February 03, 2013

Ignorance is Bliss - A Tale of a Transfer Case

( I'm not changing the direction of this blog to be a mechanic site, it's just a good place to vent, and detail information as much for me as the next guy who's doing the same research. )

There's a lot of truth in the saying "ignorance is bliss"; I'd rather not have to know anything about the transfer case under my 1999 GMC Suburban 6.5L Diesel.  I'd rather it just worked.  Unfortunately, it doesn't, and I'm trying to repair it for the least cost possible.  I'm already in the hole for $200 (+ install) for an encoder motor, which wasn't needed, but that's part of the troubleshooting process, right?  Right?  Yeah.. I'm a little bitter about the encoder motor.

Credit: HowStuffWorks.com
Anyhow, for my folks, who may not know what the function of a transfer case is, I offer Wikipedia, and a short explanation.  In a rear-wheel drive (RWD) vehicle all the power of the engine goes to the back wheels along a central drive shaft, and power is transferred to the rear axle by the differential, located at the T between the drive shaft, and axle.  The transfer case is a box with gears in it, located right behind the transmission, which is right behind the engine, and transfers power to another drive shaft which points forward to the front axle, and another differential at the T between the front axle and the front drive shaft.  It's that simple.

When I bought the Suburban, I specifically asked the previous owner, who's son is a local aviation mechanic (which worries me more than a little), if he'd changed the fluid in the transfer case.  Yes, he said.  I asked if it worked, he thought so, he was driving around with it in 4WD-Auto. He did say when putting it in 4WD-Lo, it made a clunk that he was worried about.  No problem, I said - we'd figure it out; as long as it was working in 4WD, that's all I cared about...  That was last October, right before driving ~1500 km into Quebec and Northern Ontario...  Luckily, my mechanic immediately found that there was absolutely fluid in the transfer case - none at all.  So he topped it up, and I took it for a spin.  It made sounds when I put it into 4WD, but I couldn't tell what kind of power I had to the wheels.  From the sounds that changed, I thought it worked.  In hindsight, I had no reason to believe that other than wishful thinking.  While in the middle of nowhere at the end of October, I knew I would only put the vehicle into 4WD if I absolutely had to, since I wasn't sure if it would come out of 4WD, blow up, or whatever.  I was very careful NOT to use it for the entire trip - good thing too, because it wouldn't have worked...

Anyhow, we fast forward to winter in Ottawa, after the first freeze, thaw, freeze when the driveway turned to ice.  I was backing the Suburban up the driveway, which has a slight incline, when I lost traction and couldn't make it up the hill.  No problem, I hit the 4WD-Hi button and the little 4WD-Hi light lit up.  I tried again, no luck.  I stomped on the gas, spun the tires, and still nothing.  So I got out and checked my tread marks.  I was only getting power to the rear, even with the 4WD-Hi button pressed, and the light lit up - the light was lying to me.  Bastard.

My mechanic mis-diagnosed the issue as the encoder motor, so I bought one from Summit... sadly, that didn't work.  So now I'm looking for a whole transfer case.  But which one?   Off to research the issue on the internets...

I had no idea there were so many different options for transfer cases in GM 4WD trucks.
www.car-parts.com helped me figure things out somewhat.
I have a push-button operated transfer case, with a button marked "4WD-Auto" - so  simply put, I need an "automatic" transfer case.  Which one?  The NP246 is what I have under the vehicle.  This I can tell from the NP8 RPO option as shown on the RPO sticker in the glove box, and I have a 32 spline input shaft (not the 27 spline input shaft) because I have a heavy duty 4L80-E (rather than the lighter-duty 4L60-E) transmission.

Prices range from ~$300 for one that just fell off a truck (literally) to ~$2000 for a fully rebuilt GM warrantied one, plus install.  It seems the previous seller isn't eager to help out on this one either, despite there being a lack of fluid in the transfer case when I bought it.  I'll chalk it up to an expensive life lesson I guess?  Once it's fixed I'll bring up the topic again.  Buying a used vehicle as-is, means it's as-is, no matter what was said, so I have no legal recourse.

One option is to say screw it, and leave it as a 2WD vehicle.  Unfortunately, that's not really an option.  The vehicle is almost 7000 lbs, and just from going up and down the driveway I can tell it needs a 4WD option to get out of trouble; and trouble is what I expect I'll get into in the middle of Northern Quebec or Northern Ontario.  Before making that trip, I need 4WD to work again. 

I have found an NP246 for sale ~3-4hrs from Ottawa, unfortunately it has a 27 spline input shaft that would need to be swapped for a 32 spline before install.  That's a problem, as the transfer case currently under the Suburban is... under the Suburban, and unavailable for take-apart.  Hmm.  Still working out that logistically.
NP246 Transfer Case|
Photo Credit: Pirate4x4.com

NP246 Transfer Case|
Photo Credit: Pirate4x4.com

NP246 Transfer Case|
Photo Credit: Pirate4x4.com


  • Break apart diagrams of the NP246 and parts for sale here 
  •  RPO decoder here

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