* Weak or failing alternator: This is the top suspect. The AC compressor draws significant power. If your alternator is weak, it can't keep up with the demand, causing the voltage to drop too low for the engine to run. When you turn off the AC, the load decreases, and the alternator can maintain sufficient voltage. Have the alternator tested for output at various engine speeds, especially with the AC on.
* Bad battery: A weak or failing battery might not have the reserve capacity to handle the extra load of the AC compressor. This would show similar symptoms to a failing alternator. Have the battery load tested.
* Wiring issues: Corrosion, loose connections, or damaged wiring in the charging system (between the battery, alternator, and starter) can increase resistance and lead to voltage drops, especially under heavy load. A visual inspection of all wiring and connections is necessary.
* AC compressor clutch: While less likely to cause a complete stall, a problem with the AC compressor clutch (the part that engages the compressor) could cause excessive load. A faulty clutch could draw excessive current.
* Poor ground connections: A poor ground connection can disrupt the electrical system, leading to erratic behavior. Check all ground straps and connections for corrosion or looseness.
* ECM (Engine Control Module) or related sensor issues: While less probable, a failing component in the engine control system could be misinterpreting the increased load from the AC as a critical fault condition. Diagnosing this would require more advanced diagnostic tools.
Troubleshooting Steps:
1. Visual Inspection: Start by visually inspecting all wiring and connections related to the battery, alternator, starter, and AC compressor. Look for corrosion, loose connections, or damaged wires.
2. Battery and Alternator Test: Have your battery and alternator tested at an auto parts store or mechanic. This is the most important step.
3. Check Ground Connections: Clean and tighten all ground connections, paying special attention to those near the engine and battery.
4. Check AC Compressor Clutch: Listen for unusual noises from the AC compressor clutch when it engages.
If you're not comfortable performing these checks yourself, take your Cadillac to a qualified mechanic specializing in older vehicles. They will have the diagnostic equipment to identify the exact cause of the problem more efficiently. Describing the symptoms precisely – that it only happens with the AC on and restarts immediately after turning the AC off – will be very helpful to them.
