Need help diagnosing notable driveline clunk in 2002 Suburban

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Ive had a metallic clunk in the drivline of this truck when shifting into Reverse for a while. I dont drive it much, and its got almost 300k, so I just dismissed it as an internal 4l60e issue that I was gonna just ride out until it something caused me to pull the tranny. If I quickly bypass Reverse to go to Drive, no clunk. But stopping the shift lever in Reverse produces the clunk.

Yesterday, I crawled under it to change the T case fluid, and I noticed that I think the noise is actually coming from the front drive shaft output area of the T case. I could duplicate the noise by trying to rotate the front driveshaft. To my surpise, the front driveshaft seems to be engaged, even though the truck is in 2H. Im not sure, but I think some GM trucks of this vintage are designed to have the front driveshaft rotating all the time so I dont know if that is indicitive of any issue. The front drive shaft has about 3/4" of rotational play. Not sure if that play is from front diff slop or Tcase slop. Not sure if its supposed to be rock solid tight like the rear is.
All U joints, front and back appear tight with no play. If I jack up a front wheel, they turn independently in 2H. If I jack up a wheel in 4H, you can tell its engaged, and I can also duplicate the clunk by trying to turn a front wheel.

The previous fluid was at the appropriate level and still bluish. Ive done some reading about these auto systems sticking in either 2wd or 4wd. Next week it is going on a lift and Im gonna thorougly put the 4wd through its paces. Hopefully I can get a scanner to see if Tcase engagment motor is working properly. Any thoughts?
 

D60

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So Autotrac II fluid...you have the NP246?

If so it should release the front d/s in 2hi.....

BUT FIRST, just so we're clear and at the risk of insulting your intelligence:

Unless you have one of the weird full-time 'cases 4wd requires TWO distinct things to occur for 4wd to happen:
1) the t-case must engage the front output
2)your front axle CAD must tie the front wheels together

#2 is what is happening when you engage 4wd, based upon your description. It sounds like #1 is always engaged, which is odd if the vehicle is off BECAUSE:

the NP246 achieves "old school" 4hi by simply mashing a bunch of clutches (basically auto trans friction plates) together tightly in a basket. The shift motor does the "mashing" and then just holds it there.

In "auto" mode it can vary the degree of "mashing" to adjust f/r bias based upon WSS's and maybe other stuff, kinda like slipping a clutch for the front wheels

I suppose it's possible the shift motor has gotten stuck in the 4hi position, but I'd expect to see some Service 4wd messages or similar

Furthermore, all of my reply is predicated on the presumption you have the NP246. The manual shift was the NP261, the basic "non-Auto4wd" electric shift was the NP263, but there were some weird full-time options that never even offered a dedicated 2hi button and if you've got that I'm no help.
 

D60

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Also IIRC even the NP246 was not immune to pump rub issues. I've heard rumors/theories before that you can hear the pump ring clunk when going from forward to reverse if it's eaten into the 'case enough. Honestly....not sure how I feel about that theory....

If you do have the 246 and you go into it there's also an upgraded rear bearing to get you from a plastic-caged bearing to a metal cage

I'm hoping @clinebarger will stop in to mop up any inaccuracies in my CRS-afflicted memory
 

N Heat

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I suppose it's possible the shift motor has gotten stuck in the 4hi position, but I'd expect to see some Service 4wd messages or similar
Holy crap. I cant believe I left that out of OP. Yes, I have gotten a random service 4WD light. Its only happened 2 or 3 times, then went away on the next trip. Im only driving this truck about 500 miles per month. Also, no codes from your standard ELM OBDII code reader. Thanks

And yes, Ive got the standard push button 4wd with the auto option on top.
 

D60

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Holy crap. I cant believe I left that out of OP. Yes, I have gotten a random service 4WD light. Its only happened 2 or 3 times, then went away on the next trip. Im only driving this truck about 500 miles per month. Also, no codes from your standard ELM OBDII code reader. Thanks

And yes, Ive got the standard push button 4wd with the auto option on top.
Just to be clear, you do have a "2hi" button, yes?
 

N Heat

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The clutch pack is preloaded even in 2 HI, Takes roughly a 2 foot bar to overcome the clutch preload. That's what I use to make sure the Front Differential is disconnected.

Could be excessive chain slack, Excessive wear/slack between the Main Shaft & Clutch hub inside the Transfer Case.

High mileage 4L60E's are known for the Center Support to "eat" into the Case Lugs, Once it wears enough.....The Support will snap against the Lugs.
Forward & Reverse load the Support in opposite directions & the snap/clunk will be most audible when going from Park/Neutral to Reverse.
The Anti-Clunk Spring preloads the Support against the Lugs to prevent clunking when going from Park/Neutral/Reverse to Drive.
 
