Okay, i got an 87 iroc with ls1 with 4l60e behind it, when i first got it started and drove it it only had 2nd and 3rd gear. We figured out guy sent use the wrong PCM, he sent me the one for a 6 speed.. So i had my tuner guy do a segment swap on pcm which changes it to an automatic. the other day i put pcm back in and drove it, now it only has 1st and 2nd gear.. i put it in drive or overdrive and the rpm just shoots up and isnt catching. but it starts out it first in drive and OD. i dont knowwhat to do!! lol please help

 

If the fluid is still good, and it was working ok before then the computer could still be the problem. If the fluid is burnt or there is material on the bottom of the pan, then the 3-4 clutch pack could be gone.

 

You can unplug the transmission and you will have manual 2-3 R. If you have manual 2-3 and with the trans plugged back in you only got 1-2 either the B solenoid in the trans is stuck on which is unlikely or ECM problem. Do as Probuilt said and pull the pan look for garbage.
Rob

 

fluid is clean and nothing in oil pan, its gotta be pcm, i hope. my tuner is gonna check on his computer and see what it says. is there anyway having no speedo would be doing something? the vss is plugged in but i dont have speedometor yet

 

The speedometer not working will cause this.

 

Like Probuilt said no speedo will cause this. The pcm has to see speed to know when to turn solenoids on and off. Also the right gear ratio and tire size input has to be in the pcm or the shift points will be way off.
Rob