i have a 06 5.9 mega cab. I bought it w it having a aftermarket converter and a rebuilt tranny. So now to the confusing part. When the truck is could it will shift into OD and lock the converter but, when it gets to running temp it will not lock the converter. so it runs about 2k at 55mph. What could b the issue.

 

So it doesn't go into overdrive? Probably a blown overdrive piston seal

 

it shifts into od just doesnt lock the converter.

 

Time to order some parts! soleniod?

 

Does the converter lock at all?

 

The converter locks in OD when it's cold and it does shift into OD more when in t/h mode. It will lock the comerter in 3rd w/ noproblem at running temp or when cold.