All, Just did and engine swap on a 2001 blazer i bought. Runs great, but on a test drive, it wont shift. The transmission was replaced at chevy dealership right before the engine blew. The truck sat for a few years before i bought it. Truck has the 4WD Auto, 2HI, 4WD hi and 4WD Lo. I was under the truck looking for the Vehicle speed sensor and found 2 on the transfer case. One on the passenger side of it and one on the top(this one was not plugged in) I borrowed a friends snap on solus scanner, can i run any tests to diagnois with this? Also, the speedometer is pegged at 100MPH. I do have an ABS light, and intermitant BRAKE light. Can the TPS sensor cause issues like this? Feels like the truck is in Limp mode. If i try and shift manually, i can go into first and second, but when i put into 3rd, it goes back into 1st. TIA!!!

 

You mentioned two speed sensors on the transfer case, one of which was not connected. This could very well be the cause of your shifting issues. Speed ​​sensors (or VSS - Vehicle Speed ​​Sensors) play a crucial role in the proper functioning of the automatic transmission, as they inform the ECM (Engine Control Module) of the vehicle's speed and help the transmission shift properly