1. The Obvious Checks:
* Parking Brake: Is the parking brake fully engaged? Release it completely.
* Shift Lever: Try gently rocking the shifter back and forth while applying slight pressure in the desired gear. Sometimes it just needs a little persuasion to overcome binding.
* Transmission Fluid: Check the transmission fluid level. Low fluid is a very common cause. If it's low, add the correct type of automatic transmission fluid (consult your owner's manual). If it's dirty or smells burnt, that's a more serious issue requiring professional attention. *Never* overfill the transmission.
* Engine Running: Is the engine running? Some transmissions require the engine to be running to shift into gear.
2. More Involved Troubleshooting:
* Shift Linkage: This is a mechanical connection between the shifter and the transmission. Inspect it for any broken, bent, or loose components. Look for anything that might be binding or interfering with its movement. This often requires getting under the car.
* Shift Cable Adjustment: The cables connecting the shifter to the transmission may need adjusting. This is usually a more advanced procedure and may require a repair manual or professional help.
* Vacuum Modulator (if applicable): Some older automatics use a vacuum modulator to help with shifting. A leak or malfunction in this system can prevent shifting. This requires more advanced diagnostics.
* Internal Transmission Problems: The most serious possibility is a problem *inside* the transmission itself. This could be a variety of issues, including worn clutches, a broken valve body, or low fluid pressure. This is usually indicated by other symptoms like slipping, harsh shifting, or noises from the transmission.
3. Seeking Professional Help:
If the above steps don't resolve the issue, it's crucial to seek professional help. A mechanic with experience in older vehicles can properly diagnose the problem. Ignoring a transmission problem can lead to very costly repairs.
Important Considerations:
* Safety: Never attempt repairs if you're not comfortable working on cars. Transmission work can be dangerous if not done correctly.
* Repair Manual: A factory service manual for your 1990 Lincoln Town Car is an invaluable resource. It provides detailed diagrams and instructions for repairs.
* Professional Diagnosis: The cost of a proper diagnosis from a mechanic is often much less than the cost of potentially replacing parts unnecessarily.
In short: Start with the easy checks. If those fail, inspect the linkage. If you're unsure about anything, don't hesitate to get it checked by a mechanic specializing in older cars. A transmission repair can be expensive, so accurate diagnosis is key.
