exactaly what is a tranny flush?

[spongebob]

im having a cooling line replaced under warranty, and was thinking about having the fluid changed..i drained the pan and added dextron 3 and then found out it should have been 4...anyway would a flush bring me back where i need to be as far as the type of fluid is concerned?

[BozDMAX]

Drain and fill on the Allison will only replace a certain % of the total fluid - maybe 50-70% or so. A proper flush will pump the new fluid through the entire transmission while pumping out all of the old stuff - probably replacing 99% and requiring 22 quarts of fluid.

Search the forums for Allison flush and you will find a lot of info