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Driveline noise: Transmission or dif? 12 bolt or 10?

yakmastermax

yakmastermax

1/2 ton status
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Feb 4, 2012
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Albuquerqu NM/ Austin TX
I bought my K5 knowing that the transmission was needing replacement. PO said that it made noise in 2nd and 3rd gear. I drove the truck a little before dropping the trans and sure enough it made a terrible clunking noise in 2nd and 3rd when applying gas. noise got faster and worse as speed went up but was non existant in low and 1st. I got a used SM465 in a trade and my mechanic inspected it visually and by hand. He said it looked good. Paid him an unholy ammount of money to "pressure wash and install a seal kit":mad:. first off, the seals are not even working (he must have only used the paper gasket, no sealant) because the transmission is leaking gear oil from the two bottom covers. :doah::angry1:

anyway, as it turns out the old adapter/tailhousing between the SM465 and NP208 was cracked

20120806_181712.jpg


i spent a couple of days looking through junkyards for a later year tranny crossmember so that the short foot adapter/tailhousing could be made to work but i had no luck. i ended up machining a spacer which has worked out nicely.

20120806_2310511.jpg


20120808_195358.jpg


20120809_191855.jpg


the kicker is that 3 weeks and several hundred dollars later: THE TRUCK MAKES THE SAME NOISE:chainsaw2:

my friend and i reasoned that there is no way in heck that a different transmission could have the exact same problem. right now i'm pointing fingers at the transfer case, u joints, or rear dif. it really does sound like it is coming from the tranny.
TLDR; clunking noise that seems to be coming from tranny in 2nd and 3rd gear only when you're on the gas (nothing when coasting on in 1st or 2nd no matter how hard you gun it). replaced tranny, noise persists. :confused:

any help appreciated!
 
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Don't you just hate it when that happens??

Actually it is possible for two same model trannys to make the same noise if its a common wear problem for that model.
But, unlikely.
Do the usual stuff. Check the driveline u-joints. Look for play in the rear end yoke.
If the 4wd works, try dropping the rear shaft and driving in front drive a short distance to see if the noise goes away or changes.
Your transmission mounts should be ok, since you just put it in, but check the rear spring bolts.
 
I chased noises on my crew cab and just knew it was a tranny or transfer case, however, after riding in the back of the truck, and listening better I determined it was either the drive line support bearing, which I replaced and the k5 doesn't have, or the rear end. Well it was the rear end! Its amazing how sound can travel up the drive line and make one think the tranny is the culprit. The scenario and noise you speak of sounds a lot like when my rear end pinion bearing was worn out, no noise on decelerating only when accelerating. Good luck in the hunt!
 
Just based on your description, I'm betting rearend. Does the noise fade in when you hit second or third, or is there zero noise then a noise of noticeable volume? If it fades in and gets louder with speed (like 2nd or 3rd gear) it's probably rearend.

If the noise is consistent when on the gas and when coasting, it's probably a bearing (somewhere). If it is, say for example, loud when on the gas and silent when coasting, it's more indicative of a gear mesh issue.

I agree with Fordum and K5-CJ5 as far as try different tactics to narrow it down. Riding in a different seat may surprise you. The sound may be totally different. Good Luck.
 
fades in, volume and frequency of the clunks directly related to speed. tomorrow i'll try to FWD trick. that seems like a good way to narrow it down. if it is the rear end would it make sense to go to a 12 bolt axle or just rebuild the 10 bolt dif?

any recommendations for vendors of new ring and pinion sets? i'm on a time crunch so i gotta order stuff soon
 
I don't know the history of your truck or what you use it for, but you're already out some serious cash on the tranny and you think it's the rear end I think it'll be a hell-of-a lot cheaper to just rebuild the 10 bolt. A 12 bolt is only marginally stronger than a 10 bolt IMO and with a new set of gears, master rebuild kit, limited slip and new stronger metal shafts all upgrading to 30 spline, you will have accomplished your 12 bolt strength if not surpassed it.

