My tranny was acting up on my 6.5, it shifts from first to second then once you hit 60km/h it shifts to neutral. Before I would get up to 80km/h and it would start revving up nad down about 300rpm while accelerating and my speedo would go down. was told it was bad transmission, so I swapped in another 4l80e I had and after all the work its doing the exact same thing. Anybody experienced this?

 

What condition is the output speed sensor in? When that thing starts to go you get all kinds of wacky shift issues and the speedo will drop to zero and your cruise control will cease to function.

It doesn't just go either, it can work intermittently. Mine wouldn't even shift out of first on occasion.

Hard to mess that 4l80e up, let alone two of them.