Chevy & GMC Vans

I've got an '04 Astro AWD and my 'Service AWD' light has been on for a long time. The other day I was out in a snow storm and the van was all over the road, when I got home I got stuck because all the power has been going to the rear wheels only...I kinda think it's been doing this for awhile because I've gotten stuck a few times in the past couple of years with the same issue.

Also having an issue especially in parking lots making hard turns at full steering wheel lock where I can feel the transfer case slipping and grabbing.

Any ideas what the problem might be? I've been thinking it's the viscous coupling in the transfer case but I haven't turned too many wrenches on the newer computer controlled stuff. The transmission was replaced 100,000 miles ago.

Chances are the "service AWD" light is on either because the shift motor is dead or the clutch pack is fried. There is no viscous coupling in the 99 - 05 AWD's. You can buy 4 quarts (liters actually) of the Auto Trak 2 fluid from GM or if a parts house (our local NAPA carries it) carries it and flush the case out. To do this, drain the t-case, refill with 2 quarts of fresh fluid. Drive in figure 8's in a paved lot. I then also drive a few hundred miles to make sure it is flushed throughout the t-case. Then drain and refill again. If that doesn't work, you may want to look at a replacement transfer case if you have a ton of miles. I am guessing you do as you stated the tranny was replaced 100k ago meaning your van probably has 200k on it. It is cheaper to buy a used one than rebuild yours.

173K on this one.

The flush and figure 8 procedure reminds me of my BW1339 Jeep Quadra-Trac. By any chance is the recipe for the GM transfer case oil 3 parts 30wt non-detergent to one part friction modifier?

How do I check the shift motor?

no.

nothing but auto trac II.
anything else could make it worse than it is now.
there was a service bulletin for our vans describing what you are, low speed parking lot maneuvers, front tires fell like they are binding and skipping . solution was flush with auto trac II, and refill after 100miles.

check you tcm fuse, and maybe separate an clean the connectors on the encoder, and relater harness.

to check the encoder motor on the case requires dropping the front driveshaft to get the bolts out. I think its 11mm on the front axel and 15mm on the encoder motor. 3 bolts on the encoder, 4 on the front axle side of the front d.shaft.

when you have it out, take a 9/16ths wrench and rotate the cam the encoder controls, if it moves freely and springs back into place on its own, the t case is probably not the problem.

id bet on the encoder motor being shot. worst case tccm. (transfer case control module)
I've done two myself in my 03, pretty easy actually.


oh, disconnect the battery for 30 mins to reset the system and clear the codes. easy to do while installing new encoder.

An easier check of the encoder motor can be done be disconnecting the plug on the end of the motor and measuring the resistance of the motor. Should .5 to 50 ohms.

With the Service AWD light, it should be throwing a DTC code. If you can borrow a scanner to get that code it will give you an idea of what is going on.

It seems in a lot of cases the encoder motor is the problem however, if there is a front or rear shaft sensor the code will show that. It will also point out motor encoder shorts/opens and sensor speed mismatches.

I've plugged in with several different high end code readers but I can't seem to pull the codes for the AWD.

It will take a more high end scanner(snap-on modis?) to get the AWD codes. They cost close to $1,000 dollars, hence the term "borrow"! I am still pondering on buying one for the family.

I know a few transmission shops in my area that have them. Not sure if your local Oriellys would carry something like that in their collection of scanners to use at the store.

Still the resistance check is worth doing for starters.

The blue connector is were I disconnected and checked the motor resistance.

Hello Astro fans. I thought I would reply to this thread as I have a "Service AWD" light issue. First a little background. I just got this 2000 AWD Astro: kind of a good deal but now thinking maybe it wasn't - that's another story. I suspect the AWD is NOT working but am not certain. Does the "Service AWD" definitely mean the AWD is dis-engaged (that info from another source)? Is there a way to tell when the AWD is engaged and working (other than waiting for the snow)? From the fine advice on this and other posts, today I will be doing a Auto Track II flush and measuring the resistance on the transfer case motor. How often is the transfer case control module to blame and lastly, what is the best way to determine if the transfer case itself is shot? AWD DTC codes? Thank you kindly.

About the only way to tell if AWD is truly working is to find some gravel and see if you can get the tires to spin. Other than that, if you put it up on jack stands, all 4 tires will spin even with the AWD fuse pulled. There is enough preload on the electric clutch for this to happen. I have not heard of a TCCM failure mostly the encoder motor.

Thanks for the reply LiftedAWDAstro. I believe the AWD is not-working. My driveway is dirt/gravel and the rear wheels spun getting up on the ramps - not the fronts. Anywho, the resistance on the transfer case motor was 10.7 ohms however the fluid that came out of the transfer case was horrendous: black as a moonless night with a strong 'burnt' aroma. I suspect this is strong evidence of a toasted transfer case. I saw some decent units on ebay and suspect this may be my best option. Opinions?

If you can find a used unit at a reasonable price I say go for it. With fluid that burnt, chances are the clutches are fried to the point they may never work again.

Roger that. We got snow last night so it was fairly easy to determine if the AWD was working: it ain't. So yeah, I got a unit w/ 132,000 miles on it coming from eBay. The saga continues.

Nice thing is you will have some spare parts for the new one if needed. Hopefully the encoder motor is good as well as the sensors.

If you are going to stick with the NP136, rebuilding isn't a bad option. I have seen the rebuilt and it looks pretty straight forward. If you buy a used T-case, as I did, you will be buying someone else's neglected Tcase.

X2 on the rebuild. They aren't that hard to drop and all you will need are the friction discs and some seals. If you are any kind of shade tree mechanic and have someplace warm for a weekend you can have that satisfying feeling of doing it yourself. Lots of forum posts on the how to's