The first thing to look at if your Sentra's standard transmission is not shifting properly is the oil level. Standard (or manual) transmissions use gear oil to lubricate and cool them internally. If this fluid is allowed to go below an acceptable level or run dry altogether, the heat will cause the metal gears to expand, making for a rough-shifting car. The standard transmission on a 1993 Nissan Sentra has two holes in the case. One is for filling it with fluid, the other for draining. The fill hole is located on the front driver side of the transmission, near the front wheel.
Worn Clutch Components
Another reason that your Sentra could be shifting rough is the clutch itself. Over time, the clutch disc and pressure plate can wear out or become badly adjusted, which could in turn cause inadequate shifting. Sometimes bolts even shear out of the casings, causing the clutch components to literally wobble around inside the transmission. This could not only cause shifting issues, but also bad vibrations when the car is moving.
Damaged Shifting Forks (Internal)
Inside the manual transmission, there are "forks" that slide around and select the proper gear depending on where the shifter is pointed. If these become warped, bent or broken, it will definitely cause the car to shift rough, at the very least. Sometimes, damaged shifting forks will result in not being able to shift the car at all. Unless you are very adept at rebuilding manual transmissions, messing with the shifting forks should be left to a professional. It involves complete transmission removal and possibly teardown.
Shift Linkage, Cable or Lines
If the linkage between the shifter inside the passenger compartment becomes maladjusted, it may cause the transmission to have trouble going into any or all gears. Likewise, if the clutch cable becomes loose, the clutch may not engage or disengage all the way, which will make it very difficult to shift the Sentra normally. If your Sentra is outfitted with a hydraulic clutch, check the fluid in the master cylinder to make sure there is no air in the line, and bleed it if necessary.
