* Low Refrigerant: While this might seem unrelated to a clicking sound, low refrigerant can cause the compressor clutch to cycle rapidly and make a clicking or chattering noise. It's crucial to check this first. A qualified mechanic can easily diagnose this with refrigerant pressure gauges.
* Bad Blower Motor Resistor: The resistor controls the fan speed. A failing resistor can cause intermittent operation and clicking. This is a relatively inexpensive part to replace.
* Failing Blower Motor: The most likely culprit is a worn-out or failing rear AC blower motor. The clicking might be the motor trying to engage but failing to spin properly due to worn bearings or a seized rotor. This usually requires motor replacement.
* Wiring/Connectors: Loose, corroded, or damaged wiring or connectors to the blower motor can also cause intermittent operation and clicking. Inspect the wiring harness leading to the motor for any signs of damage.
* Clutch Relay (if applicable): Some systems use a relay to control the compressor clutch. A faulty relay could cause clicking.
* Compressor Clutch (less likely for just a clicking): While a failing compressor clutch can cause clicking, it's usually accompanied by other symptoms like lack of cold air. A clicking sound *only* is less likely to be the compressor clutch.
Troubleshooting Steps:
1. Check Refrigerant Level: This is the easiest and cheapest check. Take it to a mechanic or use a refrigerant gauge if you have the experience.
2. Listen Carefully: Try to pinpoint the exact location of the click. Is it coming directly from the blower motor itself or somewhere else in the system?
3. Check Fuses: Ensure the relevant fuses for the rear AC are not blown.
4. Inspect Wiring: Examine the wiring harness for any signs of damage, corrosion, or loose connections.
5. Access the Blower Motor (if comfortable): This usually involves removing some interior trim panels. Visually inspect the motor for obvious damage. If you're comfortable with automotive repair, you might be able to try manually spinning the motor shaft (after disconnecting power!) to see if it's seized. Caution: Be careful working with electrical components.
Recommendation:
Unless you're experienced with automotive AC systems, it's best to take your 2004 Expedition to a qualified mechanic for diagnosis and repair. Attempting to diagnose and fix this yourself could lead to further damage or injury. The mechanic will have the tools and expertise to accurately pinpoint the problem and recommend the appropriate solution.
