Nuts & Bolts: AutoTrac Transfer Case DeletePosted in How To: Transmission Drivetrain on September 17, 2016
I have built up my 2006 1500HD Chevy with a 7-inch BDS lift, 35-inch Mud Grapplers, a Detroit Locker, Cognito steering, 6.0L tuned up a bit, and a 4L80E transmission. The truck has been very reliable but does have a weak link with the AutoTrac transfer case. I have gone through a few sets of clutches, and I have even bought a brand-new complete T-case right from GM, but it still didn't last very long. I never use the auto 4WD feature, but I still seem to go through them. I would like to swap to a transfer case without any clutches or at least one that doesn't use the clutches once it is locked in 4-Hi or 4-Lo, but I'm not sure which one would be the best bang for my buck. I have thought about an Atlas, but I think that is overkill for what I need. Which factory T-case would be the easiest to swap in? I have no problem cutting a hole in my floor for a true manual shifter, and I have performed drivetrain swaps before on older, simpler vehicles. I am unsure how to make it work with the ABS (which I need for our long brutal winters up here in northern Canada) and to keep the computer happy so I don't get a dashboard filled with warning lights.
I have no plans on selling this truck, so I want to make sure that I do this right so I can continue my overlanding, mudding, trail riding trips, long highway drives, towing duties, and daily driving that I put my truck through. Can you point me in the right direction?
The NP246 AutoTrac transfer case has a checkered reputation for durability, especially with a modified truck such as yours. Fortunately, converting to a true part-time transfer case is fairly easy. You won't need to go the route of an Atlas, which is a great transfer case but, as you said, is probably overkill for your needs.
AutoTrac was an option when your truck was built, so there are similar Chevy trucks to yours equipped with both pushbutton and floor-shifted transfer cases. These trucks were equipped with NP243 (electrically shifted) and NP241 (floor shifted) transfer cases, and both of these are readily available in junkyards everywhere. We have always preferred floor shifters, so we would recommend sourcing an NP241C from a similar year (2003-2006) Chevy truck. You can use one from an older truck, but staying close to your vehicle's year will net you components that will be more compatible with your truck's existing parts and electronics. You will also want to grab the front driveshaft, shifter mechanism, floor bezel, VSS, and shifter switch wiring pigtails from the donor truck. Keep in mind that your 4L80E will have a different output shaft than a 4L60E, so be sure your donor has the right transmission to avoid needing to swap the transfer case input gear.
Once you have the parts in hand, the mechanical portion of the swap is pretty much a remove-and-replace affair. You'll need to cut a hole in the floor for the shifter, but there should be an indent already in the floor to take the guesswork out of where to cut. There are reports that you can even make the original front driveshaft work, but having the front shaft from the donor truck is a safer bet.
Wiring, on the other hand, may be a little tricky. First you will need to address the VSS, which is located on the tailhousing of the transfer case. Depending on the year of your donor transfer case, the VSS plug in your truck's harness may or may not match. If they don't match, you'll need to splice the wires in your truck's harness to the pigtail you grabbed from the donor truck. The wire color codes themselves (green and purple) should match regardless. What remains is telling the computer that the truck is in low range and controlling the actuator for the front differential. The computer needs to know the truck is in low range to control the transmission shift points. Assuming your donor is close to the same year as your truck, all the functionality you need will be built into a switch on the NP241 near the shifter. This pigtail will have three wires: blue, white, and brown. Ground the blue wire. Unplug the TCCM behind the dash and run a lead from the gray/black wire on pin F7 (if you have a factory service manual) to the white wire. Lastly, connect the brown wire to the light blue wire on the front differential actuator. This should make everything function normally and automatically when you shift the truck into 4WD. If your donor NP241 doesn't have this three-wire pigtail, then you'll want to run the gray/black wire on the TCCM connector to a switch on the dash that goes to ground in the “on” position. Grounding this lead tells the ECM that the truck is in low-range. You'll need to remember to flip this switch every time you put the truck in low-range or it will not shift out of First gear. You can run a separate 12-volt-powered dash switch to the light blue wire on the 4WD actuator that engages the front differential. Or you can go with a full manual actuator such as the 4x4 Posi-Lok (4x4posi-lok.com). Regardless, once you're done you'll have a much more reliable transfer case without a viscous coupling to worry about failing down the road.