It is most likely the air gap on the compressor. What year/mileage is your S40?

 

 

 

On a vehicle of that age, the clutch gap is usually the culprit. What happens over time is that the clutch engages and disengages. Each time you wear away a tiny bit of the metal hub. As time goes on, the gap gets larger and larger until you have too much resistance in the clutch coil to close the gap.

The solution is to remove the outer clutch hub, remove the shim, then reinstall. I doubt that you will find anyone who will do this.

As an alternative, if you or anyone in your area cannot do this, a reman compressor will do.

 

Deshimming is super simple and takes minimal tools. On my 2005 I removed the front pax side wheel then removed the plastic inner wheel liner. Then I removed the center clutch face bolt. Next I removed the clutch face carefully to avoid losing the 3 shims. (You don't need to remove the serpentine belt or need to use a hub puller, the clutch face easily comes off) Look at the shims carefully as they are all different thicknesses. Remove the thinnest shim and replace the other two back into the center clutch hub. Now reassemble everything and see if it works correctly. It takes 20-30 minutes going slow.