I'd have to recommend a conversion to R134a refrigerant for this car, along with fixing whatever leak there is.
The factory kit for the 740s is only about $70, and it's a straightforward installation. A Variable Orifice Valve is supposed to help efficiency- see the 700/900 FAQ for lots more on A/C.

You're likely to need 1 or more major components- a compressor, condensor, or a hose (all of which run $100-300 for the part). After it's mechanically sound, an A/C shop can pull a vacuum to evacuate the system then recharge. Hopefully all will go well, but I'd realistically expect it to take two tries- that's just the way old A/C systems always are. The shop probably sticks a big disclaimer on every job, and there's a good precedent for it.

Realistic total price for the work is $600-800. If it gets done for less, consider yourself lucky.
--
Rob Bareiss, New London CT ::: '87 244DL/M47- 228K, 88 744GLE- 212K, 91 244 183K-SOLD! Also responsible for the care and feeding of: 88 745GLE, 231K, 88 244GL, 146K, 87 244DL, 239K, 88 245DL, 246K

Agree. 1st check the freon level. There is a low pressure switch that prevents the compressor from engaging if the freon is low because the freon itself is part of the cooling system for the compressor.

Have the system pressure checked.

You can not get R-12 off the shelf anymore. Auto HVAC shops carry it. You need a liscense to buy it and its installation and removal requires special government mandated equipment. Expect to pay around $70 - $80 per pound evac'd and installed. A reputable HVAC person will only charge you the net added.

If the freon level is good, bypass (jump) the A/C control panel and low pressure switch to see if the compressor engages. If the compressor engages, that's a good thing as you have freon and a working compressor.

Next, just bypass the A/C control panel. If the compressor engages, your low pressure switch is good, and in that you have checked the fuses the likely suspect is the circuit board that controls the A/C. The solder drys out and cracks over time. There are great posts in the FAQ as to how to resolder. I am solder impaired and did it. If you go that route, I would highly recommend a lighted hobbyists freestanding magnifying glass as holding a flashlight in your teeth a soldering iron in one hand and a magnifying glass in the other is damn awkward and makes for degrading statements from your teenagers...DAMHIK!!

If you decide to just relace the board there is a place out of Texas. I can not for the life of me find it on the web anymore but they have been in business for years. They provide pretty much all the remanuactured boards for Volvo's in the country. Before I soldered mine two years ago, the price was $175 net. Huge core charge immediately refunded upon receipt of your board, or you could send the board direct and not incur the core charge.

I will see if I have the company name at home.

HTH
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D.P. Mulvey in Central VA. 1991 744 NA, 146K Miles

You will get more sophisticated answers, but the first thing to check is the freon level. The highest probabilty is that there is a leak. If it is small you may need to refill every season. If you have a R12 system, you can't buy R12 anymore, so you may want to convert to R134.