91' 300e hardshift
 

* First I would check the vacuum available at the vacuum modulator (on the left side of the trans). For a thorough check, you can watch the vacuum as the cold idling engine warms to operating temperature. If there's not manifold vacuum available at the modulating valve, find the vacuum leak (broken/disconnected hose, etc.)
* If , however, the vacuum available is the same as manifold vacuum you need to check modulating pressure. For that you'll need a trans pressure gauge and modulating pressure specs for your transmission (note the test conditions which need to be met before taking a pressure reading). The plug (12mm hex head) right next to the modulating valve is where you hook the pressure gauge. You'll need a gauge with a hose long enough so that you can read the gauge while driving.
* If the modulating pressure is correct, that takes care of a lot of variables and the problem is internal. If the modulating pressure isn't correct, then it's on to checking some other (external) things.
* If you know of an independent shop that you trust (won't try to sell you a transmission regardless of what the problem is), it might be more sensible to pass on diagnosing this one yourself.

I know you have already done it... but check the fluid level again at a warm temp on a level surface.

this happened to me all of a sudden in the 260e... and i thought i was losing the tranny.

just a low level... quart low