I'm really glade you posted your findings, many think they are safe with regard to transmission temperatures when in fact they are indeed at risk. The most accurate readings are those taken from the transmission cooler lines as it exits the transmission on it's way to the coolers (yes their are two). This is the area that will tell you what temperature the transmission is seeing. I have been researching transmission cooling temperatures, and the effects of additional cooling for almost two years in an effort to combat the problems associated with high load/high power use. The sender in the pan only shows the fluid temps after cooling, which is only half the picture, and very miss leading. Transmission fluid has a set life cycle, at 190 degrees with everything being normal, it should last for 75,000 miles. Unfortunately, transmission fluid is severely affected by temperature spikes, and most all users will not or should not allow so much time to pass before replacement. Transmission fluid has it's useful life cycle reduced by 1/2 for every 10 degrees the fluid goes over 190. That means if the fluid goes to 200 even once, the life cycle is shortened to 37,500 miles. Let it rise to 210 and it goes to 18,700, even worse let it get to 220 and that now drops to 9,375 miles. The problem most have, is they monitor the fluid temps after they have been cooled, which is sort of back wards. I have experimented with 2qt and 4qt transmission pans, extra coolers both front mounted and else ware, synthetic fluid and conventional. Fluid temperatures for our transmissions are very easy to manipulate, they as expected run their coolest at highway speeds, with the TC locked. Under these conditions, temperatures of 140-150 (pan) and 170-180 (line) are very common and very acceptable while both towing and hauling large loads. The problems start as we decrees the speed, city traffic, rush hour conditions, or anything that has us stopping and starting without longer periods of sustained speed. As expected, you will build temperature faster while towing or hauling heavy loads, but most seem to think they are OK while running empty, which is not the case. I did alot of testing with a 16,000Lb trailer, and as you would imagine temperatures are pretty easy to heat up with such a load. But surprisingly enough, they are also fairly easy to heat up while running with-out a load. Running stock coolers, temps over 250 (line) are far more common then most would imagine, while towing. It only takes about 3 miles of stop and go traffic on a 85-90 degree day to obtain. The absolute worst condition for high transmission temps are backing, here because of the design very high temperatures occur in a very short time span.
The use of over sized transmission pans is not a cure, they do hold more fluid which acts as a buffer between the extremes, but they do nothing to cool the fluid. Yes the fined aluminum pans look good and are an improvement over stock, but they did no better then the stock, thin steel pan. If anything they hurt, by giving the user a false sense of security. We need to understand that we expect an awe full lot out of our 40+ year old automotive automatic transmission. Looking on the insides (yes I took it apart) damage from heat is not only apparent, but the major component to failure. I found that heat was generated the most from the TC and the overdrive unit in normal conditions. The good news is with a few minor changes to the cooling system itself, and some driver education the heat can be controlled and or minimized. Frankly, I am amazed that this old slush box performs as well as it does, considering it's ancient design, the fact that it is a gasser trany put behind a diesel and rated to tow as much as it does, and how small most of the inside parts are.
Attention: TDR Forum Junkies 
What gear were you in and how fast were you pulling? Were you in lockup? That temperature is very high in my opinion. Most report that temperature is less that 180 pulling. Unless you are in stop and go traffic and/or unlocked pulling up a large long hill and/or backing your trailer up a hill the stock cooler seems to be adequate to maintain temperature. In OD lock up there is very little heat generated in the transmission. Ken Irwin
So if you won't install an in line temperature sending unit and it is the most accurate temperature, how do you obtain the fluid temperature on an automatic transmission? Your statement doesn't make any sense from an engineering stand point. If the probe doesn't hinder fluid flow through the coolers, then why are you against it? Ken Irwin
