Tsb On Auto Trac Clutch Slipping Problem

Mad Chad · December 14, 2008

Hey everyone,

I have a '99 Silverado 1500 with the NP246 (NVG 246) transfer case. It is the Auto-Trac one. It has 2HI / 4Hi / 4Lo / Auto Mode. There are no error codes from the TC / No "Service 4WD" messages and the front actuator seems to be working fine. The problem I'm having is that when 4Hi or 4Lo is selected the front wheels will turn if it's on a lift but not when there is a real load on them - seems an awful lot like the clutch pack is slipping. I saw the TSB about that problem a while back (#990421003C) which stated the clutch pack had worn out prematurely due to cold weather and said to rebuild it. Being in MN I figured that may be it.

Here's what gets me - when I use Auto Mode the encoder motor will engage the clutches just fine and I've just used that for the last year. Pulls through snow - up hills - everything just fine, but it won't lock the clutch pack together - it will release and then engage over and over again when going up a hill for example. I know that the encoder motor will put a specific torque on the clutch pack in 4 Hi / Lo so as to lock the pack together and will vary it as necessary in Auto mode. So my question is - could the clutch pack be worn even though Auto mode works perfectly? Or could it be a sensor or something else that doesn't let the encoder fully engage the clutches when in 4Hi/Lo? I've had 2 shops look at it and they don't really know - just suggest replacing with rebuilt. I know some of you work on these and know their ins and outs. I searched hundreds of articles and posts and haven't found this specific condition. I would greatly appreciate any insight someone may have who is familiar with the inner workings of this transfer case.

Thanks

Chad

Mad Chad · December 18, 2008

Well, I think I may have diagnosed what the problem is but need more advice. It seems the encoder motor brake mode of the encoder motor (sometimes called "Transfer Case Lock") is not engaging to lock the clutch pack in 4 Hi / 4 Lo. That's why Auto 4WD works since it's varying the amount of pressure applied to the clutch pack. In 4WD I hear the encoder motor apply the clutch pack but apparently it doesn't hold it there and that's why it slips. Anyone with knowledge about whether replacing the encoder motor would br the fix? I know they've been problem prone. I still don't get a "Service 4WD" light or any codes so maybe something broke inside the motor but it still thinks it's working. Any thoughts?