* Alternator: The most probable cause. The alternator is responsible for charging the battery and powering the electrical system while the engine is running. If the alternator is failing, it may not be providing enough power to run the AC compressor and the turn signal flasher module simultaneously. The accessories work when the engine is off because they're drawing power directly from the battery, which is still charged.
* Battery: A weak or failing battery might not have enough reserve power to supply both the AC and the turn signals under load when the alternator isn't providing sufficient charging current. Even if the alternator *is* working, a weak battery could struggle to handle the combined current draw.
* Fuses or Wiring: A blown fuse specific to the AC compressor or a short circuit in the wiring supplying power to the AC or turn signals when the engine is running could be the problem. This is less likely to affect *both* the AC and turn signals, but it's possible depending on the wiring configuration.
* Ignition Switch: While less likely, a faulty ignition switch could be failing to properly supply power to the relevant circuits when the engine is running. This would be more likely to affect multiple systems, however.
* Power Control Module (PCM) / Body Control Module (BCM): In modern vehicles, these modules manage power distribution. A fault within these could selectively cut power to certain accessories when the engine is running. This is less common but possible.
Troubleshooting Steps:
1. Check the Battery Voltage: Test the battery voltage both when the engine is off and when it's running. A significant drop in voltage when the engine is running indicates a failing alternator.
2. Check Fuses: Inspect fuses related to the AC compressor and turn signals (consult your owner's manual).
3. Alternator Test: Have the alternator tested by a professional mechanic. This is the most likely culprit.
4. Wiring Inspection: If fuses and the alternator check out, a qualified mechanic should inspect the wiring for shorts or damaged connections.
Don't attempt complex electrical repairs yourself unless you have the necessary skills and safety precautions. This problem could indicate a more serious electrical issue that requires professional attention.
