* Short Circuit in the AC Compressor Wiring: This is the most common reason. Worn insulation on the wiring harness leading to the compressor can cause a short to ground or a short between wires when the compressor's high current demands kick in. Look for frayed, melted, or chafed wires, especially near the compressor itself and where the wiring harness bends or rubs against other components.
* Faulty AC Compressor Clutch Coil: The clutch coil engages the compressor. If this coil is shorted, it can draw excessive current and blow the fuse. Testing the coil's resistance with a multimeter is necessary to diagnose this. A shorted coil is a common failure point.
* High-Pressure Switch Issue: This switch prevents the compressor from running if the system pressure is too high. A faulty switch (either stuck closed or intermittently failing) could cause the compressor to draw excessive current or run when it shouldn't.
* Low-Pressure Switch Issue: This switch prevents the compressor from running if the refrigerant pressure is too low. A faulty low-pressure switch might be incorrectly signaling that the system has sufficient refrigerant, leading to the compressor running with insufficient lubrication and increased load, drawing more current and blowing the fuse.
* Cycling Switch Problem: Some AC systems have a cycling switch that controls the compressor's on/off operation. If this is faulty, it might cause the compressor to draw excessive current.
* Problem with the AC Control Unit: While less likely, a malfunctioning AC control unit could send incorrect signals, causing excessive current draw.
* Overloaded Circuit: The circuit that powers the AC compressor might also be carrying too many other electrical devices. Checking other components on the same circuit would help rule this out.
Troubleshooting Steps:
1. Check the Fuse: Make sure you're replacing the fuse with the correct amperage. Using a higher amperage fuse is a fire hazard.
2. Inspect the Wiring: Carefully examine all wiring connected to the AC compressor and its clutch coil. Look for any signs of damage.
3. Test the AC Compressor Clutch Coil: Use a multimeter to check the resistance of the clutch coil. Consult a wiring diagram or a repair manual for the correct resistance value. A significantly lower or higher resistance than specified indicates a problem.
4. Check the High and Low Pressure Switches: You'll likely need a manifold gauge set to check the pressure and test the switches, or a multimeter to check the switches electrically.
5. Consult a Wiring Diagram: A wiring diagram for your specific 1998 Volvo S80 is essential for tracing the circuit and identifying the components involved.
Important Note: Working with automotive AC systems involves handling refrigerant, which is harmful if inhaled or exposed to skin. If you're not comfortable working with AC systems, it's best to take your car to a qualified mechanic for diagnosis and repair. Improper handling of refrigerant can result in serious injury or environmental damage.
