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Old 01-15-2016, 12:32 AM   #38
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Join Date: Sep 2013
Location: Vacaville CA
Posts: 510
Re: 4L80E & TH400 Tech.

Here's my mod report. I hope this helps someone with a more stock build like mine.


2003 Yukon XL 2500/8.1/4L85E/4.1 at 120K miles. Original owner. Started with a good working trans, just very soft shifts.

Sonnax boost valve 4L80E-LB1
Sonnax TCC valve with the .300 OD spring.
Drilled feeds:
1-2: .070" to .085"
2-3: .075" to .093"
3-4: .050" to .085"

8 check balls in the original location, but I used Torlon balls that I had. There was minor/acceptable wear on the plate so maybe Torlon will slow this down.
No accumulator mods other than cleaning them thoroughly. There was a bit of the black paste that had "accumulated" in there. Otherwise, the trans really looked good. Almost nothing in the pan.

Before, the trans seemed to "work" fine and there were no codes, but you really had to pay attention to know when it was shifting. I don't like that in a truck, especially when towing. It also seemed to be getting softer and softer, but it's hard to remember how it shifted when new. Like I said, it's always worked and it's done a fair share of medium duty towing over the years with never even a hiccup (20 ft boat, motorcycle trailer, and a pretty heavy horse trailer).

Now, you can feel it, but it's still very acceptable as a daily and a people hauler. It could even be tuned to be more solid without sacrificing comfort. I made the largest change to the 3-4 feed hole to get it in line with the others and because it was the softest shift. They all feel pretty equal now. The torque converter clutch doesn't seem noticeably firmer on engagement, but it does seem to hold better. I swear it now runs about 100rpm lower on the freeway once it's locked up. Hard to imagine that it would have slipped that much all these years?

In Tow/Haul there is the usual slight increase in firmness, but still far from harsh. However, in 4Lo it's now a solid bump. Weird. If these mods had made it that firm all the time, it would have been too much. In the dirt in 4Lo it's fine. I wonder if the electronics firm things up in 4Lo.

Overall... I'm happy, but if I had to do it again, I'd go even larger on the feed holes. I was very conservative because I started with feeds that were much smaller than I expected. Going from .050" to .085" on that 3-4 really made me nervous... now I wish I'd gone larger. Seems the changes I made got the trans to what I'd expect from a good OE tune rather than the slush box I was driving. It's "on the map" now. I hope this helps someone.
picklito is offline   Reply With Quote