why my transmission dont change gears

There could be something wrong with the gears internally for the transmission. I'd have that checked out, transmissions are alot of money.

The good thing about your problem is that it doesn’t seem like it involves the transmission at all. That said, though, there are likely two problem areas for you to look at. The first involves the air conditioner clutch and the second involves the battery. In order to work properly and save gasoline, you car’s air conditioner compressor has a special type of clutch that allows it to kick on and off periodically while your car is moving. The device that enables your car’s air conditioner compressor to turn off and on is a clutch that equalizes the rpm between the compressor and your car’s driveshaft. When they are in sync, the clutch goes clunk and the compressor engages, cooling your vehicle. By the same token, when the clutch disengages, usually with another clunk, the compressor is no longer taking power from the engine and your mileage increases. It seems quite likely that your compressor clutch is not disengaging from the engine. In those circumstances, the vehicle will quickly lose mileage and power. This can be fixed for far less than a transmission ($400 to $800, possibly less). The other area to look at is the alternator and battery. If your battery is on the rapid road to failure, then it is likely that your vehicle’s alternator is remaining on all the time as it not only tries to recharge the now-failing battery charged but also is trying to keep your vehicle’s electrical and ignition system working correctly. The washout is that with your battery unable to hold charge, you car is working off the alternator at all times so that your engine is constantly working, staying on, if you will, all the time. The washout of this is that with all of the extra work your car’s engine is doing, it is little wonder that it feels as if your car is always “in low.”