Perhaps it is actually OBDII?  ScanGuage claims their product is compatible
with the '94 850 turbo 2.3L - see http://tinyurl.com/y5jjby   They want $170
US for their product, but if it is actually a standard OBDII interface you
can do better. Most auto parts chains in the US now offer free OBDII code
reading, so that would be a good way to tell without putting up money.

Mike

The best reader is already installed.  It's a bit tedious, but:

http://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm

Even though this is for the 700/900, procedures for your early 850 are
the same.

One of the better things about the 94 is that you don't need a code
reader.  Look under the hood, on front of the computer housing.  Two
connector blocks, A and B, with a wire, LED, and button.  
To read the engine codes, plug the wire into A2, and press the button
once.  Watch the flashes (3 times, pause, 2 times, pause, 4 times =
3-1-4.  
http://www.volvospeed.com/850slight.php

Press it again, and you get the next code if there are more than one.
You can also use it to clear the service light.  
One good source of info is here:
http://www.volvospeed.com/maintence.htm

If you plan on keeping the car for a while and maintaining it
yourself, I strongly suggest getting the excellent factory service
manuals, which contain troubleshooting procedures for each code.  
If you have the original paperwork, there should be an order booklet
for the manuals.  (They used to be available via their website, but I
can't find the link right now.)

(Note, the manuals and code data aren't available to owners on new
models, which is why I own an Audi now.)

--
Email reply: please remove one letter from each side of "@"
Spammers are Scammers. Exterminate them.