I had read lots of posts on the internet that debated where to put the transmission temperature sensor. Lots of debate and lots of incorrect information. Ran across more than one post that suggested putting it on the fluid return line, which is going to give you the cooled-off fluid temp as it comes back from the fluid radiator, which is useless! Likewise the pan is where the return fluid collects before being sucked up back into the transmission to be heated up again, so it also is not a very good place to find the max temp of your transmission.
Finally had input from a fellow who claimed to be a Ford transmission engineer. He said the test port was fine.
The test port is a dead-end port, so you won't have fluid passing by it. However, the aluminum transmission conducts heat well, so the test port will accurately reflect the overall temperature of the transmission. But, even being a dead-end port, as you can easily see when starting the engine from cold it does not take long for the sensor to start noticing temperature increases and come up to operating temperature. Likewise, downshifting on hills quickly shows up on the test port sensor also.
A port in the outgoing fluid line will give you the transmission fluid maximum temperature, and one would expect it to more rapidly track changes in fluid temperature than the one in the test port, which would lag behind. But the test port sensor does not lag too badly, as I noted above.
I went with the test port because it was super-easy to install. Remove the port plug, screw in the sensor, and you're done. Tapping into the fluid outlet port requires a T fitting and making sure you don't cause a restriction.
Steve
