Hi I have a 94 S 10 blazer 4.3 vortec with auto transmission, that the engine was just replaced. The replacement motor was out of a 1995 S 10 Blazer. After it was installed it ran fine for a few minutes then started to run bad. the ignition modual repaired that problem. However the problem is in the transmision shift points. Once in a while it will work correctly. Most of the time it will start out in second gear then shift up ok, never using first. Other times it will find first and second, but not overdrive. Basically it's all over the board. Maually using the gear shift has no change in this behavior. There is no slippage or anything abnormal noted other that the shift pontsThe transmission was ok prior to the motor change so I beleive its a adjustment or wire related cause of some kind. There is also a kind odd whine I can hear from the transmission area. Fluid is up and looks clean. 4 wheel drive seems to work finethanks in advance

 

Sounds like you need to get it to a professional for diagnosis. It is very difficult to diagnose hit-n-miss problems like this over the internet.

One thing that does come to mind would be corrosion in the connector on the side of the transmission. Just seems odd that it would start with a "new" engine.