* AC Clutch Relay: This relay acts as a switch, energizing the compressor clutch when the AC system is activated. A failed relay will prevent the compressor from engaging, even if the system is calling for cooling. This is the most common cause.
* Low Refrigerant: While the compressor runs when jumped, a severely low refrigerant charge can cause the system's pressure switches to prevent the compressor from engaging normally. The system may have a leak.
* Pressure Switch(es): The system has pressure switches that monitor the refrigerant level and pressure. If the pressure is too low or too high, these switches will prevent the compressor from turning on to protect the system. A faulty pressure switch can give a false reading.
* Cycling Switch: Some systems have a cycling switch that prevents the compressor from constantly running. A faulty switch could be preventing it from turning on.
* Wiring and Connections: Check all wiring and connections between the AC control system, the relay, and the compressor clutch. Corrosion, loose connections, or damaged wires can interrupt the electrical signal.
* AC Control Module/Switch: A problem with the control module or the AC switch itself could prevent the signal from reaching the relay.
* Compressor Clutch Coil: While less likely since it runs when jumped, the clutch coil itself could have a high resistance or intermittent short.
Troubleshooting Steps:
1. Check the AC Clutch Relay: This is the easiest and most common fix. Locate the relay (often in the underhood fuse box) and try swapping it with a known good relay of the same amperage. If the AC works, you've found the problem.
2. Check Refrigerant Level: Have a qualified technician check the refrigerant level. Low refrigerant is a major problem, and running the compressor without enough refrigerant can damage it.
3. Inspect Wiring and Connections: Carefully inspect all wiring and connectors related to the AC system for corrosion, breaks, or loose connections.
4. Check Pressure Switches: This requires specialized tools and knowledge. A mechanic should check these.
5. Test the Compressor Clutch Coil: This requires a multimeter to measure the resistance of the coil. A value significantly different from the manufacturer's specification indicates a faulty coil.
Important Note: Don't continue jumping the compressor clutch. Doing this bypasses important safety features, and you could damage the compressor or other components if the system has a problem like low refrigerant. Have a qualified mechanic diagnose and repair the issue.
