Last spring I put on a transmission temperature sensor on the hot side line going to the factory cooler in the radiator end tank. Very interesting to watch how it behaves. Factory cooler is actually quite good, but does have its limits. In stop and go traffic with a heavy load or with a small half ton trailer, ATF will rise to around 105 C / 221 F, even on a mild day. Lack of airflow causes coolant in the radiator to heatsoak, which prevents it from extracting as much heat from the ATF. Once the car is moving, coolant temp drops on the radiator exit side and the transmission temp will go back down eventually. The other case where the factory system is overwhelmed is when running at sustained high load in very hot temps. Climbing a grade in the southwest on a 43C / 110 F day I saw the ATF rocket up to 115 C / 240 F in short order with the converter unlocked. Coolant temp only hit a max of 107 C / 224 F, so overall cooling capacity was adequate, but heat transfer from ATF to coolant wasn't enough. Locking the converter in 3rd let the temps fall back down, and I didn't let it get that hot again. I knew I needed a little more cooling, so I added a little Hayden 676 plate and fin cooler in series with the factory cooler. I know part of it is blocked by the bumper, but that was the best place to put it for hose routing and preventing air pockets. Load varies from "just me" to "stuffed to the gills", and ambient temp ranges from chilly New England winters to blistering desert summers, so I also installed a Derale 13011 Fluid Control Thermostat to bypass the external cooler until the ATF warms up. A few percent flow is allowed to stabilize pressure and help bleed out air. Only had it together a couple days and the difference is huge. System warms up plenty quick as the thermostat bypasses the external cooler, but temp rise slows way down once over 80C as the thermostat begins to open. Since temps would still want to climb when stationary due to heat soak, I set up my custom gauge controller to force the radiator fan to run on low once the temp hits a certain threshold. I'm still playing with the fan setpoint, but at last I feel confident I won't cook my transmission. As part of this upgrade, I also rebuilt the original cooler lines which were starting to leak, replaced all the o-rings, and did a D&F with Valvoline Maxlife. 3.75 qts out, 4.25 qts back in. Fluid looked decent, and I'll be sending in a sample for UOA. These units can go over 300k miles and I'd love to have it last that long.