I have a standard transmission 1995 Nissan Sentra. I am still doing great in first and second gear as well as reverse with no changes. However, I have just abruptly lost third and forth speed. The shift lever will go in fine, but nothing happens but reving when the gas is given. It is just like neutral in both thrid and forth. What is up???
Essentially, your transmission has worn out. It will happen to all transmissions eventually.
There are two synchromesh gears in a four-speed manual; one is engaged for 1st and 2nd gears, the other is engaged for 3rd and 4th gears. It appears that one of your synchromesh gears is not sliding into position. I think your problem is with the shift linkage, and not with the transmission itself.