1. Fuses: This is the easiest and cheapest place to start. Your owner's manual will show you the fuse box location and diagram. Look for fuses specifically labeled for headlights (often "headlamp," "low beam," or a similar designation). Check for blown fuses (broken filaments). Replace any blown fuses with the correct amperage. Important: If a fuse blows immediately after replacement, you have a short circuit somewhere in the system and need to proceed cautiously.
2. Bulbs: The low beam bulbs themselves could be burned out. Access the headlights (usually from behind the front bumper or through the engine bay) and inspect the bulbs. Look for a broken filament inside the bulb. Replace with the correct type and wattage bulb. (Check your owner's manual for specifications.)
3. Headlight Switch: The switch itself could be faulty. It might be internally broken or simply stuck in the "off" position. Testing this requires some electrical knowledge or a multimeter. A simple visual inspection might show obvious damage.
4. Wiring and Connectors: Look for any loose, corroded, or damaged wiring in the headlight circuit. This includes the wires running from the battery, through the fuse box, to the switch, and finally to the headlights. Pay close attention to the connectors at each point. Corrosion is a common problem, especially in older cars. Clean and tighten connections as needed.
5. Headlight Relay (if applicable): Some vehicles use relays to control the headlights. These are electrical switches that protect the headlight switch from high current. If your car has a headlight relay, it might be faulty. Relays are relatively inexpensive to replace. Check your owner's manual or online resources to identify if your car uses a relay and its location.
6. Multi-Function Switch (Turn Signal/Wiper Switch): On some vehicles, the headlight switch is integrated into the multi-function switch. A failure in this switch can affect headlights as well as turn signals and wipers. This is a more complex component to test and replace.
7. Ground Connection: A poor ground connection can prevent the headlights from working. This involves tracing the ground wire from the headlight housing to a suitable ground point on the car's chassis. A poor connection will need cleaning or repair.
8. Dimming Switch: Some vehicles have a headlight dimmer switch (for low/high beam). If the low beam is out but the high beam works, the problem could be this switch.
Troubleshooting Tips:
* Start with the simplest checks: fuses and bulbs first.
* Use a multimeter (if you have one): A multimeter can help you test voltage at various points in the circuit to isolate the problem.
* Consult a wiring diagram: A wiring diagram specific to your 1990 Lincoln Town Car will be very helpful in tracing the circuit. You can often find these online (e.g., Haynes manuals).
* Safety First: Disconnect the negative battery terminal before working on any electrical components to prevent accidental shocks.
If you are uncomfortable working with car electrical systems, it's best to take your car to a qualified mechanic for diagnosis and repair. Attempting repairs without proper knowledge can potentially cause further damage.
