My 1992 Maxima with 101,000 miles is having transmission problems. With overdrive enabled, It downshifts prematurely and often doesn't upshift until the engine is turning at well over 5,000 RPM, even I am accelerating slowly. This problem occurs at any speed, from freeway speeds to accelerating moderately from a stop sign. The odd thing is that this problem does not occur at any speed if I switch off (disable) the overdrive with the button on the shift lever.
Does this sound like a major mechanical problem in the transmission or maybe the throttle position sensor or other problem? Why would it behave as it normally would with the OD disabled when it has problems when OD is enabled? The car has not been driven hard but there did appear to be some degree of discoloration in the transmission fluid (more light-medium brown than red). The fluid has been changed with no effect.
Any ideas about the cause and solution to this problem would be greatly appreciated.