I have a 95 Buick Riviera S/C the other day I was driving and the low voltage light came on and then went off and a second later the check engine light came on and the speedometer jumped around then hit zero and I only have 2nd and 3rd gear now, the check engine light is still on and nothing changed after I have replaced the speed sensor and looked for any broken wires in the wire harness leading to it, I also checked all the fuses and replaced the negative Terminal on the battery the transmission fluid and oil was changed a week ago any help would be appreciated

 

Many vehicles today will not function correctly if there is a low voltage problem ..
Suggest you have the charging system checked out.
Systems will go into limp mode where they will not shift out of 3rd gear. That is to encourage the operator to seek repairs.
It is a protective state to prevent any damage to vehicle components.
There may be a code or two in the vehicle's computer to provide some direction.

 

The battery reads 12.76 when the car is off and 14.7 when it running and i don't have an obd 1.5 scanner

 

If,,,,,,,,,,,,,you have no loads requiring power AND that alternator can only put out 5 amps, the reading you received looks OK, but could be faulty.

The system has to be loaded, hi beam, A/C on, full blower speed then see what the alternator is doing. This only tells you it is capable of delivering a charge to the battery.
Even if it is capable, now you must verify the charge , all of it, is getting to the battery. There is more to checking a charging system then checking a few voltage measurements.