Transmission Related:
* Vacuum Leaks: The transmission's control system relies heavily on vacuum. Leaks in the vacuum lines connected to the transmission modulator, throttle body, or elsewhere in the system can cause erratic shifting and high idle. A faulty modulator itself is also a strong possibility. The modulator adjusts shift points based on throttle position and engine load. A malfunctioning modulator can lead to improper shifting and high idle.
* Transmission Linkage Issues: Problems with the linkage connecting the throttle to the transmission can prevent the kickdown from engaging properly. This could be due to broken or bent linkages, a loose connection, or adjustment issues.
* Faulty Transmission Solenoid(s): The transmission uses solenoids to control shift functions. A stuck or faulty solenoid can cause problems like high idle and lack of kickdown.
* Low Transmission Fluid: Low fluid levels can cause the transmission to overheat and malfunction, resulting in erratic shifting and high idle. Check the transmission fluid level and condition. Dirty or burnt fluid suggests a larger transmission problem.
* Internal Transmission Problems: In more severe cases, internal damage within the transmission (e.g., worn clutches, bands, or planetary gears) can lead to these symptoms. This is less likely to be a primary cause, but is possible if other issues are ruled out.
Engine Related (Indirectly Affecting Transmission):
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate engine load information to the computer, which in turn affects the transmission's shifting behavior. An incorrect signal will prevent the kickdown from functioning correctly.
* Idle Air Control (IAC) Valve: A malfunctioning IAC valve can cause a high idle by allowing too much air into the engine. While it doesn't directly prevent kickdown, a very high idle could overwhelm the transmission's ability to downshift.
* Vacuum Leaks (Engine Side): As mentioned above, engine vacuum leaks can also indirectly affect the transmission control.
Computer/Electrical Issues:
* Transmission Control Module (TCM): Although less common, a faulty TCM can cause a range of transmission problems, including the symptoms you're describing. This is more likely if other components check out ok.
* Wiring Problems: Damaged or corroded wiring in the transmission control system can interrupt signals and cause malfunctions.
Troubleshooting Steps:
1. Check Transmission Fluid: This is the first and easiest thing to check. Low or dirty fluid is a significant problem.
2. Inspect Vacuum Lines: Visually inspect all vacuum lines connected to the transmission and engine for cracks, leaks, or loose connections.
3. Check Transmission Linkage: Make sure the linkage is properly adjusted and not damaged.
4. Test the Throttle Position Sensor (TPS): This often requires a multimeter and some knowledge of electrical testing.
5. Check the IAC Valve: This may require cleaning or replacement.
6. Scan for Diagnostic Trouble Codes (DTCs): If your Thunderbird has a diagnostic port, a code reader can provide valuable clues.
It's highly recommended to consult a repair manual specific to your 1989 Ford Thunderbird or take it to a qualified mechanic. The interconnected nature of these systems makes diagnosing the problem without specialized tools and knowledge difficult. Attempting repairs without proper knowledge could lead to further damage.
