2000 Tahoe
2 Answers
Power needs to be going to the small wire on the starter solenoid when turning the key to the start position. The positive battery cable always has power down at the starter. Anyway, since you replaced the transmission, I suspect a problem with the neutral safety switch. Check out the neutral safety switch.
I also changed the switch on the side of the transmission. That detects the gears are in. Installed it while in neutral.