2000 bravada smarttrak binding / engaged 100%

[matt_o_70]

Hello,
just acquired a 2000 bravada. I notice the vehicle struggle when backing out of the drive way as soon as you turn the wheels, I assume this binding is the AWD engaged when it should not be. The same thing happens when going forward and turning the wheel sharply. the previous owner said that this was "normal" I know better, and am looking for help to correct this.

Can anyone who had the same problem comment? I don't yet have a service manual to fully understand the system. I believe that the engagement of the smarttrak happens at the transferr case? and that the encoder motor is responsible for this process? If this is true, I am guessing that my transferr case is engaged 100% of the time rather then on command when rear wheel slip is detected.

If I jack up the front of the vehicle and turn the front axel it should rotate freely? if not can I pull the encoder motor and then check that the front axel should then turn freely.

I am assuming the front differential is a simple diff and nothing fancy going on inside?

thanks for any help.
Matt

[Chris Stewart]

Here's a thread with your symtoms, the last post is relevent.
http://www.automotiveforums.com/vbul...d.php?t=651801

An option to determine whether your clutches are stuck is to remove the AWD fuse then test drive.

[matt_o_70]

thanks for that link Chris.

what is powered by this fuse? I want to understand what am I really testing?

I did change the fluid and it seems better, I still have to put some miles on it.

I would like to know is the motor there to change / vary the amount of force applied to a clutch pack? or is this an on off type of arrangement?

I would like to test the transfer case motor but see that it says "running motor off of the transfer case will damage it? Is there a way to test the motor with out damageing it?

what is a normal range of motion for this motor?

[Chris Stewart]

My knowledge of the encoder motor is severely limited...I didn't even think there was one on a '98 & newer Bravada and I'm still not convinced because I think there's an electro-mechanical clutch that can do all the engaging and disengaging.

Pro's, please correct me if I'm off base here!!!

The Automatic Transfer Case (ATC) fuse is supposed to provide power and removing it should stop the automatic function(s).
BIV wired a switch on his and the Pro's say change the transfer case fluid. If I buy a '98 & newer Bravada ( wife misses the '97 terribly), it'll get a coupla transfer case fluid changes with the ATC fuse removed then if snow ever returns to Middle Tennessee, I'll plug it back in.

[matt_o_70]

Chris,
there is a motor on the side of the transfer case. It looks just like the encoder motors. online web site's call it a transfer case motor. I do not see an encoder/speed sensor hanging off the bottom of this case so it may be as you indicate a simple motor to engage the transfer case (see "Dorman part# 600-906").

I thought that the smarttrak was "This system drives the rear wheels until slip is detected, then automatically engages the front axle to maintain traction,"

My thinking is if the motor is bad it is now stuck in one position. So how can I test the motor? and transfer case? Can I manually turn the shaft on the transfer case to some home position, to disengage and then test drive? and then turn the shaft to another position to engage and test drive?

At the same time with this motor off test to see if it is working, with out damaging it? (the case of the motor has a warning not to operate it off of the transfer case)

I still can not find any really good description of this system operation, I checked the hayes manuals that the stores sell and there is ZERO info on the transfer case operation. Does anyone have a factory manual to help?

I did chang the fluid and it did make a difference, I will continue to drive and see if the fluid fix really holds up.

[Chris Stewart]

Matt, I stand corrected through some kindly info passed to me (thanks Blazee!) that there is an encoder motor along with some other info.
It looks like if there's an electrical failure with the encoder motor, there should be a Service AWD light come on the instrument panel. If there isn't one then maybe it's "stuck" and your recent transfer case fluid should help that along. Testing for being stuck "on" should be as easy as raising one front wheel off the ground and seeing if it will turn fairly free.
You are correct with the operation with your Smartrak, sensors at the front and rear of the transfer case supply info to the transfer case module which then activates the encoder motor to engage or disengage the gearing.
If it's still engaged, it'll need to be disengaged to keep from damaging it further. I'll see what I can find.

[Chris Stewart]

One of our pals, Detectingcoins performed a trick. The info shows some of encoder motor input to the transfer case.

http://www.automotiveforums.com/vbul...d.php?t=656997

[matt_o_70]

Chris,
thanks again for the help. I don't have any indicator lights on.

I am new to GM's but not to 4x4 (I worked for toyota for 7 years). So right now I am still trying to learn the differences between these systems.

Do I read that the autotrak cars have 3 positions 4lo, 4 hi, AUTO? These transfer cases must then be different? I don't have a low range on the bravada.

the transfer case I have says New process gear 136 GM, What is the unit in the 3 position cars?

