Vacuum Modulator.

[Phreetogo]

I have an 82 300TD Wagon. It is slow at start-up for the first few seconds. Then the power kicks in. It shifts fine through all the gears except 3-4. It won't shift into 4th unless I let off the gas a little and then in clunks into 4th. Also it will shift through all the gears quickly when going slow and I let off the gas.
Any sugestions as to why this is happening. In some posts they mention adjusting the vacuum modulator. Could this apply to this situation? Where is this located? Thanks, John

[Phreetogo]

Also is there another name for the vacuum modulator? I can't seem to find it on any diagram. thanks. John