* AC Compressor Clutch Relay: This is the most probable cause. Even with a good low-pressure switch, a faulty relay prevents the compressor clutch from engaging. The compressor won't run, even if the system is otherwise functional, and the fuses won't necessarily blow. This is a cheap and easy part to replace.
* AC Compressor Clutch Coil: The relay might be fine, but the coil within the compressor clutch itself could be burned out or shorted. This would prevent the clutch from engaging, leading to the same symptoms. Testing the coil's resistance with a multimeter is necessary to diagnose this.
* Wiring Issues: Look for broken, frayed, or corroded wiring between the AC compressor, the relay, and the low-pressure switch. A short circuit in this wiring could blow fuses or prevent the compressor from receiving power. Careful visual inspection is crucial, along with possibly using a multimeter to check continuity.
* Faulty AC Pressure Switch (despite replacement): While you replaced the low-pressure switch, there's a *possibility* the replacement is defective or that there's another pressure switch in the system (high-pressure switch) that's malfunctioning. A bad high-pressure switch could also prevent the compressor from engaging as a safety precaution.
* Refrigerant Level (despite "full" light): Although you say the refrigerant light is on, it's *possible* the system is still low on refrigerant or has a leak. While a low refrigerant level *usually* results in a flashing light, it's worth confirming the system's charge with a qualified technician using gauges. A faulty sensor could also be falsely indicating a full system.
* Other Electrical Problems: A problem with the AC control unit itself, a faulty ground connection somewhere in the AC circuit, or a problem with the main AC power supply could also be responsible. These are less likely given the other symptoms but should be considered if the above steps fail.
Troubleshooting Steps:
1. Check the AC Compressor Clutch Relay: Replace it with a known good one. This is the easiest and cheapest first step.
2. Visually Inspect Wiring: Carefully examine the wiring harness going to the compressor, relay, and pressure switches. Look for damage or corrosion.
3. Test the AC Compressor Clutch Coil: Use a multimeter to test the resistance of the clutch coil. A manual for your car will provide the correct resistance value.
4. Check the High-Pressure Switch (if applicable): Consult a wiring diagram for your specific model to locate and check the high-pressure switch.
5. Professional Diagnosis (If Necessary): If you're not comfortable with electrical diagnostics or if the above steps don't solve the problem, take the car to a qualified auto AC technician. They have the tools and expertise to diagnose and repair complex AC system issues. They can also accurately check refrigerant levels and identify leaks.
Remember safety first when working with automotive electrical systems. Disconnect the battery negative terminal before working on any wiring to prevent shorts and injury.
