NP246/266 custom build

[gearguts]

A bit of feedback on how this mod has worked out for me, I have around 1500 k on the auto trac now and it works great! I have been watching the transfer case temps with a gauge that is installed inline with the external cooler and filter and it reads the hot flow into the cooler. Heres what I have found. When in auto mode the diesel will heat up the oil fast if you drive hard off the line in the rain or gravel, Its about 8c outside here and I have seen temps as high as 77c at the case before it cools down and stabilizes at about 45. You can essentially see the auto trac working by watching the temps. If I run in 4x4 the case operates at almost the same temps as 2wd, somewhere around 45c and it does indeed hold back the diesel without slipping, I haven't tested this to the extreme yet but I can climb hills until I get stuck and spin all four tires without any noticeable slipping or increase in temps. But lets not forget to consider that this is a duramax tahoe and for guys with heavier vehicles things might be different. I'm going to keep beating on this thing and see how it goes, so far i'm very happy with the whole conversion.

A big shout out to all the guys who take the time to put together threads like this, great work Chrisuns. Very good reading material for me as I wasn't too sure about doing this until I ran across your thread. :thumb:

 

[MichaelPrichard]

Chrisuns, I've just put a 246 into my LBZ Yukon XL (which is what it had originally, ironically) using 1450 slip yoke and shims on 263XHD input gear. I've never driven an Autotrac truck before but was looking forward to smooth low speed cornering and appear to have too much pre-load as I can feel the driveline binding up just short of actual crows-hop in Drive. Oddly, it's silky smooth in reverse. Do you know if the pre-load is programmable, or if there's some way to calibrate a new clutch pack? I'm assuming that when one rebuilds the clutch pack that the pre-load position needs to be reset somehow, no? I put in a new Delco encoder in my shift motor just to rule that out with no improvement. Haven't tried taking it to dealer to have TCCM software updated yet.

 

[chrisuns]

What year is the 246 out of? If it's starting to bind up right when you turn, it sounds like the clutches are pre-loaded too much, and it is year dependent on that factor - That and I need to check the specs once I am around my home computer. Earlier model 246's had the clutch basket pre-loaded, where the later models did not, that was one of the reasons for the autotrac fluid - to modify the friction coefficient of the clutches.

If it is going into full bind, then the issue is related to the tire size and/or pressure, then the speed sensors could be a culprit too.

To be frank, that is an issue that requires and bit of finesse to alleviate.

 

[MichaelPrichard]

It's a later model 2006 unit and the clutches were super loose when it was out of the truck; I could easily turn the front output by hand without dragging either input or rear output with it, and you could hear the clutch pack jangling steels to bands. It just seems like the TCCM is calling for too much pre-load in Drive. Is this just a fixed position on the encoder motor? Seems like this has to adjust with clutch pack wear and presumably there's some zero/start value for "new" clutch packs. I'm assuming it doesn't really know how much torque the encoder motor is exerting, but is instead just using some feedback from the last time in noticed slippage. I haven't spent a lot of time in the dirt flooring it to see if that might not "calibrate" it.... but I'm guessing the firmware in the TCCM isn't eager to go backward on pre-load since that's not how clutch packs wear. I'm hoping that perhaps a Tech II can reset the pre-load to "factory" start or something like that.

It's not going to full bind, just a lot heavier, and it'll stop the truck on it's own when you turn the wheel sharply whereas the truck will roll backward in reverse at idle with no binding. In fact, if you turn sharply while going forward and come to a stop, when you pop it into reverse you can feel the encoder motor release the clutches and the whole drivetrain relaxes. I should probably get some scanner which will show me the front/rear output speed sensors to confirm that it's not just getting bogus data from one of them. Will my EFI Live box do that? I can also confirm it's the later model EAU style due to the star shaped shift shaft, not the older "D" shaft.

Thanks for your help. Seems like you really understand these boxes.

