06 Tahoe Transfer case issues

[salvo2002]

06 Tahoe 5.3L with NVG 246

Symptoms: In 2HI with no load (e.g. traveling downhill) it seems the transfer case drops to neutral. Transmission is still in drive and no torque is being delivered to the rear wheels. Just 'revs' the engine. Light on the switch still shows 2HI.

I remedy the situation by switching to A4WD. It switches fine and also functions properly (auto engages the front when the rear slips). I can also switch to 4HI which functions properly as well.

I can then switch back to 2HI from either A4WD or 4HI and it will engage and function properly until I travel downhill and not apply any acceleration.

No service lights or codes.

I replaced the 4WD selector switch module in the dash. Still have the same problems. Next on my list is the encoder/motor. Seem about right? Or is there something else that might be the issue?

 

[IllogicTC]

We might be able to help here. Most of us have a GMT360/370/305 vehicle, and the 226 is pretty much the little brother to your 246.

This only happens under coast conditions? I'm also wondering if the encoder is going to pot. I would think it should trip a service 4x4 code, but I'm not sure if the encoder is actively monitored unless movement is commanded to determine if it just kind-of wanders by itself, and if it doesn't monitor while just sitting there not being instructed to move, it could wander all over. The other thing may be something in the shift linkage is getting loose, 2HI sits pretty much on the cusp of neutral -- rotate one way for clutch application in 4HI, rotate the other way and it'll disengage the HI gear (neutral), and then engage the LO gear in the planetary. If the linkage is worn or in some other way damaged, I would imagine the encoder could be parked just fine and the HI gear may "wander" out of its proper position unnoticed.

While we're at it, what's the transfer case service history? AutoTrak II replaced every 50k miles?

 

[salvo2002]

Thanks for the reply IllogicTC

We might be able to help here. Most of us have a GMT360/370/305 vehicle, and the 226 is pretty much the little brother to your 246.

This only happens under coast conditions? I'm also wondering if the encoder is going to pot. I would think it should trip a service 4x4 code, but I'm not sure if the encoder is actively monitored unless movement is commanded to determine if it just kind-of wanders by itself, and if it doesn't monitor while just sitting there not being instructed to move, it could wander all over. The other thing may be something in the shift linkage is getting loose, 2HI sits pretty much on the cusp of neutral -- rotate one way for clutch application in 4HI, rotate the other way and it'll disengage the HI gear (neutral), and then engage the LO gear in the planetary. If the linkage is worn or in some other way damaged, I would imagine the encoder could be parked just fine and the HI gear may "wander" out of its proper position unnoticed.

While we're at it, what's the transfer case service history? AutoTrak II replaced every 50k miles?

Cool, I also have a 2003 TB with the little brother 226.

Yes, it is strange, I've read all over this site for the various problems that occur with the 226 and even the tech docs on offroadtb.com, and it doesn't seem to fit into any of the common ones.

It only happens when I am traveling downhill. I can coast (foot off the pedal) on a flat road in 2HI and it won't disengage (I can also switch between A4WD, 4HI, & 2HI no problem). As soon as I take my foot off the pedal to coast down a ~15-20% grade hill I'll hear somewhat of a howling noise that seems to be coming from the front end (even thought to me that doesn't make any sense) for a few seconds and then it is somehow in neutral, or some other position where if I try and accelerate it just revs the engine. When I try and put it back into A4WD or 4HI, I sometimes hear a grinding noise, but its very rare; I can't seem to pinpoint what situation causes the grinding (i.e. the order of buttons I push or traveling forward or backward when trying to re-engage).

Something I forgot to mention in the first post is that I can't get it to engage into 4LO; I followed the procedure in the manual.

I bought the truck with 79k on it, was able to get around to change all the fluids (front & rear diff, t-case, and transmission) at 83k. T-case fluid looked fine on the way out, a little dirty but could still notice the blueish tint. Currently has 87k on it.

Would I need to take the t-case out to replace the shift linkage?

Edit: Looks like I have the NVG 246 EAU since I have the RPO code "NP8", which from here shows the following differences:
■ A new encoder motor for faster operation in the AWD mode.
■ The control actuator lever (3) is a new design with different cam angles.
■ The shift detent plunger and spring is no longer used.
■ The clutch assembly (1) uses a new style return spring and clutch washer.
■ A new rear output shaft (2) no longer uses a retaining ring by the oil pump.
■ The range shift fork (4) is a newer design.

 

[IllogicTC]

Interesting. Those design changes I would expect would be to address reported issues (or perhaps it's cheaper to manufacture this way)...

There's only one thing that would grind in the transfer case, which would be the planetary gearset. There's also a chain drive that runs over to the side (but that's permanently engaged, ideally), and then the clutches, and clutches don't grind. I would pop the fill plug on the transfer case and try nabbing a sample of the fluid, and inspect for metal particulate.

The noise may sound like it's coming from up-front but may actually be happening right under you -- it's just that in vehicles the glass seems to be the weak-point for sound, generally anything above an econo-box these days will have some sound-deadening material behind the dash and under the carpet and in the doors as a cheap solution to provide the idea of luxury and quality, so a noise right under your feet may instead sound like it's coming from elsewhere.

I personally don't even know how to go about checking the linkage. It's all internal to the transfer case, and yeah it'd have to be removed because the t-case would need to be opened up. At that point it'd be a crap-shoot between buying parts to rebuild, or just getting a different unit out of a junkyard, both would probably end up being the same price in the end but a unit off a junker would be out-and-in rather than dealing with disassembly and reassembly also. I'd say check that fluid first thing, maybe do another full fluid change so you can get the "full picture" on what metals may or may not be present. If there's shavings, I'd say you need a different t-case as your planetary gearset may be getting ground down.

 

[salvo2002]

Thanks again for the reply.

We just had a ton of snow here (Papillion, NE) so hopefully I can get a chance sometime this weekend to pop the fill plug and check. I've been driving around in 4HI with no problems at all, so still really confused at what the problem is. Crossing my fingers I don't need a new t-case.

Dealer wants $120 just to look at it and I don't even know if they'll tell me exactly whats wrong with it. So its a crap shoot, I might just buy a new encoder/motor for $200 and replace it myself to see if that fixes it.

I have noticed while driving around in 4HI I hear a different "howling" noise from what I mentioned in post 3, almost like I need new hubs/bearings. Correct me if I am wrong, but wouldn't I be able to hear that even if I wasn't in 4HI?

 

[IllogicTC]

There's more stuff moving with 4HI. If it works like the TB/Envoy, the specific parts would be the front output on the transfer case, the front prop shaft (so, at least one more bearing and two u-joints so far), and then the pinion and carrier in the differential.

It'd be worth following up with the dealer if you're comfortable spending the money, talk with them and see what all's included in that price. If it's just a code scan, forget it. If it's actual diagnosis where they at least find the problem (and of course they'll recommend a fix because they'd like your repair bill money too) that'd be a start.