The band is used for 2nd and fourth. If you now have 2nd then the band and servo function just fine. If you changed valve body it should rule out a majority of other parts. Does second seem to come in at the right speed ? PCM could be a possibility but I've never had one bad for a no 4th issue. Did you change the right solenoids ? Simple explanation of the gears in this Trans is simple.
1st: forward clutch and low one way mechanical clutch
2nd: forward clutch and 2/4 band
3rd: forward clutch and direct clutch
4:th direct clutch and 2/4 band
Rev: rev clutch and low reverse clutch
Low 1st: forward clutch and low reverse clutch.
So for not having 4th you know that all the apply components inside should work because you have 3rd and 2nd from what you are telling me. Already.changed valve body and light doesn't come on till already driving tells me the wires to the Trans from PCM should be ok because it should be able to detect a actual wiring issue just running not moving. So either something is being overlooked or haven't found problem yet.
Would it be possible to test drive and post a video of the tach with good sound of the engine to try and determine the shifts when and how they happen?
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