I have a 98 v70 non-turbo. Noticed the ac clicking on and off when driving and yesterday it stopped clicking on at all. I googled around and some people say that it just needs a charge. Got a 12 oz can of r134a and charged it. Had to jump the low pressure switch to get it to pull the r134a in.

After the charge, the ac then would cycle on and off every second or 2 when the car was not moving as was. I guessed that it needed another can? I don't think I emptied the can completely. Fog came out after i popped it and it felt like there was some liquid in it. I finished late so I decided to get a second 12 oz can in the AM.

When I shut off the car, I could hear a hissing noise coming from the air vents. I didn't listen at all the vents, but I could hear it coming out of the dash vent on the far passenger side.

I went to bed and when I tried turning on the ac in the AM, the compressor clutch would not engage.

I'm guessing that something under the dash has a major leak? is it common to hear a hissing noise after shutdown? Should I waste money getting another can or should I get a can with dye or sealant in it?

Got the car about 3 months and this is my first real fix ac has been blowing cold the whole time. Upgraded from a 94 945 turbo. So far things actually seem to be easier to work on than a 940. Cool beans.

 

Don't put sealant in. Probably have a leaking evaporator. Very common

 

Evaporator sounds like a great guess

They are located, well, inside the dashboard and it is a long process to pull it all apart to replace. Online write-ups and tutorials are out there.

Also, this part commonly fails on the era car you drive.

So many of the ones I check out for purchase have hot AC.
If there is no history of the evaporator core being replaced or if there is only a sticker under the hood showing dye has been added to the refrigerant and the air is still hot, you can safely bet that it is bad.
People seem to trade them instead of spending that kind of money at a shop to fix the air.

 

I've replaced more evaporators from Volvos of that vintage than I can count. I would not even do any more checking but spend time changing that evaporator.

Even for an experienced person, this is a lot of work. Plan on spending a weekend doing this yourself, or more. Very doable.