I just got my 4l80 back from being rebuilt, got it stabbed and put back together but when I went to test drive it would only go about 20mph. If I went any faster it would basically lock up the tires like a hard down shift (did this once and I took it back home). I tried to pull codes and there were none, I checked voltage on pin E and had my 12V so no fuses are blown. I am using a Frost Harness with the 4l60 tune that worked with my previous 80e.

Anyone have anything like this or know anything else to check? I am taking it to the trans builder, but want to make sure there is not something I missed or something I can relay to him to check. I think the Frost Harness threw him for a loop when we talked about it on the phone.

 

Sounds like something is wrong with the valve body, sort of like a transbrake locking up 1st and Reverse at the same time.

 

Well after further investigation all the gears are engaging properly except 2nd gear. I can get it up to about 30mph and shift straight into 3rd bypassing 2nd and it will continue to go to 4th and lockup. Transmission guy is repairing tomorrow.

 

The most likely scenario, with this sort of uncommon malfunction, is that during assembly they misplaced a check ball in the channel casting of the case.

 

We are thinking the sprag may have rolled.

 

Its possible however I have seen where a builder forgot to put in all the check ***** and the trans shifted really hard. Did not bind up only shifted hard and this is a vehicle I personally road tested in a shop I was working in.

This is what I first thought and definitely a place to look . Just an FYI if the sprag is in backwards you should hurt the direct clutches.