2007 Chevrolet Impala LS 3.5 transmission has been rebuilt and there is no 4th, it goes out of gear and revs freely above 40 mph until I put it in 3rd or reduce speed below 40 mph. What are possible problems and solutions? The sun gear and 4th clutch are good. It would be great to hear from someone who has resolved this issue.

What was wrong with it before it was rebuilt?

Was it rebuilt by a professional rebuilder?

If you get going hard in third gear will it upshift?

I bet it'* the 4th hub splines all stripped off.