Hmmm, I just replaced the radiator on my '91 300SE. I took the old trans cooler out just to look it over. It's just a big metal tube with what appears to be some spiral-wound metal inside (kinda like Christmas tree ropes). No thermostat or valve or restriction whatsoever. I ran water through it with no resistance at all. In fact, I actually ran the water hose through the new one, followed by an air hose, and then some ATF, just to be sure there was no residual crud collected there (the new radiator came witn no plugs on the cooler fittings). The water and the ATF ran through with virtually zero resistance.

DG