Don't know if this will help or not as your problem sounds more severe:

About 1 moth ago I lost my 1st Gear after the Trans would warm up. It would not even shift into 1st when I moved the Shift Lever to Low.

Everthing pointed to the B2 Piston which I hesitated to replace as I wanted to take a peak at it first befor buying a new one or hunting up a used one. (We were still using the Car by manually moving the Shift Lever to 2nd and then to Drive; no Hills in the areas we drive.)

In the Mean time I changed my Transmission Filter. Now when I put it in Low with the Shift Lever low gear works.
It also appears to be working in Drive not but I have not had time to get the trans really hot to see if that will cause the problem again.

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84 300D, 82 Volvo 244Gl Diesel