I undertook this swap due to limitations of the 6L80. The cost to rebuild the 6L80 with no guarantee was not inspiring, especially when I'm looking to push a good bit more power then I am now once I get the trans and fuel sorted out. I looked at 4L80's but didnt want to deal with electronics again interfering with my shifts.
My buddy Mike offered up his TH400 that last lived life in his 80's Suburban. I couldnt think of a good reason not to. I ordered an SFI flexplate, B&M shifter, and other small parts to make the swap complete.
We compared the lengths of the 6L80 and TH400 which were surprisingly close. Once the 6L80 was out of the way we mocked up the TH400. Lengths end to yoke were within a 1/4". I installed a Hughes Performance SFI Flexplate and converter spacer. Then bolted up the TH400, which all but three of the trans bolts went in. Obviously there is a slightly different bolt pattern between the trans, mainly the two oilpan bolts and the top center bolt the 6L80 uses. The existing crossmember that supports the 6L80 works for the TH400 without drilling any holes. I used an Energy Suspension TH400 mount which bolted right up to the cross-member.
I took off the 6L80 yoke adapter plate from my Driveshaft Shop driveshaft and removed the u-joint. I then installed a new 1350 u-joint to the driveshaft and a Hughes performance TH400 slip yoke to the u-joint. The driveshaft is the perfect length, carrier bearing is in the same spot. All is well there too.
Then installation of the B&M shifter. I used a piece of plate steel to block off the hole from the OEM shifter. I used existing bolt holes to secure the piece. Then bolted the shifter to the blockoff plate once I found a place that worked for my arm length. The only hole I had to drill was for the shift cable that went down to the trans. It's a 1" hole about 4" forward of the shifter. Ran the cable down to the B&M shift bracket and bolted it to the trans. Ran through the gears to make sure it was adjusted properly.
Removed the OEM trans cooler lines and installed a new trans cooler. Ran new 3/8" line from the cooler to the TH400 which worked great, took about 10 feet of line to run it cleanly.
I had to remove the TCM from the 6L80 and plug it in for the car to start. I ran the 6L80 trans harness up into the engine bay, put some heat resistant material on it where it came near the header. I notched the plastic area that holds the cabin air filter and ran the cable into this area. I connected the TCM and put it in this free space. really you cant even tell its not supposed to be there.
Car started up and runs fine, went and did some donuts for good measure to test it out. Really not a bad swap, surprised more people havent done it. Took about 7-8 hours of mechanical work taking our time, BS'ing, running to the store for stuff we forgot, looking outside at the snow, etc.. then about an hour of figuring out what the electrical system needed for the car to turn on, start, etc.
I adjusted my HPtuners tune and only have one code right now. That was only a quick look, I'm working on getting rid of the remaining code so I can be check engine light free with this swap.
Comparison pics... 6L80 and TH400
TH400 in the car:
Stock cross member works!
All the crap that comes out of the 6L80 that needs to be plugged in:
Fitting B&M Shifter:
Driveshaft Shop 9"
Roll bar designed to allow rear seat use:
And the engine bay!
Then a little test: http://www.youtube.com/watch?v=GoBiY...layer_embedded
I still have a few tweaks to make in HPtuners that I found. We are going to pull the trans back out to build it for some power, this was just a test to see that it would work and wouldnt have any gremlins holding us back.