4WD not functioning correctly

[EvoSpyder]

Ok. So a couple days ago my 4WD went out. I was able to go through the guide and flow charts, but still not exactly sure what is going on, and haven't had a chance to pull things apart yet. Money and time are tight right now, so looking for a little help getting to the next step.

'07 TB LT

Here's the situation. I'm only getting power to the rear wheels. No service lights. Selector switch will not stay lit on anything other than 4HI or A4WD. Attempts to go to other positions will flash and return to 4HI.

The front actuator stays engaged, as the TCCM thinks it's in 4WD, and will only make noise if I full the #8 fuse. It makes noise upon pulling it, and again upon replacing it. Lifting the front passenger tire up, leaving all others down, i get the wheel to turn and the front propshaft also turns.

I'm not getting any noise from the front while trying to go from 4 to 2, and underneath I can hear a click, followed by a low hum from what sounds like a bound encoder motor. If it was actually bound in the 4HI position as it thinks it is, I should be in 4WD and unable to go to 2HI. Since the vehicle is mechanically acting as though it is in 2HI, but electronically thinks it is in 4HI, what are the chances of it being an electrical (encoder motor failure), or internal mechanical failure (TC clutches?) I've checked the electrical circuit best I could, and have not found any signs of that being the issue.

Any thoughts?

 

[The_Roadie]

You may indeed have two layered problems, one or two mechanical and zero or one electrical.

What's the mileage, maintenance history, how long have you owned it, and has the transfer case fluid been religiously changed every 50K? What's the current state of the level and nature of the TC fluid?

 

[EvoSpyder]

We bought it from the dealer about 40K miles ago (around 120,000 now). I haven't personally changed the fluid, dealer service record indicated it had been drained and filled during their service when they also put on new brake pads and new tires at the time we purchased the vehicle.

It's mostly my wife that drives it, and being new to 4WDs it's been mostly left in A4WD. I'm relatively new to the whole electronic 4WD scenario. My other vehicles have always been AWD, and the only 4x4's I've spent time around still had locking hubs.

 

[IllogicTC]

One thing I should mention, don't leave it in A4WD if you don't need it. Lots of reports of it tending to wear out the clutches in the transfer case a lot quicker (plus a small reduction in gas mileage under "normal" road conditions, varies by use). Bummer is those who got the "AWD" versions of the 360 platform (I think except the TBSS) got stuck with a transfer case that does only this, be glad you don't have that at least.

 

[Playsinsnow]

http://gmtnation.com/f79/4wd-not-working-step-inside-107/

I work overnights but not tonight so I had some time on my hands. I posted this link for future searchers as it is virtually the same write-up i used that any oil change DIYer can follow with a "competent" assistant.

OP: You didn't state which fluid they changed. The transfer case fluid, autotrakII, should be changed every 50k miles religiously. The differentials vary, some say 50-100k, but I do it every 40-45k with the TC. Please check the condition of the fluids if you never have yourself.

The pics are of the disconnect. If you have an assistant with you, checking the 4wd is SO much easier. Crawl under in 2wd, safely!, and turn the ignition to accessories but do not start.

Two sounds. Do you hear anything immediately? Secondly, and highly suspect, is the actuator for the disconnect. It is attached to the small hole at 4o'clock in first pic. Pull that out and leave it plugged it but away. Turn it to 4hi and jam a tool or screwdriver in there until you physical feel yourself push the fork an inch in. There is a small spring, any man with two working arms can do this. With the screwdriver pushed in have your assistant spin the front driver wheel forward.

Does the passenger wheel spin in the opposite direction? Yes? Then it is your black actuator. No? Then it is the disconnect or other easily diagnosed by flow chart, search 4wd not working step inside.

The first pic is what it looks like from the wheel with tge actuator and cv axle removed. The second are the "guts", and the third is a closeup.

I have a feeling mine just need some quality lube. It was not like this but dry and dirty. I replaced the whole unit as I did not want to get in over my head with a rebuild having never done so. I'm holding onto this
Oh yeah and I kept my original actuator to save $$ and it still works.Might be slowing down a bit as I have a 2second delay. Tops.

Crawl under and locate the noise, or lack thereof, is the encoder motor or actuator.

 

[EvoSpyder]

Ok, I was trying to be clear, but must not have been as clear as intended. First of all, I'm an automotive powertrain engineer, but have so far focused mostly on engines and forced induction systems. I have a lot of experience on AWD cars, but haven't played around much with 4x4's.

http://gmtnation.com/f79/4wd-not-working-step-inside-107/
OP: You didn't state which fluid they changed. The transfer case fluid, autotrakII, should be changed every 50k miles religiously. The differentials vary, some say 50-100k, but I do it every 40-45k with the TC. Please check the condition of the fluids if you never have yourself.

When I bought the vehicle, I had the dealership service all fluids including transfercase and differentials. This vehicle was going to mostly be for my wife, and I didn't have any attachment to it at the time. I've always done all of my own work on vehicles, except for this one, and now I'm really regretting not being as involved as I am with my other vehicles.

The pics are of the disconnect. If you have an assistant with you, checking the 4wd is SO much easier. Crawl under in 2wd, safely!, and turn the ignition to accessories but do not start.

Two sounds. Do you hear anything immediately? Secondly, and highly suspect, is the actuator for the disconnect. It is attached to the small hole at 4o'clock in first pic. Pull that out and leave it plugged it but away. Turn it to 4hi and jam a tool or screwdriver in there until you physical feel yourself push the fork an inch in. There is a small spring, any man with two working arms can do this. With the screwdriver pushed in have your assistant spin the front driver wheel forward.

Does the passenger wheel spin in the opposite direction? Yes? Then it is your black actuator. No? Then it is the disconnect or other easily diagnosed by flow chart, search 4wd not working step inside.

The first pic is what it looks like from the wheel with tge actuator and cv axle removed. The second are the "guts", and the third is a closeup.

I have a feeling mine just need some quality lube. It was not like this but dry and dirty. I replaced the whole unit as I did not want to get in over my head with a rebuild having never done so. I'm holding onto this
Oh yeah and I kept my original actuator to save $$ and it still works.Might be slowing down a bit as I have a 2second delay. Tops.

Crawl under and locate the noise, or lack thereof, is the encoder motor or actuator.

As I explained in my OP, I have no noise from the front, however, I cannot get into 2WD from the switch, due to the encoder motor being bound in what it is sensing as the 4HI position. Therefore, I believe it is failing before it tries to engage the front actuator, thus no noise. I'm thinking the encoder motor, as it certainly is acting as though it has, but also thinking I have mechanical issues internally in the TC, and this might be what is causing the encorder to be bound. Guess I won't know until I pull it off and see, once I get some time.

 

[linneje]

It sounds like you are doing a good job with the diagnosis. Let us know how it works out, as your problem can help others with similar problems once you get it fixed.

 

[The_Roadie]

It's mostly my wife that drives it, and being new to 4WDs it's been mostly left in A4WD.

That may be your root cause. It's been reported that the NP226 transfer case that you have (with the LO range) may kill their clutches with as little as 5K miles of full-time A4WD mode. Members have reported melt-downs with 10K of wifely abuse exactly this way. 40K would be extraordinary!

The NP126 transfer case used in the full-time AWD models like the Bravada are designed for this full-time use, and the hardware design may contribute to the success, and the software algorithms in the TCCM may be another contributor. I'm guessing a bit less torque bias when there's no wheelspin keeps the Bravada running longer without destroying their clutch packs.

The fluid change interval is critical on both the NP126 and 226 transfer cases, but avoiding excessive and blind use of the A4WD mode is also essential.