TH400 shift problem and questions

[texasag91]

Well I need a little help. I am not at all a transmission guy so I need to be educated on the TH400 transmission. First I have a 1990 454 with a TH400 transmission. Second I want to learn a little more about the specs on how this transmission shifts. Third help with diagnosing a gear shifting problem.

What is the proper speed does this transmission shift through each gear (stock setup)?

Does the shifting have much to do with rpm?

Need some specs on this tranny?

My problem: Transmission shifts through 1st and 2nd fine at about the proper speeds but when reaching 3rd I have to be running about 70 mph to get it to shift to 3rd. Now this is not punching it but the gradually going up in speed on the highway. The engine runs about 3200 to 3500 rpm on the highway but the last few days to get it to shift the rpms are going to about 4500. Then once shifted goes back down to 32-3500 rpm. Also slowing down below 70 mph shifts back to 2nd. What's my problem you think? Vacuum Modulator? Vacuum problem?

Now I have only had this truck about a month so I am still getting the feel of it. According to the guy I purchased it from he did not mention anything about putting a shift kit or anything in the transmission. The engine is upgraded with performance intake and headers.

Sorry so long but I am just a dummy when it comes to transmissions but I am learning.