If i understand the smarttrak AWD, such that it is supposed to be 2wd most of the time then engages transfer case to drive front wheels as needed. I don't think the front axel then ever unlocks (no actuators). I have not looked for any acuators up there yet- they may be there? when I did the fluid change the front end was locked up such that the only way I could spin one wheel is with the other side off the ground so that it could spin the other side.

with all 4 in the air i could turn the front and it would turn the rears. this is what has me thinking that it is stuck engaged. Now to be fair I do not know how this "should" be working.

Is the NPG 136 a viscous coupling rather than a chain style transfer case?
that would explain why the fluid change then gives dramatic improvements to the problem. I suppose it would have to be a viscous center differential to compensate for dry pavement AWD situations?

just confirmed "the difference between the AWD t-case (NVG 136) and the other two (NVG 233 & NVG 236) is that the AWD has a viscous coupling between the front and rear axles where the other two are direct coupled. " http://www.blazerforum.com/m_17427/mpage_1/key_/tm.htm

so the Astro/safari are really where else this AWD system lives.

[Chris Stewart]

Matt, the trouble free viscous clutch transfer case is the Borg-Warner 4472 and was installed in the '91 to '97 Bravadas and many, if not all years of the Astro/Safari vans. It has a chain but no engagement gearing...the gearing is put together engaged with the necessary slippage, to keep from binding the drivetrain, being provided with the viscous clutch.
As far as I know, no engagement of front axle/differential is available.
My understanding of the NVG 233 & NVG 236 is the same as yours.
From Detectingcoins description, it looks like the NPG 136 transfer case engagement could be manually set to neutral by removing the encoder motor and turning the gear slot one notch although I wouldn't attempt to apply great force in fear of breaking parts. With your previous experience, this procedure with pictures could be very helpful to the rest of the gang.

[lesam1]

My husbands 99 had a kind of grinding noise when we bought it and they said everything was fine. NOT! We started out thinking it was the transfer case shudder that alot of the post talk about, but it turned out to be the wheel bearings on the dirvers side were going bad. HOpe you have better luck than we did.

[matt_o_70]

these cars definatly need a thread with some trouble shooting and problem resoloution for everyone to share. This topic comes up alot - it is clearly a problem spot with these cars. I do not

I have a 2000 bravada - it is also "stuck" in all wheel drive at all times. You can read my earlier posts. Even just driving on the highway I was having to keep on the gas pedal to keep speed. Here is what I have done so far.

after reading about the fluid change - I gave this a shot - after the fluid change it was noticable better but by no means fixed. My guess is that the viscious clutch additive package in the fluid is exhausted by working fulltime - so the fluid change gives slight improvement. In my case when I got the car it would click audibly before with the old fluid. the noises completely went away after the fluid change.

In my 2000 the fuse panel inside the drives door area has a fuse labled "4WD" I removed this fuse as mentioned in other posts. Then drove the car. INSTANTLY I notice the car feels like it should in 2WD. this I view as good news. It would make sense that the encoder motor is working. It is engaging the clutch pack with the fuse installed and not engaging with power removed. It is worth mentioning the with the 4WD fuse removed the "service AWD" "Battery" "fuel Gauge" and the "temp gauge" all did not function or showed a problem. All gauges returned to normal after reinstalling the fuse.

The next step is to get to the bottom of What the transfer case sensors are seeing causing the encoder motor to engage the transfer case motor.
1) tire PSI is set.
2) The tires on this vehicle are evenly worn but well worn. I will measure them all to get a better sense of the actual individual diameter. Can anyone comment on how much variation is allowed. There has to be some allowance for things to function normally in general use and in a turn. I was a professional tech so I have seen my share of worn tires - no GM exposure prior to this car.
3) Can anyone who has had a bravada long term comment on previous tire changes? Are these cars overly sensitive to normal tire wear?
3) I have noticed that the rear springs on my vehicle are sagging? Can anyone provide the OEM ride height specs? I do not believe that this sag is the cause - but I will address it.
4) I have read that 2000 bravadas had limited slip rear diffs? Is this standard on all or an option?

For the GM techs - Is there a common failure of the system that will not set off the service awd light? What test can be done to verify?

DOES the NVG 136 have 2 speed sensors - One for each drive shaft? IF SO I ASSUME that it compares front input to rear input and engages when the difference goes beyond a certain threshold?

Will the dealer scan tool show live data streams for these TCCM inputs when test driven?

Matt