my 85 does it errrr, check connections, tps, afm, clean fluid. ive dont all that and still cant find the issue. i believe it is electrical in nature.
My mechanic replaced the cam position sensor on my 1983 Euro-spec 635csi and fixed the downshift issue.
Make sure you have ENOUGH clean tranny fluid. You might also check the electrical connector on the transmission. Mine had a bit of corrosion and the tranny went into the default mode, preventing the shift into 4th gear.
