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So I plan I putting in a transmission temperature guage this year. Which tube do I connect to to get the fluid comming out of the transmission to the cooler, and what is the best way to tap into it. Truck is a 08 F250 w/5.4L.
I'd run the truck for a while, get the tranmission warmed up.
And feel the steel lines. One will be hotter than the other. That's the one you want.
I did this on a Ford Ranger. And it works better, it seemed to me, if the transmission isn't HOT. Then it's too hard to tell which one is cooler because they'll both be almost uncomfortably hot.
If you want to connect a temp gauge, you don't have to worry about which line is which. Connect the sensor directly to the tranny. Driver's side of tranny... upper side. There's a plug... remove cap, install the sensor. Use some plumber's putty or better, pipe thread tape, DONE!
I would install in the trans, but plumbers tape is an insulator, and most of these senders need to ground through the steel lines. Use copper thread sealant, like this Just For Copper Thread Sealant
The fitting in the lines can leak just as easily, if not more so that in the trans itself. The readings in the trans will still be more accurate than the temperature of the lines. The fluid leaving the lines will read the hot fluid as it exists the torque converter and will read much hotter than the actual trans temp. What you want to make sure of is that the heat is dissipated and that the trans as a whole is not getting too hot.
Teflon is an insulator, yes, but after instalation there will be continuity as the tips of the threads cut through the teflon and there will be sufficient metal to metal contact for sensors to be properly grounded. At least there have been in the few dozen I've installed. I use 2 to 3 wraps of standard thin tape.
The port in the trans measures right as fluid leaves TC, the peak temperature. Usually works out to be about the same as the outlet pipe to cooler.
I did use the teflon tape AND I did use the electric sensor AND I had no problem getting the ground to "connect". mwsF250 is correct.... the threads do cut thru the tape and make the ground.