I recently acquired a 1997 GMC K2500 Suburban with a 6.5L turbo diesel to replace my 1993 Chevy K2500 Suburban with a 5.7L that is worn beyond its miles and years. The GMC needed a little work and I’ve been picking through it. Occasionally around fifty mph with an idle throttle there was a whirring with a very light grinding sound. The previous owner had some ideas but indicated it had been happening for a while and he wasn’t worried about it. However, when applying torque in reverse the transfer case would disengage and grind. I suspected the symptoms were connected.

The Chevy has an NP241C and the GMC a NP243C transfer case. That’s New Process, also known as New Venture. The main difference is that the NP243 is electronic shift. Notably it isn’t autotrac or all-wheel drive, it is simply a push button activated transfer case with 2HI, 4HI and 4LOW. This system signals an electronic motor mounted to the transfer case to shift the transfer case, rather than a shift lever with mechanical linkage. It is really a terrible upgrade, in trade for a little aesthetics we lost the ability to put the transfer case in neutral and got more technology that is going to wear out and break on us.

When I got the Chevy, the transfer case would occasionally shift into neutral when the transmission was in reverse and a minimal amount of torque was applied. I pulled the transfer case and rebuilt it, finding a number of worn out parts. The NP241 and NP243 aren’t that different, so I connected the symptoms and started planning a transfer case rebuild for the GMC. However, the problem with reverse kept getting worse and a couple weeks ago I lost 2HI and 4HI. I managed to limp the truck into the yard in 4LO.

When I rebuilt the transfer case for the Chevy I did the work in my friends warehouse and had access to a lot of equipment. While the warehouse lacked a ramp, we got the truck inside by lifting it up onto the loading dock with two forklifts. Unfortunately I don’t have those resources here in Seattle. I have a pretty well equipped shop in the one car garage of my rental. I probably could have done the work in the entrance to a warehouse where I share metal shop space with some others, but if there had been any problems it would have been complicated to move to truck out of the way until I resolved them.

The GMC has running boards which makes it a little tight to get under it so I started off by putting the truck up on four jack stands. Since I’m working out of the backyard I put a sheet of plywood on the ground under the truck for the creeper to speed up getting out of the truck every few minutes for a different tool. A floor jack under the transmission pan with a board to distribute the weight is essential. I snugged this up pretty early in the process. I also have an ATV/motorcycle jack I used to raise and lower the transfer cases. This sat to the side most of the way through the process though.

My truck had a skid plate for the transfer case that took a T50 torx wrench and another wrench for the nut to remove. The NP243 has a couple more electrical connectors than the NP241. Pull these first and put them safely out of the way to the side of the transmission, but not above or below, so they aren’t harmed while jacking the transmission. Both the NP241 and NP243 have a couple speed sensors up top, but the shifter motor for the NP243 has a couple of its own. Next I drained the fluid from the transfer case and pulled both drive shafts. The front drive shaft can be unbolted at the yoke on the transfer case. The rear has to be unbolted from the yoke at the differential as the connection to the transfer case is a slip yoke. You definitely want to drain the transfer case first, otherwise you’ll get a mess when you pull the rear drive shaft. This is a problem for off-roading because it prevents you from unbolting the rear drive shaft if something goes wrong with the rear axle and driving only the front axle with the transfer case in 4WD. Fortunately this isn’t an incredibly common circumstance.

With the transmission supported I was ready to unbolt the transfer case. Unfortunately the six bolts are difficult to get at. You need a stubby 15mm wrench or you’ll be there all day. I removed the two bolts holding the transmission to the rubber mount then carefully jacked up the transmission, raising the entire drivetrain. Then I unbolted the transmission mount and removed it. This gave me a little more working space and access to the bottom bolt. Then I removed the crossbar that serves as the transmission support and slid it out of the way. I removed the bottom bolt, lowered the transmission again, put the lift under the transfer case to take up some slack and slowly removed the remaining bolts. This may not be necessary as the transfer case hangs well on the input shaft. I slid the transfer case back off the transmission and lowered it out with some wrangling and a friend operating the jack.

I had to swap the sensors and the shift motor over from the old transfer case to the rebuilt one. I had left the truck in 4LO so I had to use some channel locks to change the gear of the rebuilt transfer case to match the shift motor. The rebuild didn’t have a boot on the rear output shaft, which was a little disappointing. Installation was, as they say, reverse of removal. I almost forgot to scrape the old gasket off the transmission and should have done it while the transfer case was still on the ground. I put a half a quart or so of Lucas oil stabilizer in with the ATF to prevent misting. My heart goes out to Lucas for their squirt tip fitting the ATF oil bottles as well.

It took me a minute to remember that despite all the technology I still had to put the transmission in neutral to get the transfer case out of 4LO. Otherwise everything went well, despite it being cold and lightly snowing out in the process. It is nice to have all the gears back and reverse working right. Additionally, the whirring sound is gone, confirming that it was the transfer case slipping and grinding. Out of habit I am still a little easy on reverse, but it’s great having trust in the transfer case again.