I don't know if there's any "official" specification on this. I think the theory for the pan is that tells you the fluid temp entering the trans. I like pulling the temps before it hits the coolers because to me that's more representative of the operating temperature.
I tend to agree. I don’t think there’s one perfect location for the temp probe, and each has a valid reason for selection.
Pre-cooler will tend to be higher, but is probably the most accurate read of demand on the fluid itself. I don’t like using the return line since it a lagging indicator of what’s going on inside the transmission and of demand on the fluid. I have come to prefer putting the probe at the back of the pan, where I get a good average of demand on the trans as well as demand on the fluid.
My fluid runs to the external cooler, then back through the low pressure side of the radiator before returning to the trans. Cruising, the ATF doesn’t get above 180/190, and is routinely lower on long flat sections where the TC is locked, indicating that the ATF is actually absorbing some heat from the coolant in the radiator. On a long grade, 1:1 3rd, TC unlocked, my temps will move steadily up to 220 and sit there. The engine fan is usually engaged at that point, too.
With the probe at the back of the pan, my coolant and ATF temps move somewhat in parallel, with the coolant being just a bit quicker to register the immediate demands of the hill climb and the ATF holding on to that heat until back locked up and cruising. All this, and I have a 1700 stall converter.
Echoing
@mrk5, I don’t think there’s a wrong location for the temp probe (except maybe outside the fluid path), but it’s most important that you’re familiar with where temps should be at a given load/speed/rpm. A 4L80E, with its lockup and control features will change the game for sure.
One other thought - Dex6 can take repeated beatings and shrug it off. When
@Spork929 and I ran Pikes Peak, everything was hot and stayed hot - no boiling or burning; just operating at the very top of the normal range before you might consider pulling over. That same fluid is still bright red and running in the suburban. I don’t see old school Dex3 performing the same.
David