I cut the metal line going to the cooler....added in some hose to the 2 straight sides of a tee fitting with hose barbs in them. Screwed the sender in the other hole....had to run a gound wire from the metal tee to a good ground because there hose between the tee and the metal line...no signal can make it to the gauge otherwise.
This will tell you the temp of the hottest fluid in the trans as it heads to the cooler...thats what I want to know.
You could get fancy and have another gauge in the return line and see how much of a drop you are getting in the cooler.
Mine rarely exceeds 140 driving at speeds over 50. Less that that and it occasionally will hit 190 in traffic at low speed.
I do have a thermal switch in the line to the cooler....until the fluid gets hot enough to open that switch, it is returned without going to the cooler. Once it gets up to temp, then it goes through. This keeps it from super cold winter air...if there is any water...humidity or condensation...in the trans fluid, one runs the risk of that freezing in the cooler and bursting the thing...I have seen it happen.
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!