J changed the course for the ac 2 days ago, and last night it blew..., why?

 

Either a short to ground or a very high draw across the clutch coil.

 

hi Tony, I appreciate your answer, but I don't understand, not Volvo savvy, ;(

 

These are general electrical terms. A short to ground is another way of saying that the positive 12 volt wire is rubbing against the steel body, creating a short circuit to the grounded car body. That condition will pop a fuse.

The other is the clutch coil on the compressor. They can go bad and draw a very high current, drawing enough power to pop a fuse. They can also become internally short circuited.