4L80e problems

[Ctinman]

I have bee doing a 6.0 LQ4/4l80 swap on my 70 c10. This is my first ls swap so it's been a huge learning experience for me. Anyway, I now have the truck running again and went to drive it and the 4l80 is acting up. It goes into reverse and drive fine but when you start moving it shifts very quickly(as in it is shifts into second when you are just idleing along) and when it does the engine tries to stall. I haven't driven it very far but I'm pretty sure it isn't shifting past second either. And if you put the shift lever down in first, it starts in first but still shifts to second very quickly. A little background, it is a freshly rebuilt tranny, I have filled it to the proper level, and it has no obvious leaks. I purchased it from the same vendor I purchased the engine, and wiring harness/pcm from. The pcm was supposed to already be programmed to run this engine and tranny. The check engine light came on and I had a P0740 code. From everything I see when I search this code, it looks like this should not be active with a 4L80e. That seems to me like it should be impossible to get this code if the computer is programmed correctly. Is this correct? Any help would be greatly appreciated, I'm just trying to figure out what my next step should be in figuring out the issue.

[Clyde65]

Holler at member Clinebarger, he is the resident Tranny guru.

[Custom 68]

check this thread out. http://67-72chevytrucks.com/vboard/s...ighlight=4l80e
This is something you will need to check inside the tune if you can or have your tuner check it. These transmissions are electronically controlled so they must have the correct speed input.
As noted in my other thread my speedo worked correctly however the one I circled is what the trans uses.
look these over if that is not it then I will look more.
There is also "limp" mode but I would check here first. Mine was doing the same thing however when it shifted so early my TC was loose enough it almost wouldnt pull itself.
Dave

[Ctinman]

Thanks for the replys. I talked to the place that programmed my ecu and they looked at my file and discovered that they had a 4l60 tune in it instead of a 4l80. So I'm sending them the ecu to reprogram it. Should fix the problem I hope, keeping my fingers crossed.

[Custom 68]

Yes it will or should anyway. I am sure they know what they are doing but there is what they call a segment swap in HP tuners. If they are going to use the same tune they will need to do this swap. I am sure they have done lots.
In addition to that did you use a wiring harness for the 4l80e, they are different than the 4l60e.

[Ctinman]

Originally Posted by Custom 68

Yes it will or should anyway. I am sure they know what they are doing but there is what they call a segment swap in HP tuners. If they are going to use the same tune they will need to do this swap. I am sure they have done lots.
In addition to that did you use a wiring harness for the 4l80e, they are different than the 4l60e.

Yes, I am using a 6 liter 4l80 harness. Can't wait to get the ecu back and drive the truck. I've been working on it all year. Thank you for the help. I'll be sure to let you know if this fixes the problem.