I've been working thru trans issues with one of my w124 cars. The cable you might be referring to is sometimes called the Bowden cable, or kick down cable. It is attached on one end to the throttle linkage, the other end actuated a little lift lever in the trans. When you press hard on the throttle, this causes the trans to downshift into a lower gear.

There are several really good posts here about the vacuum systems which affect the trans shifting. I'm not able to post links to them on my iPad, otherwise I'd help you a little more.

Good luck

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1987 300TD named Klause
1987 300TD named Spouse
1987 300D named Seabiscuit
1980 300TD names Chloie (now sadly gone)