rebuilt transmission won't engage

[atomicdog]

I have a '90 automatic 240sx and needed thetransmission replaced. I had a guy put in twodifferent transmissions already , one refurbished andone rebuilt. Both times the wheels would spin while on jackstands but on the ground the car doesn't move at all. He told me it could be something wrong with the transmission electronics. I checked the ecu already and there were no error codes indicated, but i was told there was a port on the driver's side to attach a computer to to check the transmission error codes. Has anyone had a similar problem or know what might becausing my problems? Is there anyways to diagnose this myself without taking it to the nissan dealership and getting ripped off? thanks