A little bit of temp testing-maybe just muddying up the waters.

So, a local friend of mine recently put his truck back together. Basic rundown:
'78 F-150 longbox 4x4 chassis, all steel and stock looking. Heavy
557" A-head
C6 w/UCC converter for built for competitive pulling and Broader brake VB
Tube and fin style cooler in bed (approx 120 sq in) with 10" mounted fan
-8 cooler lines to and from cooler
1 electric half sweep trans temp gauge, sensor mounted in output (pre-cooler) fitting

Once he got it all back together and went for a ride, he called saying the trans get REAL hot fast. Not really thinking about where he mounted the sensor right away, I said I'd be over to check it out. We took a ride with a high quality (at least better the mine) infrared gun. Cruising after 5 min showed about 180.....the he swatted it a few times and watched the gauge climb to 250 within 10 seconds. We stopped quickly and started taking readings all through the system. In the end, gun would show;
200-210 at the fitting/sensor
200-210 at the cooler inlet
160-170 at the cooler outlet
170-180 at the return fittin in the case
180-185 on the steel pan
Also, over the course of 5-ish minutes of idle/park time, outlet temp (on the gauge and the infrared) would drop more rapidly than return/inlet temp. Also the pan remained steady about 180
Basically, the cooler was dropping temp in the 30-50* range, although skewed #'s from the gauge. Now I understand the infrared only shows surface temp, but does anyone know or have insight how much the variance between fluid temp and the surfaces?
Sorry for the novel, just thought this may be handy to others thinking about where to put a sender and what effects location may have on readings
In the end, we are going to leave the system as it is and keep monitoring, possibly adding one more sending unit in the pan or return line.

The temp gun I use will give a false reading on any reflective surface such as metal. Looks like the cooler is doing it's job though. I'd say the only temp. that matters is the fluid temp. Also you mentioned he is putting the truck back together. Is it getting hotter now than before?

Sensor is in a different location, so I wouldn't say it's a fair comparison. It was inline post cooler last time. As far as the temp gun, I realize its only so accurate, but his steel pan, brass case fittings,and flat black coolers aren't what I'd call reflective. If nothing else, reading different spots with the same gun should be some indication of temp differences. That said, we see 40-60* drops at cooler in/out fittings.
Like the title, I maybe be muddying up waters here, but my overall take is the coolers are working, and the gauge gives a general "idea" of the fluid temp. I think it'd more important to watch for change in the temp, now that he has a baseline reading. More than anything, I was trying to share info with anyone having similar readings. With the sensor in this location, I believe it'll fluctuate more, and reach higher #'s than other common sensor locations.

Do you still have any fittings or any type of restriction in the cooler line? If so, remove that and any other restriction in your cooler circuit as this is your converter feed oil and will cause heat and efficiency issues.

The fitting for temp sensors were a T style and don't actually get inline of the fluid path.