I am going to check the FAQ after this post, but wanted to do this while I was thinking about it. My air conditioner has never worked in my 745 Turbo. I am NOT a proficient, well-versed mechanic, but want to learn how to take care of my baby! I have no clue if my car is R12 or 134A, compressor is original.
I had a gentleman try to get the compressor engaging, and he "jumped" something under the hood that made it engage, but still got NO cold air out of vents, and I had PLENTY of pressure in the system, so, plenty of coolant was there. I am looking at replacing all that I can in the near future. Is there a complete walk-through for that process anywhere? I am not afraid to fix something that I have been without for years anyway :) just don't want to WASTE money on something that I don't need or will ruin! Appreciate any help you could give, thanks!

Funny you should bring this topic up. I also have a 91 745 turbo that I have driven for years without the AC, and have been trying to have it fixed for the past two years. Trouble is, no one here will work on it, so I drive to Kelowna (11 hours away) once a year to have it checked out. Past two years I have replaced hoses from the compressor, but still no AC. I am working down the line of things that could be wrong with it.
Mine was switched over to 134A many years ago, but of course the system is now empty, with the leaks I have had.
Where I live (northern BC) I don't really need AC, but it's something that should work, so I am working toward that goal. Might actually have it working in a few more trips, a few more years!!
JD

best of luck with yours. I am going to try to do it all myself.... hahaha, this ought to be REAL fun!!

My 1990 740 AC would not work , not wanting to get involved I took it to an AC shop , they told me that it had R12 and the compressor was suspect. If the compressor turned out to be OK it was going to cost $1200.00 to "Fix" the system , if compressor was bad $1750.00.
So I decided to open the windows a bit more
Good luck with your decision,
Cheers

There is alot of auto ac troubleshooting advice and tips on the internet. Most systems are all the same. You can get the compressor clutch to engage by disconnecting the pressure switch plug on the expansion tank (silver tank on the passenger side) and jumpering the two spade connector plugs. You really need a set of gauges to tell of you've got "plenty of coolant" or the right pressure in the system.

I had a 90 740 that I successfully converted for less than $300, and I have a 91 740 that was successfully converted before I got it. Converted in approx '04, still works, not as fantastically well as it did some years ago but it has been 7 years, and I think a little more juice would do it, I just don't need top perf right now.
I do use PnP parts, so that counts. In the 90 I used an already converted used compressor, the Volvo conversion kit which was quite cheap at the time, and followed the directions in the FAQ. I will do the same thing again on my 92 745, with converted parts all from PnP.
From following many stories it became clear to me that 'plenty of coolant' was not sufficient. It has to be measured and within an oz or so of correct, for the system to work right. Many are in the 34oz range but the exact car specs have to be checked, and don't forget that if you use other parts Those specs may need to be taken into account.

OP has a 91; even though the compressor came on when the pressure switch was jumped, there was no cooling maybe because the controller in the dash wasn't working. These are a problem in that era, I have fixed several, also using the procedure in the FAQ, with resoldering and replacing the little relay on the board. It is not sufficient to reflow the solder IME, it has to be taken out and new solder used.

thank you all for your input. I can safely assume that my system is set with R12 and still needs to be converted. Currently reworking the rear brakes, and when more money comes in, will try to work some magic on cooling system. Have a great evening