I just replaced the tranny in my 81 300SD and I believe the transmissions work the same way on both. Here is the skinny for the SD.

The system produces about 21inches of vac. There is a proportioning valve at the back of the fuel injection system which regulates the vac going to the transmission. The transmission must upshift with zero vacuum. The system is designed so that the vac bleeds off instantly when you open the throttle. Downshift is different - it needs vac to downshift.

The vac modulator is on the transmission. It adjusts how sharp or soft the shifts are. Gently remove the white cap (old caps are black) and turn the tee - just a little, like a quarter turn - CCW for softer shifts - but this will eventually lead to flares - or CW for sharper shifts.

The proportioning valve is attached to your throttle linkage. If you pull the linkage to WOT you will see the valve rotate toward a stop. The gap between the stop and the lever at WOT should be 0.5mm. You can just pop the lever off the knuckle on the engine, adjust it and put it back. I broke the plastic piece that holds the lever to the proportioning valve. Be careful, it pops open and slips down and can be reused. The new one cost $5 on this forum but $11 at the stealership.

Use a mityvac to see how much vac the proportioning valve is giving. Mine over the years had dropped to 9inches. It should be between 10 and 15 inches. And it should drop immediately to 0 inches when you open the throttle.

Adjust the proportioning valve by removing the plastic dome from the driver's side. It is in with just a press fit. You can carefully remove it by wiggling with a set of needle nose pliers. The dome extends about 3/4 inch into the proportioning valve. When the dome comes out just move the bottom screw slightly; if I remember correctly, CCW gives more vacuum. Replace the dome, start the engine, and test the vac with the mityvac. I reset mine to 15inches.

I do not know what are your symptoms. Mine were a harsh upshift and a bad clunk when it downshifted. I got it just about perfect using the procedure above. The indy that installed the used transmission had no idea what to do to smooth it out and in fact installed two used transmissions before I took the car home and adjusted it.

One way to figure out the problem is to disconnect the vac line to the transmission and drive around. If it upshifts OK with the line unattached but you are getting a harsh upshift with the line attached, then the system is not bleeding down the vac properly. If it does not upshift properly with the vac line unattached, you can try slight adjustments to the vac modulator. Once you get smooth upshifts, then do the opposite. Drive around with the mityvac hooked up to the tranny (disattached from proportioning valve) and inside the cabin. After it upshifts, pump up the vacuum and come to a full stop. It should downshift smoothly with no clunk under vacuum. That's what I found with mine. So I knew the used tranny was OK and that vac adjustment was the only problem. By the way on the SD the vac modulator does not select the shift point for the transmission; that is done by the bowden cable. My shift points were fine so I did not fool with the bowden cable.


Hope this helps. Again this is for a SD but I think it applies to your car. I knew absolutely nothing about this before seeking help from the forum - especially from Kerry and jt20. But there is a wealth of info. All my experiences and their advice can be read at my thread which is called something like "81 SD tranny goes clunk."