too low trans flid
Shift solenoids are inside the transmission.
Shift solenoids are inside the transmission, and require disassembly of the transmission to replace.
There are a couple of reasons why a 1993 Mercury Villager is hard to shift. One reason could be a gear shift cable. Another reason could be low transmission fluid.
Your transmission might automatically shift out of overdrive while driving on the interstate because of low transmission fluid. This issue might also occur because of a bad transmission sensor or overdrive sensor.
The gears in your transmission may be sticking or worn out. This would cause your transmission to not be able to shift.
Allison transmission not shifting
Leaky transmission lines would cause the fluid level to drop and eventually cause the transmission not to shift and go.
what would transmission not to shift in to overdrive
Front of the transmission, the thing the shift cable attaches to.
A bad transmission gear can cause your 1999 Nissan frontier to shift hard. Low transmission fluid can also cause the transmission to shift hard.
A transmission that is not engaging fully when in drive might need a new shift solenoid. There is also a chance that a band is stretched or broken inside the transmission.
Generally, one should have the codes scanned to make sure all the sensors are working correctly. Out of spec sensors can cause damage to the transmission. Make sure the alternator / battery voltage is correct too as I've had hard shifts when my alternator died. Failing that, you'll need to have a transmission shop check it out, working on external checks first before tearing it down.