* Alternator: This is the most probable cause. The alternator charges the battery while the engine is running. If the alternator is failing or completely dead, the battery might have enough juice to start the engine, but turning on the lights (which draw significant power) drains the battery beyond its ability to keep the engine running. Testing the alternator's output voltage is crucial.
* Battery: While less likely given the car started, a weak battery could be the problem. The initial start might have used most of its remaining charge, and the lights put it over the edge. Have the battery load tested.
* Battery Cables/Connections: Corroded or loose battery cables or connections can create high resistance, preventing sufficient current flow, especially under load (like the lights). Inspect and clean the terminals thoroughly.
* Wiring Issues: A short circuit somewhere in the wiring harness could cause a sudden voltage drop when the lights are turned on. This is less likely but possible. A thorough visual inspection of the wiring, particularly around the battery and lights, is warranted.
* Starter Solenoid: While less probable given the initial start, a failing starter solenoid could be contributing. It might be able to engage the starter motor for a brief moment (enough for the initial start) but then fail under additional load.
Troubleshooting Steps:
1. Check Battery Terminals: Clean and tighten the battery cable connections. Corrosion is a common problem and can significantly hinder electrical flow.
2. Test Battery Voltage: Use a multimeter to check the battery voltage both with the engine off and then with the engine running. A low voltage reading (below 12.6V) when the engine is off suggests a weak battery. A low or unchanged voltage when the engine is running points to a faulty alternator.
3. Test Alternator Output Voltage: With the engine running, measure the voltage at the battery terminals again. It should be around 13.5-14.5V. Anything significantly lower indicates a problem with the alternator.
4. Visual Inspection: Examine the wiring harness for any obvious damage, fraying, or loose connections.
If you're not comfortable performing these checks, take your Bonneville to a qualified mechanic. Attempting electrical repairs without proper knowledge can be dangerous. The issue is almost certainly electrical, and diagnosing it safely and effectively requires the right tools and expertise.
