If yours is like my '81 300SD, remember that the transmission needs vac only for downshifts. The vehicle is designed to have about 12inHg of vac at the VCV, but as you apply the throttle, the vac almost immediately drops to zero. Yes, it is theoretically a gradual release but if you hit the accelerator it goes to zero in about one second.


In fact you can drive with the line unhooked to the trans modulator and you will get smooth upshifts and clunky downshifts.

What exactly is your problem? What are the symptoms and why are you thinking vac modulator?