transmission not working right

i have an 88 e350 c-6 i just put a junkyard tranny in it. it will not go over 60 mph. does any one have any ideas.

Does this happen when driving on a flat surface highway or on a little steep highway?

All the time. It will not go over about 60 mph. The engine is screaming.
I have changed torque converter with same results.

The vacuum line to the moduator is either plugged, not connected, or there is a cap on the modulator nipple. The modulator *thinks* you are wide open all the time (no vac signal) therefore it will NOT upshift to 3rd.

It is shifting into all the gears, just as it should.

You said: "It will not go over about 60 mph. The engine is screaming."

That would indicate it's not getting into 3rd. Try a "D" to "2" downshift when it's doing this. If the RPMs do not increase you were NOT in 3rd, but in second.

Thank you for the info. I will check the vac. line in the a.m. I did try the downshift and it did exactly what you said. Thank you very much for taking the time to respond.