99 Bravad AWD won't engage

Hi,
I have a 99 Bravada that the AWD won't engage. When I first noticed the problem I checked both fuses, they were fine. Put it on an engine code scanner, only code listed was for a bad wheel sensor, replaced that, code cleared. The service AWD light has only come on twice, both times only for few seconds on some very slick road conditions, no AWD and no codes stored. Have put the unit up on a lift and the front tires turn, but without any kind of power (can stop them w/ foot). Have replaced the encoder motor, and am going to try doing flush and fill on transfer case. Could it be the clutch pack assembly in the transfer case..............any other ideas on what might be the problem.

Thanks

Welcome to AF.
You can not check the good on a lift.
We need the tag no off the back of transfer case.
On the dash shift buttons does it have a auto button or just 2W and 4W?
It takes a transmission /transfer case capable scanner to get the codes or do the test on on the electronic controlled ones.
Just a engine scanner will not do the job.
What kind of fluid are you using in it?

Have tried to test 4wd on icy road conditions and out by the river, always the same thing just rear tires spinning. Transfer case should be an NVG 136 w/RPO code NP4, but will verify today. Unit is automatic, no button for 2W/4W. The scanner I used was an 8K Snap On unit, definately had transmission capability, will have to check on tranfer case. Fluid is the AutoTrack II direct from the local dealer.

Did the scanner connect to transfer case module?
Does the service 4W light come on on key on bulb check and go off or stay on or flash?
Will the scanner run any test on the transfer case?
Is the transfer case module getting power?

Scanner connected to the under dash terminal. Service AWD light comes on with all other dash lights on start up. Like I said before, has only come on twice intermittently during driving conditions.

Scanner is a Snap-On MODIS, does do transfer cases through the under dash terminal, will try and scan again tomorrow.

Transfer case is NVG 136 w/RPO code np4

Transfer case module is getting power.

Additionally, fluid that came out of transfer case was BLACK. Would guess that it had never been changed by original owner (100K miles on unit).

The scanner should read out and test the transfer case module threw the OBD11 conector. If it is transfer cse capable.