Why does a 98 Volvo s70 blow the fan motor fuse?

If there is nothing else on the circuit it's a good indicator that the fan motor is failing and probably has worn bearings. Worn bearings will cause an electric motor to pull more current and often blow a fuse. ==Additional Input== The purpose of a fuse it to protect the wiring circuit which it serves from excess current flow, which could severely damage the circuit conductors [wires], OR WORSE, CAUSE A FIRE!!! When a fuse "blows," or burns out, it indicates that there was an OVERLOAD on that circuit, and it did it's job by opening up the circuit. A blown fuse should NOT BE replaced, UNTIL someone has troubleshot the circuit, identified the cause of the overload, and corrected the defect. There are three most probable causes for fuse fuse failure: [1] A short circuit between the "hot" supply wire and ground [bare or cut wire in contact with a bare and sharp metal edge], [2] A defective fuse [which is highly unlikely] [3] Something which causes a "drag" on the blower motor which causes it to draw more current[amps], so much extra, that it excedes the current [amps] rating of the fuse. When the fuse is unable to carry the excess current, it fails by blowing. There are many things which could cause the drag: [1] As described by the previous answer, worn bearings would slow the motor and fan rotation, or [2] A buildup of trash [leaves, candy wrappers, etc.] in the plenum chamber around and in contact with the squirrel cage blower blades]. There may be another, or two, causes I can't think of right now. To fix this problem, you must determine what is causing the excess current demand, and remove/correct the defect. This could include: [1] Repair of wiring involved in a short circuit, [2] Clean out of debris blocking/dragging the fan, [3] Or as a last resort, replacement of the fan motor itself. Hope this helps you to understand and correct the problem. Good luck.