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I have encountered similar problems with a 69 142 with a BW35 and an 80 GLE with a
BW55. After dropping the pans, changing the filter, and flushing, the no-shift was still there. I then dropped the pan again, removed the valve body, and disassembled on the bench, finding sticking shift valves in both trannies. I polished the "spools" and their bores with 600 grit emory paper. Cleaned scrupulously and reassembled, making sure the valves would move freely and return with the just the spring pressure. Re-installed and also installed a magnetic in-line filter in the return line from the radiator cooler to the trans. On startup did another flush to to be sure to get rid of all the little nasties. Never had another problem. I admit this is quite involved, but this is what will work if you are up to it. On the other hand, a used trans that works correctly would be a much simpler solution.