The first thing to do is to test the flow. Remove the cooler line from the rear connection on the trans. Have two containers, each at least a gallon. Put the line in one container and start the engine. When the flow is steady (should only be a couple seconds) move the line to the second container for exactly 15 seconds, then back to the first container and shut the engine off.
If fluid sprayed out of the trans where you removed the line, STOP immediately! This is a failure. If there is at least a quart in the second container it passes the flow test.
If it fails for either spraying out of the trans or not a quart in the second container, either the bypass has failed or the cooling system is restricted. A rebuild kit for the bypass is somewhere around $25, and really easy to install.