I have had my red turbobrick for a whole month now, and so far i haven't had any performance-impeding problems, but there are a few little things that don't affect functionality but still don't look/feel right.
Top of my list is an occasional lightbulb warning lamp, most often with left turn indicator, when all bulbs are functioning properly. This problem doesn't appear to be temperature / time / vibration sensitive. It has done its job correctly once, when I had a headlight connector loose, but other than that it comes on with right turn signal or the brake light or both, and stops after a while.
Second is my twitchy speedometer; most of the time it functions as it should, but on 3 separate occasions it has either decided to read random incorrect speeds or it takes a nap. I really don't want it to take a nap or freak out while I'm driving, so I'm wondering what besides the speed sensor (not the problem, because my ABS works beautifully) could cause this problem, and how to fix it.
Third, and (I think) related to the first two is a lack of warning lights coming on when i turn the car to run and/or start it. The fuel warning and seatbelt light come on, as well as the overdrive off light, even though I left overdrive enabled. Sometimes when I click the overdrive button, the lights that should have come on will do so, the overdrive light will stay on, then I click it again and it goes off, and the lights that were supposed to come on in the first place stay on and turn off in their usual order. This happens seemingly at random, and most of the time if I turn the car off and restart it, the problem goes away. Today it didn't until I started driving; the overdrive light was stuck on as I was driving, so I twiddled the button a few times to see which way was on and which was off, and as I was doing this, the bulb got brighter and turned off, then everything acted as is normal. My best educated guess says there's a loose wire or something similar.
Next is my non-functional air conditioning, but I've read a few posts, and I think I can get that nailed down on my own; it's either low on freon, has a lose wire/connection/solder joint, or a bad switch / sensor. My dad found me a schematic, so I have my work cut out for me. I just hope it's only a matter of low freon.
I also find that when I have my ventilation running while I am cruising along or at high throttle, airflow from the vents drops considerably, and comes back when I let off the throttle. I know these vents are vacuum-actuated, and this behavior is somewhat normal, but is there anything I can do to lessen this a bit?
Least of my worries, because my heater works almost too well, is the heated seats; I turn them on, they come on, I hear the alternator load down the engine, then the thermostats click off and the seats don't feel any warmer. Are the thermostats adjusted to just thaw the seats on a cold day, or are they out of adjustment/otherwise not working right, or am I just so warm-blooded that I can't tell the difference?

 

Not 100% but the dash problems sound like a grounding issue. I would try to pull the cluster and make sure the ground wires (brown in Mercedes, not sure in Volvo) are there and tightly connected to the chassis.

 

Your "no air flow from vents on acceleration" (or, basically, anytime you have high load/low vacuum) is most probably a failed vacuum motor. There are several located to the right of your accelerator pedal. The "by the book" fix is to replace it; I'm told that it is a major p.i.t.a. involving hard to reach nuts, standing on your head in various uncomfortable positions, etc. The quick fix, which I went for, is to plug the yellow hose with a glass fuse or an old spark plug. This will reduce the available air to the floor vents somewhat, but it will get you all of the airflow to the dash back, which is what you want most with the A/C. In the winter you can use the dual level position on the airflow **** to get some heat to your feet if you need to. FWIW..

BTW, I'd love to know where your dad found an A/C schematic for a '91 740.
I'm having zero luck with that one...

 

If you have no air flow ONLY during acceleration, it is most likely that the vacum check valve is bad. This check valve maintains the vacum during acceleration and you need vacum in order to keep internal vent flaps open.

It is pretty easy to check. The check valve sits right on top of the intake manifold and is usually black & whit and has a vacum line that runs to the firewall on the passenger side. Just pop it off and check if air flows both ways through it. If it does, then it needs replacement.

Pretty cheap and easy fix if this is the case.

 

okay, I have the speedometer and lightbulb warning light fixed: the lightbulb warning was a loose ground screw in the trunk, and the speedometer problem was a couple of bad capacitors (more details to come).
I just tried to fix my air conditioning problem by using Freeze 12 r12 replacement, and I ran the system while filling it, for about 10 minutes, and the clutch started chattering, slipping, and eventually burning up and making an awful screeching noise just before I cut it off. a few minutes later, I tried turning the compressor by hand, and it moved as easily as it did when the system was neutral. I hope I didnt just destroy something expensive / hard to replace