* Headlight Switch: Even though you replaced the dimmer switch, the main headlight switch is a separate component. It's possible this switch has a fault specifically affecting the low beam circuit. The low and high beam circuits are separate within the switch. A faulty contact or internal short could be the culprit. This is a common point of failure.
* Wiring Harness: Check the wiring harness between the headlight switch, the fuse box, and the headlights themselves. Look for:
* Broken wires: These are especially likely in older cars due to age and vibration. Inspect the wires closely, especially near connectors.
* Corrosion: Check connectors for corrosion or loose connections. Clean them with electrical contact cleaner.
* Damaged insulation: Look for any chafing or damage to the insulation that could cause a short or open circuit.
* Low Beam Relay (If Equipped): Some vehicles use relays to switch the headlight circuits. If your Roadmaster has a separate relay for the low beams, it could have failed. You'll need to consult your wiring diagram to find out if this is the case and locate the relay.
* Ground Issues: A poor ground connection can affect the low beams specifically. Check the ground points at the headlights and the headlight switch. Clean and tighten them to ensure a good electrical connection.
Troubleshooting Steps:
1. Wiring Diagram: Obtain a wiring diagram for your 1992 Buick Roadmaster. This is crucial for tracing the low beam circuit and identifying components like relays. You can find these online (often via forums dedicated to that car model) or at a parts store.
2. Visual Inspection: Thoroughly inspect all wiring related to the low beam circuit, paying close attention to areas where wires might be subjected to flexing or abrasion.
3. Continuity Test: Use a multimeter to check for continuity in the low beam circuit. Start at the headlight switch and trace the wires to the headlights, checking for breaks or high resistance in the circuit. The wiring diagram will guide you.
4. Voltage Test: With the headlights switched on (low beams), use a multimeter to check for voltage at the headlights themselves. If you're getting no voltage, the problem is upstream (before the headlights). If you have voltage, the problem is likely within the headlights (although you've checked the filaments).
5. Relay Check (If Applicable): If a low beam relay exists, test it for proper operation using a multimeter or by swapping it with a known good relay (if possible).
Replacing the headlight switch is likely the next step if the above checks don't reveal a problem. Remember safety first when working with car electrical systems—disconnect the battery's negative terminal before starting any work.