 

[the_ure]

The transfer case control module compares rear output and front output speeds and applies the clutches accordingly via the encoder motor. My question is, if GM didn't think this would work and last in a stock application, hence it never coming on anything heavier than a 6.0 1500HD, what's the point in making it work behind a 1000ft lbs diesel? Like, ok, different idea, innovation blah blah blah, but to me, this makes about as much sense as putting a 4L60E in it...

I have a 2004 2500HD 6.0 4l80 np246 with a AAM 11.5 rear axle from the factory.
Love hitting the accelerator in the rain.
Found out they don't actually make them that way from quite a few sources.

 

[chrisuns]

I still have that truck with the transfer case in it, I have about 40-50k more miles on it and still rolling around with a 1k ft lbs tune and no issues other than what was mentioned in the beginning to date.

I recently contacted a company and having them make a special output shaft for the Allison (32 splines, high quality steel, heat treated etc.) to try a 08-09 H2 transfer case(BW4484). It has 4-high-open, 4 high-lock, 4 low for positions. Then having them make an input shaft for the Silverado SS case (NP149) that's 32 splines, that one is a viscous clutch. Just to try them out, as you can tell I enjoy AWD vehicles and these projects. I will also update those like my current case to handle a bit more power.

This time next year I intend on building a single-cab-short-bed duramax(LBZ) - Boxed-frame, slightly lowered, big sway-bars, poly bushings, built front-end; basically make it one the of the best handling duramax's possible.

 

[Ridin'GMC]

I still have that truck with the transfer case in it, I have about 40-50k more miles on it and still rolling around with a 1k ft lbs tune and no issues other than what was mentioned in the beginning to date.

I recently contacted a company and having them make a special output shaft for the Allison (32 splines, high quality steel, heat treated etc.) to try a 08-09 H2 transfer case(BW4484). It has 4-high-open, 4 high-lock, 4 low for positions. Then having them make an input shaft for the Silverado SS case (NP149) that's 32 splines, that one is a viscous clutch. Just to try them out, as you can tell I enjoy AWD vehicles and these projects. I will also update those like my current case to handle a bit more power.

This time next year I intend on building a single-cab-short-bed duramax(LBZ) - Boxed-frame, slightly lowered, big sway-bars, poly bushings, built front-end; basically make it one the of the best handling duramax's possible.

The short bed boxed frame you mentioned, are you using a 2500hd shortened too short bed and box it or using a 1500 frame boxed up? I've always thought how cool it would be to have an AWD single cab short bed Duramax meant to handle the track or windy roads where I live. The suspension, would it be coil overs or airbags or sticking with stock torsion bars?

 

[chrisuns]

The short bed boxed frame you mentioned, are you using a 2500hd shortened too short bed and box it or using a 1500 frame boxed up? I've always thought how cool it would be to have an AWD single cab short bed Duramax meant to handle the track or windy roads where I live. The suspension, would it be coil overs or airbags or sticking with stock torsion bars?

I'll for sure be sticking with a 2500 frame and shortening that; I may even try to put a 2011+ frame on it to getting around from boxing the 01-10s. Those are the two options that are on my mind at least - I still have a bit of research to do in that department.

The front suspension I'll more than likely leave torsion; the rear: a 5 link air setup has been on my mind - maybe a Keldermans, or even custom (pending I decide to buy a CNC plasma table and CNC machine I've been looking at). Although, specifics like that will be further down the road, since this will likely be one of the vehicles I keep for the rest of my life, as a toy.

It's mostly in the planning stages/slowly looking around for the right truck when time allows at this point in time.

 

[JoeyD]

I still have that truck with the transfer case in it, I have about 40-50k more miles on it and still rolling around with a 1k ft lbs tune and no issues other than what was mentioned in the beginning to date.

I recently contacted a company and having them make a special output shaft for the Allison (32 splines, high quality steel, heat treated etc.) to try a 08-09 H2 transfer case(BW4484). It has 4-high-open, 4 high-lock, 4 low for positions. Then having them make an input shaft for the Silverado SS case (NP149) that's 32 splines, that one is a viscous clutch. Just to try them out, as you can tell I enjoy AWD vehicles and these projects. I will also update those like my current case to handle a bit more power.

