I HAVE A 1994 850 TURBO THAT WON'T BACK. HOW DO I SOLVE THIS PROB

If diesel, it could be dodgy glow plug(s). Had a similar problem with my s40 and new glow plugs solved it!

you can try some fluid but changing it will just cost you money,fill it first,but if the old fluid is burnt the trans is prob toast

Sounds like a blockage in the exhaust system. A quick fix at a local garage

Get a proper diagnostics done as it seems like one of the engine sensors needs to be replaced.

The seals inside your transmission are bad and leaking by and they don't start to seal until the tranny fluid warms up and swells the seals. To temporarily fix this problem you can pour some brake fluid into your transmission fluid. This will swell the seal in the transmission and it should start working again. This is only a temporary fix however, eventually you will ned to replace or rebuild your tranny. I used this method on a turbo trans 350 and got a year of service out of it that way before I had to replace the tranny. Just do not put too much brake fluid into the tranny, I would recommend one or two capfuls to start with.

There is a leaking coolant hose, or a perforation of the Heater Matrix.Often a temporary quick fix is to shut off the incoming Hot Coolant and crimp the return to engine hose with a Long Nose snap lock.
Regards,DT.

its prob the cooling fan its located in a housing along with the radiator fan at the front of your engine bay look down when you open the hood . and make sure the fan is spinning run the ac on high and wait ( do this while the car is running of course ) . do not run your ac if the fan isnt working . you will burn up the compressor . the noise and vibrations you feel are from the compressor turning on and off.
now if the fan is working its prob the compressor. if all else fails take it to a mech where they can test it ... its not pricey and its quick

the situation you describe is an air pocket. you need to raise the front of the vehicle turn the heat on full blast. open the radiator cap, while cold of course, and add coolant until you see not more air bubbles coming out of the filler neck.

Sounds like a connecting rod bearing problem. I would have a pro. listen to it.

I own the same 3500 GMC 6.5 diesel truck with the same problem. We replaced the brain under the upper turbo hood and it fixed it pronto. No more shutoffs