On my 300D T, when I put it in L, it does not stay in 1st gear but shifts through all gears as if it's in D, same thing if I put it in S. That's not the way it should behave?

What's wrong and what's the remedy? Thanks!

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked