So with a lot of help from guys on this site i got my 4l60e rebuilt and in the truck. It runs and sifts awesome. Except for one large problem...
SO if i run the truck on the highway for more than 20 mins something happens. I'm assuming it overheats but i'm not sure what happens.Pulling up to the toll booth it downshift fine. Pulling away in 1st till 8-10 mph then it shifts to 4th. Full throttle for 1 min of so and then back to highway speed. It also has some really small rev flairs on the way back up to 70. Im confused and hoping that it might be related to a shift solenoid. Let me know what you guys think.
also this is an 4.8 4l60e combo swapped into a 77 Chevy k10 and it has a brand new alum radiator that was replaced wen the new trans went in.

 

The flair is likely related to a Pressure leak. As for it going straight from 1st to 4th I could only assume something might not be installed correctly or its not getting a correct signal.

 

4l60e shift table
1-2 solenoid 2-3 solenoid
1st On On
2nd off On
3rd Off Off
4th On Off

If youre goin from first to fourth, id almost say you have the solenoids hooked up backwards or theres a problem with your harness.....

 

The thought that shift solenoid b (2-3) was failing under the conditions of added heat. it could cope with the line pressure for a little while wile in 1st but once the line pressure rose it failed into the off position causing 4th gear. Think that could be possible and is there any way to test this theory.

 

Very well could be a weak solenoid, did you replace them??

 

I did not. None of the electronics were replaced. Now that I think about it that sounds like a stupid idea. Should I replace all the valves and noids in the valve body? Also would it be a good idea to replace the wire harness as well while I have the pan down.

 

Test solenoids at very least, and harness is inexpensive enough, i came across a helluva deal on a new reman vb with all the solenoids included, but if i hadnt come across that i would have replaced them all....