My 1985 300TD wagon, besides normal sluggishness, almost refuses to downshift on it's own. It seems that sometimes, even from a stop, it never goes down to 1st gear. I can shift it manually, and get it where it needs to go, but even pressing the pedal to the floor seldom will get it to downshift. 308K on the odometer, and it runs great except for this issue. Any thoughts?

 

If the vacuum system checks out OK, check the vacuum line running down to the modulator valve on the drivers side of the transmission. If this vacuum line is OK ,then the diaphram in the modulator valve itself may be ruptured. The valve is not too expensive and easy to change. Good Luck Dusty

 

Here is a article on adjusting the transmission:

http://www.continentalimports.com/ser_ic20242.html

 

I have a 87 300td and mine did something similar, If I slowed down to a stop and then accelerated it would slam into 1st and then operate as usual, I replaced the transmission Vacuum Dashpot and it seemed to clear up considerably, very cheap and very easy.