Q: Yes I change transmission on 94 5.7 chevy truck 1500 after one day it started doing exactly like old transmission will not shift

Assuming the second transmission actually works, what you are describing points to a fault in engine controls that affect transmission shift points. This would include things like a throttle position sensor, speeds sensors or any vacuum operated controls. However, it is possible, of course, that the second transmission is defective as well. Generally, used transmissions are risky. It is always better to rebuild and thoroughly inspect a transmission before installing it on your vehicle. There are just simply too many unknowns and too many possible things that can go wrong. You can first check for the possibility that there is a problem with the engine controls that affect transmission shifting. If nothing is found, my recommendation would be to rebuild one good copy of the transmission (using all available updates that are out there) and solve the issue that way. It is actually easier to rebuild a transmission than an engine because you do not need any outside machine shop work. Alternatively, if you desire, you can request a transmission diagnostic which would include consideration of possible engine controls issues. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.