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I put U Joints in my 02 Yukon over a reverse clank. Didn't change the noise at all. Drove it for 4 more years. Sold it to my friend.
 

N Heat

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The clutch pack is preloaded even in 2 HI, Takes roughly a 2 foot bar to overcome the clutch preload. That's what I use to make sure the Front Differential is disconnected.

Could be excessive chain slack, Excessive wear/slack between the Main Shaft & Clutch hub inside the Transfer Case.

High mileage 4L60E's are known for the Center Support to "eat" into the Case Lugs, Once it wears enough.....The Support will snap against the Lugs.
Forward & Reverse load the Support in opposite directions & the snap/clunk will be most audible when going from Park/Neutral to Reverse.
The Anti-Clunk Spring preloads the Support against the Lugs to prevent clunking when going from Park/Neutral/Reverse to Drive.
Thanks for the input. So, given the mileage and age, just run it until something worse happens? Any issue with just pulling front driveshaft?
 
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N Heat

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@clinebarger @D60
I found a video from a guy with probably the same clunk as I have. Watch this video starting at 5:25. Thats the same clunk I get when shifting to Reverse and this guy said it was the same for him. If nothing else, I prob need to do the "pump rub" fix. Ive watched many NP 246 videos today and theyve all had the pump rub damage to some degree. Looks like Ill pull the Tcase over the winter.

 
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D60

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If you're mechanically inclined t-cases are fun and the pump rub fix is never a bad idea. I installed the MA kit in a 261 but when I tore into a 246 with 180k miles it looked good and I opted to leave well-enough alone and save the $

If you opt to replace the clutches in your 246 you quickly get into some specialty Kent Moore tools to check stack height. Yet another KM hard plastic case I have sitting around I'll probably never use again ;) ....but I'm still in the running for top 100 of He Who Has the Most Tools, albeit upon my death

Also Valvoline makes a Transfer Case Fluid that meets Autotrac specs despite being.....RED (gasp!)
 

N Heat

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If you're mechanically inclined t-cases are fun and the pump rub fix is never a bad idea. I installed the MA kit in a 261 but when I tore into a 246 with 180k miles it looked good and I opted to leave well-enough alone and save the $

If you opt to replace the clutches in your 246 you quickly get into some specialty Kent Moore tools to check stack height. Yet another KM hard plastic case I have sitting around I'll probably never use again ;) ....but I'm still in the running for top 100 of He Who Has the Most Tools, albeit upon my death

Also Valvoline makes a Transfer Case Fluid that meets Autotrac specs despite being.....RED (gasp!)
Man, I dont know if his clunk better or not. Watch at 7:05.
 
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Thanks for the input. So, given the mileage and age, just run it until something worse happens? Any issue with just pulling front driveshaft?

All the play in the geartrain can stack-up with 300,000 miles of wear. Inspecting & culling these parts can be challenging.....I use a large magnifying glass to inspect for Spline Wear.

If your truck has the front diff uncoupled (2-HI)......I don't see the front driveshaft making a difference.
 

D60

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Man, I dont know if his clunk better or not. Watch at 7:05.

So in your first video example I don't know where his clunk is coming from. Could be splines. Could be planetaries. Could be chain slack. Could be that the front and rear outputs are not located by the rear case half ('cause it's removed) so everything is a bit more sloppy than it should be.

What it is NOT is the pump because in the first video example the pump is there but, again, the rear case half is not and that's where the pump bangs around, eating thru the rear case half (sometimes)

In your second vid (quoted above) he doesn't drop the pump on until ~11:16
 

D60

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Here's an idea: find a helper you trust....REALLY trust.....and hold each d/s by hand, preloading (twisting) it a bit in one direction or the other. Then have them shift from P to R to D and see if by acting as a damper on a given d/s you can mitigate the noise.

At least that might reduce possibilities.

Is the rear end original? 300k on a 10bolt is lot....we're sure this isn't pinion bearing slop or excessive r&p backlash?
 

N Heat

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Here's an idea: find a helper you trust....REALLY trust.....and hold each d/s by hand, preloading (twisting) it a bit in one direction or the other. Then have them shift from P to R to D and see if by acting as a damper on a given d/s you can mitigate the noise.

At least that might reduce possibilities.

Is the rear end original? 300k on a 10bolt is lot....we're sure this isn't pinion bearing slop or excessive r&p backlash?
Good ideas. Im gonna try those when I have it on a lift. Also, rear is original. No slop at all in rear DS.
 
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I have a 2007 Yukon Denali awd. Replaced the front carrier mounting brackets (bushings included) and most of my clunk went away. Not sure if the 2000-2006s are similar or not?

My awd transfer case (BW4485) has 181k miles and could use freshening up too.
 
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