* Weak or failing battery: This is the most common cause. An old, corroded, or simply weak battery can't supply enough current to handle the sudden load of the headlights in addition to the engine's needs. The voltage drops below what's required for the engine to run.
* Bad alternator: The alternator is responsible for charging the battery while the engine is running. If it's failing, it may not be producing enough electricity to keep up with the demand, especially when the headlights are switched on. The battery discharges rapidly, causing the engine to stall.
* Faulty wiring: Corrosion, loose connections, or damaged wiring in the charging system (between the battery, alternator, and starter) can cause high resistance and voltage drops. This reduces the amount of power reaching the engine and the headlights. A short circuit could also overload the system.
* Voltage regulator problems (if separate from the alternator): Some older vehicles had a separate voltage regulator. A malfunctioning regulator would cause the alternator to overcharge or undercharge, leading to the same problems as a failing alternator. This is less likely than a bad alternator itself in a vehicle of this age.
* High draw somewhere in the electrical system: While less likely to cause the engine to die *just* from turning on the headlights, an excessively high current draw from a faulty component (like a short in a power window motor or other accessory) could exacerbate an already weak system and cause the engine to stall.
Troubleshooting Steps:
1. Check the battery: Inspect the terminals for corrosion. Have the battery tested at an auto parts store to check its voltage and cranking amps.
2. Check the alternator: Have it tested for output. A simple voltage test across the battery terminals with the engine running will give an indication (should be around 13.5-14.5 volts). A low voltage indicates alternator problems.
3. Inspect wiring: Look for any loose, corroded, or damaged wiring, particularly in the positive and negative battery cables, alternator connections, and starter connections.
4. Start with the simplest things first: Make sure all your lights are working, and then test the headlights individually to see if one bulb is drawing excessively more current than the other.
It's best to have a mechanic or auto electrician diagnose the problem. Trying to troubleshoot a complex electrical system without proper tools and knowledge can be dangerous. A simple voltage test at the battery terminals with the headlights on and off would be a good starting point for professional diagnostics.
