Tranny won't shift without letting up
 

My 77 300d will not shift unless I drive to red line, then let off the gas. I fixed a leaky tee, which solved the engine run-on, but still have this shifting problem. Where do I start? Vacuum to the tranny? I've read many threads leasing me to assume a faulty modulator. This is a w123 chassis, 195k miles.
Craig N

I would check to see if the bowden cable is stuck or stretched.

The 1977 300D should have only 1 rod from the transmission that controls the shift. ADJUSTING this rod should help the shift problem, make the rod longer where it connects to the accelerator linkage.

I have a 1981 300d that was shifting late in all gears and my problem was a vaccuum switch on top of the valve cover. There are 3 of them under a black cover. There is a connector to them. The one to the tranny was bad. The part number for mine is 0029975336