popping/banging when shifting from 4hi to 2wd

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My 1994 RCSB Sierra Stepside 5.0l 5 speed has a horrible noise when going from 4hi (never used 4lo so that may too) back to 2wd. Sounds almost like transmission is hitting the floor or one of the drive shafts is disconnected from the differential end and spinning free and banging against the floor or trans tunnel. No noises in either 2wd or 4hi and both seem to drive the proper wheels when engaged. I know trans mount is bad but can't see it causing this issue. I am shifting the t-case in neutral with truck stopped if that matters. I have lots of 4wd/AWD trucks but never had this exact issue. Any thoughts? Also what t-case does this have and it is manual transmission specific?
 
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I noticed if shut the truck off in 4hi and then shift it to 2wd and start it again this doesn't seem to happen. Does this help narrow down the issue?
 

TerryD

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Probably the transmission spinning the input if you are pausing in N for too long. Even out of gear and with the clutch pressed, it generates enough friction inside to spin the output of the transmission. What year truck do you have that you have to go through N to get to 4hi? Both out 93 and 00OBS you just pull the shifter back one spot, no N involved.
 
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By neutral I mean the transmission NOT the t-case sorry. I should also clarify that this noise occurs after driving truck even 25 or so feet on dry pavement or a foot of snow. The way my neighborhood gets plowed I NEED to go into 4wd to get out of my driveway or parking spot to the street. At that point I shift trans to neutral hold in the clutch and shift t-case to 2wd and then put it in first and start driving and that is when noise begins. If I back up a few times and then go forward again it seems to stop or as I found last night turn truck off in 4hi and THEN shift to 2wd noise doesn't seem to occur at all. Hope this is clearer than my original description.
 
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I crawled under this today it appears that the grease fitting for the front driveshaft where it telescopes is hitting the bottom of the transfer case shift lever ever so slightly. Saw the very tip of the fitting and the bottom edge in a spot on bottom of transfer case shifter that lines with the fitting polished like they were rubbing a bit. This is just weird but then again if I post a problem on a forum it is never anything simple lol
 

IOWNJUNK

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You would think that it would make that noise at all times while in 4wd, not just while shifting back to 2wd. Can you remove the fitting to see if the noise stops? If it goes away maybe get thrifty with a dremel tool and make some room for everything.
 
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Well that was my thought too but I am wondering if the combination of a bad trans mount and slow to retract front actuator causes the driveshaft to continue to turn for a bit in 2wd and it moves just enough from play in trans mount to make slight contact with transfer case shifter in 2wd. The fitting however is made into the slide part of the driveshaft so that leaves creative clearance time with my die grinder. Going to put it in the air on the lift tomorrow and have an assistant work everything while I am under it and see what is really going on. I have a bunch of new parts for this including trans mount and both engine mounts I was going to do anyway.
 

IOWNJUNK

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I'm thinkin a new trans mount may be most of your problem, curious to see if it makes a difference.
 
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I hope so really can't afford to put a transfer case in this right now. Hope to get it on the lift today and see what exactly is going on.
 
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OK got it on the lift today and seemed to have solved the problem. The trans mount was wasted and had about 3/4" of play in it and when I used the tall jackstand to push the trans up about 3/8" the grease fitting on the front driveshaft started banging against the shifter for transfer case when I turned the drive shaft by hand. I pulled the trans mount and put in the new one I had purchased and also topped off both differentials and the transfer case too all were a bit low. I tested it by putting it in 4wd in front of my house in some snow and when I stopped and shifted to 2wd again and started moving the truck no more noise :) Unfortunately I noticed the left inner cv boot and right outer cv boot were both starting to split so I will be putting in new front cv axles and also new axle seals in front as they are a bit wet and I will be there so I might as well take care of it all at once.
 
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