I drive a 90 Celica GT, 4 speed automatic transmission with overdrive. What I notice is that when climbing a steep hill, I usually hit the overdrive button (which drops it into 3rd gear), and after about 15 seconds I smell something getting hot. The tachs at 3200 rpm and around 85-90 km/hr when this happens. Once I get over the hill I take overdrive off and the car is fine again. Is this normal when there is more stress on the engine, or is there a problem that could be causing this? All fluids are at the proper levels, however its been a while since the tranny fluid was changed even though it still looks good. Any advice?