You write that your wiper blade is not flush ... for 15 out of the 90 degree sweep of the arm? By any chance, do you mean that the wiper blade is not pressing on the glass over those remaining 15 degrees, so that it doesn't clear the rainwater for the portion of the stroke?

If that's the case, it might not at all be a problem of the wiper arm or its tension (i.e., the amount that it presses on the glass).

I've had this problem for as long as I've owned Volvos (decades) until I found a solution this year. My cars are 240's (not your 850), but these cars have a severely curved windshield at the sides (and maybe yours does, too), and despite trying virtually every wiper blade out there (until this past year), the wiper blade (especially the passenger side, because the wiper arm's pivot is farther from that side) would not follow the windshield's curve to "squeegee" off the water -- instead, the blades tip would be off the glass, not in contact at all, at the passenger side.

Does that sound similar to your problem? If so, the solution I found this year might serve you, too. My problem was totally eliminated when I tried a new wiper blade. It's called the Michelin Stealth Ultra (that Ultra is important, because I tried Michelin Stealth blades and they didn't work). The Stealth blade only has one articulation (a joint that allows flexibility) on each end, but the Stealth Ultra has two articulations, the additional one close to the end, which allows the blade to curve more, in effect reaching around the curve, and keeping the blade in contact with the windshield regardless of the curve on the side. With these new (it's a new model that just came out this year) blades, my wipers squeegee the entire surface of the windshield that their arc covers.

Maybe this is what you need.

I Has a similar problem.Changed the blade, but to no avail. A friend took the arm off and used a Dremel tool and ground down the stops at the base of the arm.It worked! The blade sweeps the entire 90 degrees

The same thing happened to our 1995 850 turbo sedan about 8 years ago. I took it to our body shop friend, he used sufficient local 'heat', changed its shape as needed and it has been working wonderfully well ever since. The cost was about $20 and it took minutes. In our case, it did not seem to be accumulated dirt-related at all.

1995 850 turbo sedan, a really great car
2006 S60 2.5T, a really nice car my other half likes very much

You can do three things: 1) Replace the mounting arm (not the wiper arm), 2) stop it at the top of the arch and bend the outer part of the wiper arm after putting a 2x4 support under the lower portion, and 3)grind down the stops with a dremel tool. Number 2 & 3 are just temporary as the bending will continue. The passenger side also bends and that arm will eventually start hitting the hood.

What you should bend is the lower part of the arm where it is larger and close to the bolt down point. Make sure the bolt is on tight. This is a very common 850 complaint, enough so that you should keep a cheap wrench in the glove box.

Klaus
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Proud owner of a 220S. If I had more room, I would have more Volvos.

Short of bending it (which is what I was going to suggest), about the only other thing you can do is replace it. They're about $30 something dollars, and they just get out of shape and the springs wear out after many years.