Gm auto track transfer case question

Striper80 · #1 03-14-2017, 08:26 PM

I know Ferm is pretty knowledgeable when it comes to gm, among other things, so I figured I'd post here. I have an 05 Sierra 1500 non HD with an auto track transfer case. I've owned the truck since new and had very few problems over the years. Last year I started getting a service 4wd message once in a while that would clear out after cycling the ignition. After a little while the truck got stuck in 4 wheel. I tested the switch and thought I had a bad one but apparently I just screwed up testing it, because it was fine. I asked the mechanic next to my shop if he could check the codes for me so I could look into it further. He checked them, said he wanted to check it a little further for like $100 so I said sure. I didn't want to throw parts at it so I obliged. Well one thing led to another and I wound up spending a lot of money on parts that in the end I could've just bought and swapped myself.
In the end I thought it was fixed but with today's snow I noticed I have no 4wd. The lights on the dash change and auto 4wd works but 4lo and 4hi have nothing. Anyone here have a clue?

oteps · #2 03-14-2017, 08:43 PM

What did the shop replace? If im correct there should be the switch on dash, motor and a clocking ring on t case. My dad has a Tahoe that's in an auto electrical shop now because it goes into 4 hi when u start it up. Plus the dash gauges went crazy. THey said u should put it in 4 x 4 often and it will keep everything moving and working good.

Striper80 · #3 03-16-2017, 08:30 AM

He wound up replacing both actuators, the 4wd harness, and computer. I wound up paying for just the parts. I didn't want to pay, that stuff doesn't all break at the same time, but it seemed to be working.

THEFERMANATOR · #4 03-16-2017, 11:28 AM

Pretty sure the late model auto trac system uses only the encoder motor at the transfer case. Early ststems also had an actuator on the front axle, but I thought around 02 or so they did away with it for full time capable systems. The most common failures on that system are the switch. The encoders fail every so often, but not a common failure part by any means. The encoder motor at the transfer case consists of a high torque motor assembly, and an encoder switch that relays the position of the motor shaft to the tccm(transfer case control module). It has a 2 hi, 4 hi, and 4 low position as well as when in 4 high, it is also used to engage the clutch pack for the full time operation(it is an on demand system that is electronically controlled to engage the front axle unlike most other systems that used a viscous bias differential). If it has the front axle disconnect, it is a very simple setup. It has a screw motor that runs out a piston when engaged, that piston puts pressure on a spring that in turn connects it to the actual plunger that engages the disconnect. And there is also a switch inside of it to send a signal back to the tccm to tell it when the piston is extended. If the correct tccm was put back in it, then the system is really good about siagnosing itself and knowing what is and isn't engaging. The speed sensors on the transfer case provide good feedback to tell the tccm if the shafts are actually engaging or not. With that many parts being thrown at the problem, I would be very concerned if the correct parts got put back in. The np-246 and np-263 actuatorslook VERY similiar to each other, but do not interchange. And the tccm's are all the same size and I believe areeven the same part numbers, but the programming is different depending on which style it is. Buying a replacement from a junkyard, they will sell you either style tccm, it isup to you totake it to a dealer for programming.

So you're saying auto 4x4 works, but not 4hi or 4lo, BUT the lights for them change?

Striper80 · #5 03-16-2017, 01:14 PM

It has the motor on the front axle. All lights change on the dash and you hear the encoder motor changing the gears in the t case, It even goes into 4lo, but the front axle only engages in auto. I have no service 4wd message on the dash. Everything that was replaced was ordered directly from gm and is brand new.
The guy that did the work does all the work for the body shop I work at. He looked at it again but now wants to wash his hands of the job. I'm pissed that I painted a bumper for him and didn't half *** it.

THEFERMANATOR · #6 03-16-2017, 10:29 PM

Was the tccm programmed by the dealer? Pretty sure they come blank, and require service programming. When you say it goes into 4 low, does it go into low range? Trying to figure out if your issue is at the front axle or the transfer case. If it shifts into 4 low, does the driveshaft lock in and just not the front axle? There was an issue with the autotrac cases internally where the shift fork would wear out from running in auto 4x4. When it's in auto, it actually runs in 2 wheel drive, when the tccm see's a difference between the front and rear driveshaft speed, the tccm tell's the encoder motor to start applying pressure to the shift fork that engages the clutch pack in the transfer case to lock the front and rear shafts together. It's not a simple shift, it actually uses the shift motor to apply and hold pressure to engage the clutch pack via the shift fork, and overtime that fork can wear, and some have failed.

Striper80 · #7 03-17-2017, 04:48 AM

The tccm was programmed by the guy that installed it. The worn shift fork seems like a real possibility. Once the snow melts a little I'll look into it further.
One thing to note is if I leave it in auto, then shut the truck off, the light on the dash goes to 4wd when I start it up. When left in 4wd, it goes to auto on start up.
If the case needs a rebuild how difficult is it? Any special tools? Seems a little more complicated than a reg t case without clutches.

THEFERMANATOR · #8 03-17-2017, 07:31 PM

The light changing on startup would make me think tccm. The tccm reads the position of the transfer case via the encoder motor, then the tccm turns on the appropriate light. If it's changing, that would mean the tccm is either reading the encoder position incorrectly, or the encoder is bad. I seen dorman is now selling a replacement encoder because the whole assembly is so expensive, and the encoder's do fail semi often(the most common failure in that system is the encoder). As to repairing the transfer case, I've never been inside a np246. The only full time cases I rebuilt were the np242 and np249, but they were a much more basic viscous clutch system.

Striper80 · #9 03-19-2017, 10:37 AM

Was just under there changing my oil so I cycled it through a few times. The front shaft is locking and unlocking. Also didn't change when cycling the key. I couldn't jack it up today since my time was limited.

THEFERMANATOR · #10 03-19-2017, 11:53 PM

Quote:

Originally Posted by Striper80

Was just under there changing my oil so I cycled it through a few times. The front shaft is locking and unlocking. Also didn't change when cycling the key. I couldn't jack it up today since my time was limited.

But that doesn't tell you if it's the front axle locking in, or the transfer case without jacking a tire up and spinning it.