This time next year I intend on building a single-cab-short-bed duramax(LBZ) - Boxed-frame, slightly lowered, big sway-bars, poly bushings, built front-end; basically make it one the of the best handling duramax's possible.

I have a project that I want to use with the np149 behind a 6l90, not as much power as your running though.

 

[DAVe3283]

I've now done this basic conversion on both of my rigs. Did the 2003 LB7 Suburban a year or so back. Ended up paying a transmission shop to do the mechanical side of things. The electrical was easy on the Suburban, because it originally had the NP246, so basically just plugged the original TCSCM and harness back in.

I liked that so much that when I (thought I) killed the 263XHD in my GMC, I used it as an excuse to do the same thing. This time required a good bit more wiring, since the body harness didn't have the necessary wiring. My brother and I went to a junkyard and pulled the body harness apart on a Suburban, and took the complete AWD wiring out of it.

Not wanting to completely rip my truck's dash apart, I loomed the additional wires for the AWD case (front driveshaft speed twisted pair, rear driveshaft speed twisted pair, motor brake return) into their own harness and poked it through the firewall next to all my other aftermarket crap. This ended up being a much more direct route than the factory routing, so I had a bunch of extra length. Not wanting to cut & crimp new pins on (long-term plan is to loom this into the stock harness next time the dash comes out), I just made a big U going up towards the top of the dash then back down to the TCSCM, and added the new wires to the factory connectors. I avoided looping the harness to prevent cross-talk. Under hood, everything uses the stock AWD connectors, so the transfer case extension harness is all original and just plugs in.

This seems to have worked nicely, as the AWD system works perfectly! Both the transmission shop and I are a little worried about the smaller output shaft on the 246 vs the 263XHD. I plan to take it to the track next year and beat on it, so we shall see how it goes.

 

[TheBac]

Guys, one thing that hasnt been mentioned (or maybe Im dense) is, "Just how did you both actually connect the 246 to the Allison and driveshafts in the first place?" b/c the spline counts and trans output shafts are different?

Did you pretty much have to swap all the 261/263 internals over in the end?

Also, just what was involved with the harnesses? More sekrut skwirl stuff? That was another point that was never fleshed out.

 

[DAVe3283]

No secret squirrel, but you're right, this thread has been short on DIY.

The mechanical side of things is not my cup of tea (which is why I paid Ultimate Transmission to do it), but let me see if I remember what they said they did. Take this with a grain of salt, as I don't remember things I don't care about super clearly. The main shaft on the transfer case is the 2 pieces. You take the front piece out of a 263XHD and that fits the Allison just fine. The main shaft is from the 246 to fit the clutch pack. You can make the large chain out of the 263XHD fit on the 246 shaft. There is some machining work, but I can't remember if it was on the shafts to get the 2 to connect together, or on the front 246 case half to take the larger input shaft. The rear case halves are basically the same. The 263XHD back has the blanks for the speed sensors. You can just drill those out, or use the 246 back half. The 246 main shaft is smaller, so you use a larger bearing and seal on the output. I am using the smaller yolk to attach the driveshaft on both of my rigs. The trans shop thinks that is the current weak link at my power level (~700 WHP), but we will see.

The electrical side is my jam, though. And honestly, it isn't that hard if you add new wires separately, instead of adding them into the stock harness. Even then, the hard part is getting the dash pulled apart and back together.

There are only 5 wires that need added (mentioned in my previous post). I've attached the factory wiring for the 263 and the 246 so you can see where they all go. Once you add the wires, you are good to go to plug in a 246 transfer case harness, and it just works.

The 246 pushbutton module uses a different connector, but the pins are all the same. I just de-pin the 263 pushbutton connector, then put those pins and the 1 new wire into the 246 connector. Might as well take the 246 pushbutton apart and put new bulbs in at this stage. Then it just plugs in and clips into the dash.

If you have any questions about the electrical, I can help.