No 4WD High but 4WD Low Works Fine
January 21st, 2022, 2:05 PM
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No 4WD High but 4WD Low Works Fine
This is a post-diagnosis report because I was not able to find any references to this problem, just trying to help the next guy.
2004 Silverado Z71 with GM 246 (aka NP8, NVG246) Transfer Case and four-button 4WD drive switch module (Auto 4WD, 2WD, 4WD High, 4WD Low)
I inherited this truck recently and needed to do some bodywork before taking it for state inspection. I fixed all of the known problems except for the glitched-out multifunction display in the gauge cluster (it has been rebuilt since). The morning that I drove it to get inspected was the first time that I had tried to drive on snow-covered (glaze ice more like it) roads and by the end of the fairly long round trip I was very aware of the 4WD not working properly. The encoder motor made noise, the lights on the switch changed, 4WD low worked as expected but 4WD Auto and High both had issues. It would even shift the transfer case into Neutral. During the trip an error started reading on the display (in Spanish) that seemed to indicate a 4WD issue. Some power was being sent to the front wheels, there was a slight difference in traction, but not much. I estimate less than 10% was getting to the front wheels, maybe much less. The encoder motor was shifting and there were changes so my initial thought was that the clutch pack in the transfer case was burnt out but I kept looking for other causes. I decided that before I replaced the transfer case I would eliminate every other cause that I could. I cleaned the ground connection and replaced the switch, no change.
My final thing to try was swapping a new encoder motor. I ordered a Dorman one from Amazon and when I installed it it was DOA. Fortunately, I had not bolted it in yet so I pulled it back out and decided to tear it apart to see how easily it could be done. The encoder motor assembly came apart easily and would be easy to get back together so I opened my original GM one up as well. Inside I found that all three gears had teeth missing, there was a dead spot in the range of rotation. I am assuming that the clutch pack is modulated by rotating the control shaft to progressively engage it. This finally explained why 4WD Low worked and there was a little power going to the front wheels in 4WD high, it was able to engage the clutch pack just a little bit on the 4WD High side. The portion of the gear that would have fully rotated the control shaft was stripped out so it was not able to fully engage the clutch pack.
I swapped the gears from the Dorman encoder assembly into the GM one (they looked identical) and put it back together. 4WD works exactly as expected now.
Anyway, I hope this saves someone a bit of head-scratching at some point.
2004 Silverado Z71 with GM 246 (aka NP8, NVG246) Transfer Case and four-button 4WD drive switch module (Auto 4WD, 2WD, 4WD High, 4WD Low)
I inherited this truck recently and needed to do some bodywork before taking it for state inspection. I fixed all of the known problems except for the glitched-out multifunction display in the gauge cluster (it has been rebuilt since). The morning that I drove it to get inspected was the first time that I had tried to drive on snow-covered (glaze ice more like it) roads and by the end of the fairly long round trip I was very aware of the 4WD not working properly. The encoder motor made noise, the lights on the switch changed, 4WD low worked as expected but 4WD Auto and High both had issues. It would even shift the transfer case into Neutral. During the trip an error started reading on the display (in Spanish) that seemed to indicate a 4WD issue. Some power was being sent to the front wheels, there was a slight difference in traction, but not much. I estimate less than 10% was getting to the front wheels, maybe much less. The encoder motor was shifting and there were changes so my initial thought was that the clutch pack in the transfer case was burnt out but I kept looking for other causes. I decided that before I replaced the transfer case I would eliminate every other cause that I could. I cleaned the ground connection and replaced the switch, no change.
My final thing to try was swapping a new encoder motor. I ordered a Dorman one from Amazon and when I installed it it was DOA. Fortunately, I had not bolted it in yet so I pulled it back out and decided to tear it apart to see how easily it could be done. The encoder motor assembly came apart easily and would be easy to get back together so I opened my original GM one up as well. Inside I found that all three gears had teeth missing, there was a dead spot in the range of rotation. I am assuming that the clutch pack is modulated by rotating the control shaft to progressively engage it. This finally explained why 4WD Low worked and there was a little power going to the front wheels in 4WD high, it was able to engage the clutch pack just a little bit on the 4WD High side. The portion of the gear that would have fully rotated the control shaft was stripped out so it was not able to fully engage the clutch pack.
I swapped the gears from the Dorman encoder assembly into the GM one (they looked identical) and put it back together. 4WD works exactly as expected now.
Anyway, I hope this saves someone a bit of head-scratching at some point.
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