Speed Control Operates But No Resume, Acceleration, or Coast

1. Check to ensure proper operation of SET-ACCEL, COAST, and RESUME switches and slip ring circuits. If satisfactory, proceed to step 2. If not satisfactory, service or replace switch and/or circuit as necessary.
2. Ensure that all system vacuum hoses are tightly connected and are free of cuts, cracks and kinks.
3. 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 steps 4 or 5. If vacuum cannot be pumped or held, replace check valve.
4. 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 3. If vacuum cannot be pumped or held, replace aspirator check valves as necessary.
5. On all models, perform tests on servo. If satisfactory, proceed to step 6. If not satisfactory, replace servo.
6. 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.