* Weak or failing alternator: This is the most common cause. The alternator charges the battery and provides power to the electrical system while the engine is running. If it's failing, it might not be able to supply enough power to keep the engine running when a significant load (like headlights or brake lights plus the engine's needs) is added. The engine stalls because the voltage drops too low.
* Bad battery: A weak or failing battery can't hold a sufficient charge. When the extra load of the headlights or brakes is added, the voltage drops below what the engine needs to run, causing a stall. This is less likely if the problem *only* happens when the extra load is applied, as a completely dead battery would prevent the engine from starting.
* Poor battery connections: Corroded or loose battery cables or terminals create resistance and reduce the amount of power available to the system. This is easier to check than alternator or battery issues.
* Faulty voltage regulator: The voltage regulator controls the output of the alternator. A malfunctioning regulator can lead to overcharging or undercharging, ultimately causing the engine to stall under load.
* Wiring issues: A short circuit or faulty wiring somewhere in the system could cause a sudden drop in voltage when extra power is demanded. This is less common but possible.
* Ignition switch problems: Although less likely, a failing ignition switch might not be able to handle the added electrical load, leading to a stall.
Troubleshooting Steps:
1. Check the battery connections: Clean and tighten the battery terminals. Corrosion is a common problem.
2. Test the battery: Have the battery tested at an auto parts store to check its voltage and cranking amperage.
3. Test the alternator: This requires a multimeter or a load test at an auto parts store. The alternator's output voltage should be within the specified range while the engine is running.
4. Check the voltage regulator: This is often part of the alternator assembly, but sometimes it's a separate component. A mechanic might need to test this.
5. Inspect wiring: Look for any damaged or frayed wires, particularly around the battery, alternator, and ignition switch.
It's recommended to have a qualified mechanic diagnose the problem if you're not comfortable working with car electrical systems. Ignoring this could lead to further damage, stranded vehicle and potential safety hazards.
