace as necessary.
9. Perform tests on servo. If servo operation is satisfactory, proceed to step 10. If servo operation is not satisfactory, replace servo.
10. Perform tests on speed sensor. If sensor operation is satisfactory, proceed to step 11. If sensor operation is not satisfactory, replace sensor.
11. Replace amplifier with known good amplifier. If system operation is satisfactory, replace amplifier. If system operation is still not satisfactory, ensure all system electrical connectors have proper contact.