1. Battery Voltage: A weak battery can sometimes confuse the anti-theft system. Have the battery voltage tested. A low voltage reading (below 12.4 volts, ideally closer to 12.6-13 volts when the car is off) could be the problem. A jump start might be necessary, but even a fully charged battery is preferable.
2. Key Fob (if applicable): Does your Eldorado have a key fob for remote locking/unlocking? Make sure the battery in the fob is good. A weak fob battery *might* interfere with the anti-theft system's recognition of the key. Try locking and unlocking the car with the fob to see if it responds reliably.
3. Passlock System (The Usual Suspect): The 1991 Eldorado likely has a Passlock system. This system uses a resistor in the ignition key to communicate with the car's computer. If the resistor is damaged or the key is not properly programmed, the system won't recognize the key, even if it turns the ignition. This is the most common cause of anti-theft issues in these vehicles.
* Check for obvious key damage: Examine your keys for any signs of damage or wear, particularly on the transponder chip embedded inside.
4. Vehicle Security Module (VSM): The VSM is the brain of the anti-theft system. It could have failed or malfunctioned. Unfortunately, this usually requires professional diagnostics and potentially replacement.
5. Other Electrical Issues: While less likely, a problem with other electrical components (like a bad ignition switch or wiring) could interfere with the anti-theft system. This is more likely if you've experienced other electrical issues.
What to Do Now:
1. Try a different (fully charged) battery: This is the easiest and cheapest first step.
2. Professional Diagnosis: If the battery is good, and you've tried both keys, your best bet is to take the car to a mechanic, preferably one experienced with older Cadillacs or those specializing in automotive security systems. They'll have the tools and knowledge to properly diagnose the problem. They might be able to bypass the system temporarily for testing, or reprogram keys if needed. A code reader that can communicate with the VSM is essential.
Important Considerations:
* Towing: Do NOT attempt to tow the car unless absolutely necessary and you understand the implications for the anti-theft system (some systems trigger when they detect unusual movement or activity).
* DIY Repair (Not Recommended): Attempting to bypass or disable the anti-theft system yourself is risky. You could damage the car's electrical system or even create a situation where the car is more vulnerable to theft.
A professional is your best bet for a safe and efficient solution. The cost of professional diagnosis will be far less than the potential cost of damaging the car trying to fix it yourself.
