Diesel Suburban: Transfer Case Modifications

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As I've previously mentioned, I had decided to use an NV4500 manual 5-speed transmission in my project to install an Isuzu 4BD1T turbo diesel into my Suburban.

I chose to keep the NP246 transfer case that came with the Suburban. The wiring harness and controller were already there, and the transfer case appeared to be in good working order. This would avoid the need to cut another hole in the floor and the addition of a linkage for a manually operated transfer case. If the NP246 gives me any problems, I can switch to one of the other units later.

The NP246 is an interesting unit, in that it is electronically controlled and uses a couple servomotors to engage four wheel drive. It is an Autotrac transmission with pushbutton controls on the dash. One of these buttons is for automatic 4wd operation. In this mode it engages the front axle via an internal clutch pack if slippage is sensed. I noticed that it has a pair of speed controllers to monitor the front and rear output shafts. I believe that if the control unit sees a sudden difference between front and rear shaft speeds, it will engage the clutches, instantly putting the vehicle in 4wd.

Because I used an NV4500 from a 2000 Chevy 2500 4x4, it had the standard GM rear bolt pattern for a transfer case. It wasn't going to fit, though. The automatic transmission that came with the Suburban was a 4L80E with a 27 spline (male) output shaft. So, naturally, the transfer case had a 27 spline (female) input shaft. The NV4500, being designed for heavier-duty service, came with a 32 spline output shaft. I did some research, finding people who had converted GM pickups from this era to manual transmissions. They tended to have other transfer cases in the same family, and remarked that they had switched to a 32 spline shaft that fitted. I found out that Drivetrain.com had a lot of useful information on this transfer case and others, including the input shaft. I've purchased some parts from them, but I found a 32-spline input shaft that was in very good condition on eBay for considerably less.

The transfer case has a magnesium case, so it is quite light. I picked it up, put it on my workbench, and started taking it apart. I had some issues getting the case to separate, until I pulled a rubber plug in the tail housing and opened a retaining clip. When I had it apart, I was impressed with how clean it was inside. I didn't see much wear, either. It was very impressive, in fact, for a vehicle with 151,000 miles on it. I figured that the vehicle had been well-maintained. Perhaps this transfer case had even been replaced at some point.

I carefully removed the chain, the main shaft, the clutch pack, and noted where all the parts came from. Then I saw the fork inside--blocking access to the planetary gear assembly.

I had to buy a very large Allen wrench to remove the bolts that hold the fork from the outside of the case. My local stores didn't have any this size, so I ordered it online. When the tool arrived, I was able to remove the planetary gears and removed the input shaft from the center of them. Here are both shafts sitting next to the planetary assembly. After replacing the front seal, I got out the synthetic assembly grease and started to reverse the process. When I got to the point where I was ready to install the rear case, again, I reached into my supplies and pulled out my case saver. The aluminum pumps are known for rubbing on the rear case and, soft as aluminum is, they actually wear through the magnesium cases, causing a leak. The location of these leaks is in the pockets around the shaft. The case saver is the steel ring shown here that sits in those pockets and provides wear protection to the magnesium case.

Looking at these pockets, again, showed very little wear on the magnesium. Curiously little wear, again, for a high-mileage vehicle. However, with the long life offered by a robust diesel engine, I hope to get hundreds of thousands of miles out of this transfer case. These things cost $40, which seems kind of silly for a piece of sheet metal that was cut and bent to shape, but it's cheap insurance in the long-run.

When reinstalling, I set the case saver directly on the pump as shown here. Be sure not to forget the lock ring that holds the chain sprocket on the forward output shaft on the upper-left corner of this picture. I scratched my head on this one for a bit before I found the lock ring and realized what it was. Be very methodical about disassembly and reassembly. If you get the Allen wrench ahead of time, weeks won't pass during the process, so you'll be less likely to forget anything.

When I was done with this, I was able to bolt it to the NV4500 and trial-fit it into the Suburban as shown.

From this point, I located where I'd need to cut a hole in the tunnel for the shifter. There was already a factory-stamped outline for the cut, but I had to shift the pattern and cut it in a custom location. I chose to keep the transfer case where it was, axially, so that I wouldn't have to extend one driveshaft and shorten the other. Fortunately, this puts the engine in a nearly ideal location. The engine, transmission, and transfer case wound up mounted in a position 1.5" to the right, but the driveline angles didn't change significantly (fellow engineers will note that the cosines were pretty dang close to 1).

Note that these articles go back months in time from the present, because I've only recently begun to write this blog. Right now I actually have all of the hardware installed in the vehicle and am working on the wiring harness. With luck, I'll get the vehicle running and drivable this weekend!

-My recipe for success: free markets, more diesel, and less government!