Vehicle history: 2005 F150 XLT 2WD with a 4.6 and 4R70W. 172k miles. Has both radiator end tank ATF cooler and small aux cooler piped in series. Boss bought it used with ~40k miles; original transmission gave up around 100k miles and got replaced with rebuilt unit. Truck had some other issues at around 171k miles and got parked for a few months until I was hired and started fixing it up on the side since the boss wanted to try first instead of buying another truck immediately. Transmission would flare badly or entirely miss 2nd gear on kickdown, but would shift fine up though the gears under WOT acceleration. Fluid was dark red-brown but not gritty and didn't smell very burnt, just overdue. After a single drain and fill plus new filter it began shifting smoother and 3-2 kickdown became closer to a flare than a straight to neutral missed shift. Also installed a pan with drain plug for easier fluid changes. But after a mild 30 minute drive half on freeway and half in town, the "O/D Off" light began to flash indicating overheated ATF and I got a vague hot oil smell at stoplights. I was only a mile from destination so I limped it there letting it idle in neutral at stoplights and coasting whenever possible. Infrared thermometer showed transmission pan temp of 285, but the aux transmission cooler was ambient temp as if no fluid was flowing. All subsequent trips were short with half hour or more cooldown time so I got it back to base without any further overheat warnings. Boss remembers his other worker mentioning the warning light when he drove that truck a while ago. In tracing the cooling lines, I found a little inline filter piped after the aux cooler on the return line. Since it may well have been in place since the rebuild, I deleted the filter and replaced the hose to start. To confirm that the coolers weren't clogged, I left the hoses pointing into a bucket and started the engine. It pumped about two quarts out in 15 seconds, so there doesn't seem to be a blockage, but it seemed like a fair bit of fluid came backwards out of the return line going TO the thermostatic bypass valve. Did another drain and fill, and the cooler lines downstream of the bypass valve seem colder than I'd expect even as pan temp gets up to 180. System has a lot of thermal mass so I'll have to see after another long drive on Monday or Tuesday, but I'm suspecting the bypass valve may be jammed or lazy, so coolers don't get enough flow as fluid gets hot. Anyone else seen a problem like this on Ford transmissions with the external cooler bypass valve? I found a few anecdotes but I suspect this problem may be getting misdiagnosed as regular old transmission failure, with the bypass valve getting flushed or replaced when the the new unit goes in and never investigated as the cause of failure.