While I was able to find information on testing the ATF temperature sensor in the GF4A-EL manual I've got:
I was unable to find any info on the normal operating temperature of the transmission.
I'm guessing that normal operating temperature would be pretty similar across all ATX's, like it is for engines. IIRC, the highest I saw on mine was about 60*C.
According to this paper:
Normal operating temperature for an automatic transmission is about the same as the engine temperature, i.e., about 195°F. The temperature inside the torque converter, while pulling a big load from a standing start, could easily rise above 350°F.
EDITORY NOTE: I'm seeing this same basic range at many different sites.
The critical thing is to keep ATF temperatures less than the oxidation temperature for the given ATF. For high performance or towing vehicles, a large external radiator helps maintain a moderate temperature.
All transmissions work best with warm ATF, and some are more sensitive to cold than others. For this reason most vehicles pump the ATF through a tube in the bottom of the engine cooling radiator. It serves two purposes, one to warm up the ATF, the other to cool it. While 130F radiator fluid may hardly seem "cool" to a human, it is cooler than hot ATF in severe service conditions.
A chart (with little to no sourcing information) that gives you some idea how hot ATF runs is here: http://v8sho.com/SHO/ATFTempChart.htm
This link gives some idea how ATF's are tested: http://www.intertek.com/automotive/atf/oxidation/
If your ATF fluid gets above 275F, you are at risk of serious damage and should change the fluid ASAP and should pull over and let the transmission cool down. At 300F, you are looking at metal parts warping, seals becoming hardened/damaged, etc.
175F to 200F is considered ideal operating temperature for a transmission (some sources say 160-180), and 150F is considered minimum ideal operating temp.
If you only ever get the temp to 175F, your fluid could last 100k miles in theory; if you run at 195, you're looking at a 50k service interval. If towing, or driving hard, or driving up steep climbs with your torque converted unlocked, or sitting in hot weather in traffic often (things leading to higher temps), you should be changing your fluid more often.
Be aware that in winter, it's possible for it to take several minutes of driving for operating temps to rise to the 150F mark. It's considered ideal to let the vehicle idle until RPM's drop before driving (though newer cars don't always idle high when cold--no advice there other than get a trans temp or scangauge, or just idle for a few minutes when cold).
I've included some further reading. The first I summarized above. The second are the slides of a talk about ATF that includes a lot of valuable information and math and theory about how ATF functions within a transmission, which necessarily includes lots of information about temps. The third is the collected information experienced by a guy who closely monitored his transmission temps under a variety real world conditions carefully, plotted the data, studied it, and gave some conclusions he came to.
http://www.digi-panel.com/trannyoil.htm https://www.bobistheoilguy.com/automatic-transmissions-study/ https://www.thenewx.org/threads/thoughts-and-observations-on-engine-and-auto-trans-cooling.65980/
