Jerking, binding, shudder on slow tight turns.

[dragonvan]

FYI for all of the late model AWD astro owners. There is a common complaint of a jerking or binding feeling on tight turns at parking lot speeds. GM has a TSB on this one and it is a simple fix. The t-case has a wet clutch system in it that engages and disengages the front drive shaft. The friction modifiers in the t-case fluid break down and the clutch plates get grabby when disengaged causing the same feeling as being in 4wd in a sharp turn.

The fix is to drain the t-case and refill with Auto-Trak II. The problem started when GM was using dex-III and AutoTrak and occurred on the trucks with Auto 4wd also. They told us to flush with Saturn transmission fluid (saturn cars) and refill with dex-III or AutoTrak. That wasn't working the best. They then told us to flush with saturn ATF and refill with saturn ATF. About two weeks later they released a new fluid with different friction modifiers and called it Auto-Trak II. The new fluid is BLUE. The official procedure now is to drain and flush with Auto-Trak II and refill with Auto-Trak II.

I have found that this is usually necessary for t-cases still running dex-III or AutoTrak (both red colored). For t-cases that are already running Auto-Trak II you can usually get away with just a drain and refill, YMMV.

The Auto-Trak II turns from blue to a greenish-blue when it has broken down. Depending on driving conditions t-case fluid may need changed as frequently as every 15,000 miles.

The problem affects only '99 and up Astro/Safari vans with the NV136. It also affects other vehicles with the NV136 (Bravada, etc.) and the NV236 and NV246 which have an "Auto" 4x4 mode. Some of the NV236/246 made it into some very late '98 models but obviously this doesn't affect our vans.

The NV136 has been a quite durable design when used for it's intended purpose. As long as you are within the specs for GCVW, tongue weight, and max trailer weight, use weight distributing hitch over 200/2000lbs and elec trailer brakes over 2000lbs there should be no problems. The fluid is only $5 per quart and the t-case takes exactly 2 qt. for a drain and refill. I am in the Great White North, aka Wisconsin, and wouldn't give up the AWD and ABS for anything. Offroading is a different story altogether though. Get a real 2 speed t-case!

If you aren't experiencing the described issue you don't need to change the fluid except for preventive maintenance. When you start to feel it change the fluid to prevent premature wear of the clutch disks. It's a no brainier. I have seen the clutch disk wear out early, mostly in the NV236/246 but the fluid hadn't been changed ever and the vehicles pulled travel trailers on a regular basis and had over 120k miles.

FYI - the t-case as controlled by the PCM is not just on or off, the pressure on the clutch is PWM so as to send no more power to the front axle than need to control slip at the rear and increase gas mileage.

The new blue fluid also fixed a driveline clunk in the full size trucks with the NV246, so if anyone has a truck with those symptoms give it a shot, it has worked on about 75% of them I have looked at and IIRC the others were the early 2000 silverados that had the bad pinion bearings that were redesigned half way through the year.

As always YMMV.

[e7ats]

ANOTHER GREAT WRITE UP!!!!

I will back you up on everything you have said. I drive a 04 AWD cargo that carries a lot of weight and a 03 AWD personal van. I got the 03 with 26,000 miles on it. It had this shuddering problem and took it back under warranty. They change the fluid and it ran just like new. I need to get it changed again. Will do it myself this time.

The 04 went to about the same mileage before it started shuddering as well. I have had the T case flushed twice now. 130,000 miles on it.

[Mbroggi1]

Not sure if i'm experiencing the same problem but from a dead stop on a steep incline i'm getting a thump , like every 15-feet once it gains momentum it goes away. I can floor it and it will not do it. I would think if it was the rearend i would get that thump all the time under hard accceleration. Just steep inclines from a standing start or under 5 mph till she gets going.

2004 AWD 140,000 miles

[e7ats]

Yep same problem. 04 with 140,000Miles!!! Wow and I thought I drove a lot with 114,000.

[LiftedAWDAstro]

Yup, my 03 did it with about 52,000 miles on her. PJ said the thumping would go away with the transfer case flush and sure enough it did!

[2000AWD]

I did the flush as well when I bought my 2000. (68K on the clock) I had a pronounced shudder in the t-case, so bad I thought something was wrong. Did the 2X fluid change with AutoTracII and it's been smooth sailing ever since.


Jay

[Mbroggi1]

My 2004 AWD had started what i would describe a thump most noticalbe from a stop then going up a steep incline. Best way to describe it was if a big glob of tar was stuck on a back wheel. In a earlier topic many had talked of a shutter in a tight turn and the fluid in the transfercase being the culprit. It was recommened by many to change the fluid. I had a hint of the shutter after backing up in a tight turn and upon pulling forward in a tight turn thr opposite way I felt a pretty good shutter like I was on marbles. It was time to give the fluid change a try.

Well 2 quarts of AUTO TRAK II and simple 10 minute change and instanlty upon driving not a hint of the thump even on the steepest hill. Another plus from this miracle fluid is I had a pretty good clunk when going into gear and though my 04 had 143k on it after owning a 93 that had 275k and going strong it had the same clunk when going into gear. THe CLUNK is gone

The drained fluid was black and did have a bit of a burnt smell and I measured the drained fluid right at 64 oz. The bottles from Chevy measure 33.8 oz each and cost me about $7.00 each.

Thanks again to all those who helped and I just wanted to pass along the good news and tip to all those who have a later model AWD.

OK one more note and I'm going to do a bit more testing to prove this one but the best mpg i have got was 16.5 since Jan 2007 I filled up Saturday after the change and topped off the tank to day since I noticed the gauge barely moving well I had 120 miles and it took 6.1 gallons to fill. 19.7 MPG !!! I'm wondering if my AWD was enagages all the time?? and now its not