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01-10-2010, 04:59 PM #1
- Join Date
- Nov 2009
Trying to understand GM 4x4 operation.
It's an 02 1/2ton with Auto 4wd NP246. 5.3L V8
The way I understand it there are 2 actuators. 1 for front diff and 1 for xfer shifts.
In A4wd the front diff actuator engages but no pwr from front dr shaft until wheel slip detected. (Does the abs provide the input for slip detection?) then the xfer act engages 4h shift.
In all other 4x4 modes front diff act engaged & xfer case act engaed to select range L or H.
Is this correct?
How about the front hubs? Do they engage/disengage by some other type of actuator? Or are they in constant mesh?
Also I was reading about the "Case Saver" from Driveline. Sounds like it may be worth the tear down to install one. Anybody use this product? How much labour time? Any way to tell how much wear there is already on the case besides tear down? Does wear depend on usage or just milage?
This is a 2 owner truck. 1st owner got it new and I got it in 11/09 (64k miles) so I don't know how the 4x4 features were used or not used 1st 7 years.
Started a thread yesterday but it was pretty lame when I looked at it today so I started this 1 to try and be more clear.
Having some problems with the 4x4 operation. Bearing-like noise from front 4A or 4H when coasting down from speed, getting louder from 45 and down. Not real loud but not sounding right.
Also front diff actuator seemed to stick once tried to turn into driveway in snow and wheels were binding skidding didn't want to turn although it was in 2WD. Shift in/out 4wd seemed to fix that prob though. Except, with the front apearantly stuck engaged , when I shifted into 4H got a bang/jolt when started to move forward. Only time its' done that.
If the 1st owner rarely used 4x4 maybe all these parts are gummed up, dry, sticking etc.? IDK
What do you guys think?
When I test drove the truck everything worked, no noise. After using it a few times started getting these issues.
Thanks for your consideration.
Last edited by 02z71; 01-10-2010 at 05:20 PM. Reason: typo'02 Silverado 1500 Z71
01-10-2010, 09:08 PM #2
- Join Date
- Nov 2006
- Canyon Lake, Texas
- Blog Entries
I wish I could help, but I have not had any problems with mine. I use mine quite a bit so that may have save it from some of the normal problems (what ever they are). Hopefully some of the more informed gear heads on site can give you a hand. Good luck!
2010 Z/71 Colorado C/C
Black 4X4 w/5.3L V8
2007 Winnebago Adventurer
w/8.1L Chevy & Allison 6spd.
Canyon Lake, Texas
01-10-2010, 10:54 PM #3
02z71- I posted that I have a similar problem in your other thread too, but since it seems you've abandoned that one in favor of this one, I'll post my follow-up here.
Do you feel like the "bearing noise" is coming from the drivers side or passenger side? Mine seems to be coming from the drivers side, but it's always hard to tell from the inside.
Does your truck pull to one side or the other at highway speeds? Mine seems to pull to the left at highway speeds, but I've had the alignment checked and was told "it's within spec." I wonder if a bad CV axle or other component on the drivers side could be dragging a bit, causing it to pull to the left.Truck: 2005 GMC Sierra SLT
Toy: 2006 Honda 599
01-10-2010, 11:08 PM #4
I can explain the t.case It's a 246 like you said the auto works with clutches that are compressed by a fork that is mounted to the case on one side and the other side is pushed in by a cam wheel attached to the t.case motor is senses slip by the front drive shaft speed sensor mounted right above boot on t.case (you have three total) the front end stays engaged the whole time 4auto is selected and it reads that input front shaft.your t.case should aready have a case saver (my 99 did) it keeps the oil pump tab from wearing through case.AND the noise you are hearing might be from chain
99 tahoe 2 dr. 4x4 7.4 4L80-e NP246, SAS, dana 60 hp, sterling 10.5 rear, gibson headers, and alot of tricks of the trade !!!!!!IF YOU'RE NOT CHEATING YOUR NOT TRYING!!!!!!!!
01-11-2010, 08:09 PM #5
- Join Date
- Nov 2009
Thanks Dog! Thats helpful. Did you mean GM did a fix or recall with the case saver?
Clone: Yes! Mine is pulling left. I haven't had alignment checked yet though. I'm having things checked out at a shop tomorrow. I really can't tell which side. Noise is fairly faint and hard to pin down.
01-11-2010, 08:58 PM #6
02z71- I'll definitely be interested to hear what you find out at the shop, sounds like we've definitely got the same issue!
01-11-2010, 10:33 PM #7
It's a little piece of spring steel bent in a square shape that its in case and gives the oil punp something to rub on
01-12-2010, 08:26 PM #8
- Join Date
- Nov 2009
Well here is is what I found out.
Noise is occurring in 4A and 4H but not 2WD. So in both cases 4H and 4A the front axle actuator is engaged (except in 4A front drive shaft doesn't send power to the axle without slippery condition).
So in both cases (4A,H) the front diff and axle shafts are "spooled up", everything is in motion, and this is when we get the noise. 2WD clean and smooth.
The noise then is isolated to the front diff/axle and probably a bad bearing but no way to tell exactly which of the 5 or six bearings it is without a tear down and inspection of the whole axle housing. Bummer!
They said also the noise could be an out of adj pinion/ring gear lash, which could be a factory defect or pinion bearing.
So there are 2 axle shaft bearings, 1 per where the 1/2 shaft exits the housing (with seals). Then 2 carrier bearings and I think 2 pinion bearings. Also the ring and pinion lash which is noisy if out of adj.
At 1st the mechanic couldn't even hear the noise so I told them to keep looking. End of the day they changed lube in xfer, front diff, topped off the rear diff. They said no metallic residue in the oil so thats good. But still said he didn't hear a noise.
Then the shop owner called me back to say he drove the truck himself and immediately heard the noise and gave me the above diagnosis. He said it would be $200 - $500 to tear it down and find out which bearing and replace it, or adj backlash or both. He recommended just drive as is for now and keep an eye on it. Since the oil was clean nothing major was being chewed up or anything and everything is operational.
So I guess I'll just baby it a bit, use 4WD only when I have to at road speed. I don't think low speed use like under 40mph or bustn snowdrifts etc will hurt it. The risk of a lot of bearing wear would be highway speed when everything is spinning fast and getting hot.
Oh yeah I also have a minor leak on driver side axle seal where it exits housing. Not sure if it's related but could be indicator my noise is coming from that axle bearing. Again no way to know for sure till I tear it apart.
And that's all I know. I hate driving the truck when I know there's a problem like this but don't feel like spending $500! Besides truck may work just fine for a long time and just make a lil noise. I don't think I'll get a sudden fatal failure or anything. Worst case is the bearing will just get a little louder over time. Have to fix it eventually of course but can wait till the wallets little fatter.
Planning to keep a close watch on the the oil in the front axle though, check for metal contamination.
01-12-2010, 08:41 PM #9
Thanks for the good write-up z. I'm with you, I will just continue to drive and avoid 4WD at higher speeds. Maybe I'll try my hand at tearing into it a little this summer, or maybe I'll just start a 4WD repair fund and have it taken care of before next winter. Either way, as long as it doesn't get any worse I'm not going to worry about it too much.
01-12-2010, 09:07 PM #10
OK take out the front drive shaft 4 srap bolts and a metal clamp that holds the slip yoke put it in 4hi and see if noise is there then you know it in the front diff.follow that with a used one from a junk yard $200 they have 12 month warranty and save the rebuild whats the shops warranty on there build or have shop R&R
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