That is, if you're not running 39's and gunning it up a rocky hill every weekend. Then I would recommend finding a 14bff and calling it a day. :)

Maybe take a look at Yukon Gear and Axle...they seem to have a decent product for the money. I have their Posi Traction unit in my 10 bolt rear and it seems very strong. If you are interested in new shafts while you have the thing apart...maybe check out Moser Engineering. I have used their products before and they're freaking bullet proof.
 
that yukon gear and axle stuff looks super stout. i'll definitely consider a strong 10 bolt rebuild using their stuff however i'd like to keep other options open. i've got a fantastic pick and pull near my house with a ton of chevy trucks. if i were to just pull an entire rear end (100 bucks or so) what could you recommend pulling from? i would want a bolt in replacement. today i'll crack the cover off and if the problem is obvious i may go the vendor route and buy a new part. thanks again everyone for the awesome replies.

AZShooter said:
I don't know the history of your truck or what you use it for, but you're already out some serious cash on the tranny and you think it's the rear end I think it'll be a hell-of-a lot cheaper to just rebuild the 10 bolt. A 12 bolt is only marginally stronger than a 10 bolt IMO and with a new set of gears, master rebuild kit, limited slip and new stronger metal shafts all upgrading to 30 spline, you will have accomplished your 12 bolt strength if not surpassed it.

That is, if you're not running 39's and gunning it up a rocky hill every weekend. Then I would recommend finding a 14bff and calling it a day. :)

Maybe take a look at Yukon Gear and Axle...they seem to have a decent product for the money. I have their Posi Traction unit in my 10 bolt rear and it seems very strong. If you are interested in new shafts while you have the thing apart...maybe check out Moser Engineering. I have used their products before and they're freaking bullet proof.
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What do you use the truck for? Any sort of wheeling and I would really suggest a 6 lug 14b semi floater.

btw does your truck sound like an 18 wheeler is riding shotgun with you doing 80mph when its making the noise? My old toyota had a shot rear for a long time. Bad pinion bearings were allowing the gears to make horrific noise at anything above 40mph. Sounded like a firetruck was driving in the bed of the truck...
 
Stomis said:
What do you use the truck for? Any sort of wheeling and I would really suggest a 6 lug 14b semi floater.

btw does your truck sound like an 18 wheeler is riding shotgun with you doing 80mph when its making the noise? My old toyota had a shot rear for a long time. Bad pinion bearings were allowing the gears to make horrific noise at anything above 40mph. Sounded like a firetruck was driving in the bed of the truck...
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hahaha:eek1: i have no idea what it sounds like but i'm sure it is terrible. my best description for the noise would be clunk/thunk who's frequency is completely distinguishable. when it first starts i can easily tap my hand to match it. in all honesty, if my trans cover was installed as well as insulation and carpeting, it would probably be a lot quieter. it really isn't all that too loud.

my truck is mainly for snow commuting, trips to the resorts, and camping and ranch cruising. rutted dirt roads on an incline is the most technical it will see.

my parts yard sells rear ends for 110 plus tax. they've got atleast 6 late 80's suburbans, plenty of 80's trucks and 1 K5 and one jimmy. new yukon ring and pinion plus/or bearings and rebuild kit looks like upwards of 200 dollars. i'll be doing U joints either way.

another thing i forgot to include is that when my rear end is jacked up, the dif behaves strangely, i think. if you turn the drive shaft by hand the passenger wheel remains stationary and the driver side rotates. also if you turn the driveshaft by hand there is a solid 5-10 degrees of rotation before anything engages and the wheel (driver) starts turning.:dunno: symptomatic of anything?
 
14 bolt is hands down strongest of the options, and if you can score a 6 lug semi floater out of a 3/4 sub at the junkyard then I would go that route, plus it will save you some cash over rebuilding the 10 bolt and never worry about it again.
 
bolts right in? like i said there are a ton of burbs. would i make use of my stock springs or pull those too? anything quick ways to ID the 3/4 ton axles other than by the bolt count? years that will bolt on?
 
yakmastermax said:
bolts right in? like i said there are a ton of burbs. would i make use of my stock springs or pull those too? anything quick ways to ID the 3/4 ton axles other than by the bolt count? years that will bolt on?
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3/4 ton axles have the same spring perch widths to bolt right it, don't need to change your springs. I couldn't tell ya the exact years but I believe 75-91 are all the same spring perches. Someone who knows for sure will chime in.
 
