AWD VS 4WD

[Chubbs]

I searched quite a bit on the Auto4 mode because of the experience I had the couple times that I engaged it.
From the threads I pulled up on it, persons claiming to be GM tech or otherwise have said that Auto4 mode and it's overuse and/or misuse is the 2nd biggest killer of the NP246, after the pump rub and running the case low (or dry) on fluid.
Unfortunately, I'm the 2nd owner of this truck and for all I know the p/o could have driven in a mode & manner detrimental to the case but I don't have everything necessary to complete a proper diagnosis. With that being said, I don't have the luxury of debating or disregarding someone who calls himself a GM cert tech that claims frequent & unnecessary use of Auto4 will result in a rebuild.

 

[Chubbs]

Well Joe, can you describe for us what happened?

When I engage it, the rpms usually go up a couple hundred rpms if on the highway. I had it active on Monday during some heavy storms but do not think it ever applied more than the minimum power to the front axle. The truck has KO2s and has thus far handled foul weather superbly.

Well, the 2 times I selected Auto4... same scenario: raining, paved street, driving in a straight-on direction about 20-30 mph.

I hear a thrashing sound from the front under carriage, such as a very light metal-on-metal noise. Rubbing 2 pieces of tin together in a circular motion is the sound.

Then, the driveline would drag like I'm braking, truck pulls to the left and continues to slow. It's all very fast, within a couple of secs before I'm like "OH $HIT!"
Stop, make a left turn off the boulevard and hit 2HI. The first time, it went right back into 2H. The 2nd time, the light flashed for a while before changing modes. I haven't messed with it since. No driveability issues in 2 so I assumed not jack with it until I had a good idea what needs replacing.

There is a TSB 03-04-21
'grinding noise heard in the front axle' where the axle shaft engagement gear & collar is inspected for damage.
At the time all of this happened the truck was brand new to me and the tires needed replacing. The front 2 were dif size than the back and I asked a couple folks if the PCM had trouble calibrating between the front and back speed output sensors. I was told that is possible and to diagnose further once I had 4 tires of the same size on it.