Jerking, binding, shudder on slow tight turns.
Posted: Tue Jun 12, 2007 8:08 am
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.
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.