6 lug 14 bolts are not a direct fit to the old body style blazers. They were made for the 88-98 body style and in 2 different widths as well. The sf14 bolt that fits our old body style blazers were in 3/4 ton trucks and burbs from the 80's and were all 8 lug.
 
Easiest solution is to build a frankenstein in the corner of the u pull :D

Take a 80s 3/4ton 8 lug housings, yank the shafts and stick 90s 6 lug axles and brakes on it. Check on your shaft lengths before taking my word as 100% though.
 
thanks a lot guys but i think i'm going to go a different route. it seems like the quickest and easiest (but most expensive) way to do things is to upgrade the 10 bolts that i have. I'm looking at new ring and pinion front and rear, i'm going to lower my ratio for more friendly highway driving but i plan to eventually put in a NP241. the 4L reduction gearing should reclaim my offroad capability. I also discovered that my rear dif is open:eek: so i'm thinking about some type of LSD. because of money i want to stick with my stock axle shafts (28 spline?) this will determine what type of LSD carrier i get (i'd prefer new or remanufactued). are there decently strong aftermarket 28 spline axle shafts to upgrade to later on? i would go 30 spline LSD carrier and axles shafts right now i just can't afford it or rationalize it because the 28 are working fine.

right now the shopping basket looks like this: new ring and pinion front and rear (i have 3.73 right now, with my 35"s i cruise 55mph at 2k rpms. i am looking at 3.08s for a 68mph 2k rpms cruise or 3.23 for something a little lower). differential seal and bearing kit front and rear, and some sort of LSD for the rear and maybe front too. like i said the LSD would have to work with my factory axle shafts because i don't have to 500 dollars to upgrade them right now.

any suggestion on a route of LSD? maybe a reman or new posi?


all of this comes in light of me taking apart the rear dif earlier tonight after trying the recommendation to drive with only the front axle engaged. sure enough the sound was gone in all gears at all speeds. i drove the truck into the garage and started wrenching

after a downpour of muddy rusty gear oil i saw this:
20120811_221140.jpg


a little bit of work with a rubber mallet and then this:

20120812_011715.jpg


upon closer inspection:

20120812_010709.jpg
:yikes:

wonder where those came from

oh

so that's what was making that gosh awful noise. guess i'm out a transmission and a whole lot of faith in the word of the previous owner.

20120812_010835.jpg


TLDR; rear end is done and an open dif. need new ring and pinion, new carrier (for stock axle shafts and some sort of LSD) and new bearings/ seals plus all the same stuff for the front. options and suggestions? vendors? ebay? time is the biggest issue. school starts the 27th.

thanks all for the help.

oh yeah also how do i get this bugger out:
20120812_011435.jpg


also should i be looking for and 8.5 unit or a 7.5?

thoughts: http://www.ebay.com/itm/GM-8-5-CHEV...Parts_Accessories&hash=item4cf6aa63a8&vxp=mtr

usa standard gear?

maybe two of those kits plus a reman posi unit? or maybe one of these combos? http://www.ebay.com/itm/GM-8-5-3-42...Parts_Accessories&vxp=mtr&hash=item4d03a395a3
 
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You have an 8.5. Don't waste money on a set of 3.08 gears. NOBODY wants 3.08s. So half ton axles with 3.08's are worth nothing. I have a few sets sitting around, but they generally go to iron. I don't think you'll like those gears. Try 3.42.

Martin
 
gonna actually stick to the 3.73 ratio to avoid messing with the front until time is less of a constraint. seriously eyeing this package: http://www.ebay.com/itm/GM-8-5-CHEV...Parts_Accessories&vxp=mtr&hash=item4d03a395a1

any idea if shorter gears like 3.42 and 3.08 will fit that carrier later on down the line? also if anyone has a link to come sort of comprehensive guide on ring and pinion install that is real in depth that would be great. i want to make sure i adjust the engagement correctly and everything.
 
You'll be happy you stayed with that ratio the next time you go up any kind of grade in your blazer...
 
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