check your shifter cables on the left side of your engine bay. there are two cables next to each other. in a 4 spd the right selects R, 1and2, 3and4. in a 5 spd it selects 1and2, 3and4, 5and R. the other actually selects the gears. One of your cables is probably disconnected or broken. my suggestion is that you find and purchase a new cable.
I have a 1987 Pontiac Fiero GT and the shift pattern is: 1 3 5 2 4 R
Check your Automatic Transmission fluid level
Only one type of automatic transmission was used on the Fiero, the Turbo-Hydramatic-125C. The TH-125c is a 3 speed automatic. The C signifies that it has a torque converter clutch. In theory from a stop to cruising speed there should be a 1-2 shift a 2-3 shift. If you sense the transmission is hunting between 2 gears it is likely that the torque converter clutch is locking then releasing.
on the transmission
there is no fuse that makes the transmission shift the transmission shifts on cylinoids and rpms
No the transmission does not have a governor.
transmission down shift after fuild change
The gears in your transmission may be sticking or worn out. This would cause your transmission to not be able to shift.
There is no neutral safety switch on a 85 fiero with a manual transmission. There is a clutch pedal switch under the dash. The car will start in any gear, as long as the clutch pedal is depressed.
The Chevrolet Lumina shift solenoid can be found on the top of the transmission. The shift solenoid will be near the front of the transmission.
your transmission could be shot. if it will not shift at all, then you will need a new one.
what would transmission not to shift in to overdrive