Over on a Ford Forum there is a fellow who claims he was an automatic transmission engineer from 1988-2007.
https://www.ford-trucks.com/forums/1544279-can-i-install-this-temp-sensor-in-ford-e40d-test-port.html
He advises putting the sensor in the test port.
Now had originally leaned towards installing it in the transmission output line, as I agree - this is the most accurate and accessible place to measure the worst-case temperature of the transmission fluid. And I do agree that the temperatures will be hotter on the output line first before the case/test port catch up.
But I believe that the test port will suffice. In the RV, it's generally long-haul traveling and in short order the transmission itself will match the temperature of the fluid going through it.
The important thing to realize if you use the test port, I think, is that test port temperatures will lag fluid temperatures. So if you start seeing hot temperatures via the test port, realize that actual fluid temperatures are probably running hotter still.
Ultimately I went with the test port because it was super easy and did not require cutting any transmission cooling lines.
I have also ordered a 1/4" Tee and will see about putting it right in the side of the transmission where the output line currently goes, and then threading the output line into the tee and putting a sensor into the side of the tee via a 1/4 - 1/8 adapter, if it will fit. If it works, I might install another temperature gauge for fluid output temperatures and see how the two locations track one another.
Steve
