Repairing transfer case leaks (GMT800 / NP246)

Reprise

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Have recently come to the conclusion that my transfer case has at least one (and possibly up to 3) leaks - when I pull it off the driveway, I can see the accumulation in what looks like 3 spots. Recent 'finger check' shows it to be trans fluid (and I know it's not from the trans). The residue under the truck is on parts back of the tailshaft, further suggesting the xfr case.

While I run in 2wd over 99% of the time, I know I can't just forgo the case and turn the truck into a 2wd. So it has to be present (or I get a new driveshaft).

When I tested it pre-purchase, all modes worked, and it (still) doesn't exhibit any noise (except for a possible clunk under initial acceleration, which I've been attributing to a needed intermediate steering shaft replacement - I'll probably tackle that first, as it's way less time / effort.

Not many choices on my local Craigslist - the few I found are $500 and up. Car-Part looks to be starting at the same and going up from there - quickly (some running $1800 or more). So...I am probably looking at repairing the existing one.

I know it's best to crack the case and put in the preventative to guard against 'pump rub' (or fix it if it already has it). If I'm going that far, I may as well get a full rebuild kit for the thing. A shame for something I use *maybe* two days a year (but might need it while I'm out with the trailer, so I have to bite the bullet on this one.)

Anyone rebuild one of these things and can provide some advice on 'gotchas' ? I know the retaining ring that's accessed through the slot on the tailpiece is considered a b!tch to put in - anything else? Wanting to get this resolved before the snow starts falling, so I need to get started now. Outside of that, I'm not too intimidated by this - just not looking forward to yet more work before I can start using the truck for its intended purpose.

Oh, and do I need a transmission jack to get this R&R'd, or will a regular floor jack suffice? Would rather not spend the $ for a dedicated trans jack, and since I work alone, I'm guessing it wouldn't be smart (or feasible) to R&R without a jack of some type to support it.

For reference - my xfr case RPO is MT1 - has the pushbutton 4x4, and the truck has a 3.73 gear. Should be the same case as a 2500HD, although I can't provide the NP model #, as I haven't run across the tag, yet (I know I'll have to find it before ordering the rebuild kit).
 

Reprise

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Well...I'm still alive!

(PSA: I made an idiotic move of jacking up the side of the Sierra a bit, to check on the transfer case model, leaks, etc. - and didn't put jack stands underneath before crawling under. Didn't fall or shift, but I realized as I was finishing up that that was a bonehead move. Gotta remember to use those stands, even if 'just looking', and even if the tires are still mostly on the ground). :bonk:

Anyway... what I found...
I don't see any fluid leaks from the TC, anywhere. The only fluid that's on there is on the lower case housing bolts - definitely Dexron (more on this below). Other than that, the entire housing looks dry. Wasn't able to do a deep dive for 'case rub', but I don't see any tell-tale fluid leaks from the top of the tail-end housing.

Also found the model tag, and after wiping off the grease - found out it's a NP246. Good thing I checked, b/c the info I had suggested it would've been a NP263 (actually, a 263HD, and the D-Max gets an exclusive 263XHD, for reference) Car-Part suggested interchange with the 2500, and that *does* use the 263. Oh, well... (maybe it came with a 263, and got a 246 swap in an earlier life?)

So I get back online and check...and apparently, the 246 uses Auto-Trac, *not* Dexron. I'll need to get back underneath when I have the truck parked the opposite way (and jack stands securing!), track down the drain / fill plugs and check out what's in there.

Since I can shift between modes without issue, I'll assume that what's *in* there, is *correct* (but I'll check the service / owner's manuals, as well.)

Don't know whether I should hope to find AT-II vs. Dex VI - if the former, it's definitely the trans where I have a leak. If the latter, then still might be the xfr case at fault (but it would be easier to service than the trans).

Looks like my trans pan is a CCC replacement, and wondering if that's the cause of my leak source - although I'd suspect the gasket over the pan, normally. With the amount of fluid I see after 24hrs of not moving the truck, I'd expect to *see* the fluid dripping, if I waited long enough. Didn't really see any on the garage floor today - maybe it's only a "when warm" (expansion) leak scenario, as I had started the truck from cold yesterday and drove it right into the garage.

Obviously, if I use 'maybe' this many times in a post, I'm a bit unsure as to what's going on. :confused:
And the Bears have scored 17 unanswered on the Pats. Today is officially a wack day :laugh:
 
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Mooseman

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Dec 4, 2011
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I don't think Dex would have damaged the TC or caused a leak. In fact, it is believed that Auto-Trak is tranny fluid coloured blue as another member did a comparison of the two and is using Dex. My TC had something totally different in it (a yellowish power steering looking fluid) but all it had was a leak at the front output shaft.
 
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Reprise

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Ah - not suggesting the previous owner(s) substituted the wrong fluid, just that what's in there might not be what I was led to believe (the SilvySierra guys said 'Dex', for sure.)

Because of poor engineering, the tail end of the case will form a leak of its own w/o help from anything else (whether it be a 246, a 263, etc.,) as the oscillation of one of the internal components will wear a hold through the top of the tail-end case, if not addressed (apparently worse, the more 4x4 is engaged).

I wouldn't be surprised if AT-II is some sort of Dex-based fluid - it would make sense b/c of the thin consistency, & the detergents in A/T fluid by default. I won't hazard a guess as to lubricity, but a transfer case is much less complex than a typical slushbox - so the requirements should be lessened (it would be nice if it lasted for > 50K change interval, but as long as one remembers...)
 
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Reprise

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Finally got the truck back in the garage & re-examined the xfr case.

Did a pre-drain level check, and... no fluid visible on the zip tie (put about 4" of it in the case, until I met resistance near the bottom). Not good...

Felt up top for any wetness, evidence of a hole, etc. Couldn't find any. But I couldn't locate the case vent / tube, either.

Pulled the 18mm drain plug (bolt, not recessed), and got about...a pint. Mayyybe 1 1/2. At least I didn't see pieces of the case innards coming out with the fluid (which looks / smells like old Dexron...a bit purplish, but I doubt it's AutoTrac - which is what the '03 owner's manual does call for)

Cleaned off the case bottom with brake cleaner & dried it off. While I was doing that, noticed some deterioration of the seal joining the two case halves together (and the only wetness on the case is in that area, on the bottom.)

Going to put some Permatex ultra black on the outside seam, dump in a couple of quarts of the cheapest Dex I can find, and see if it holds. If not...well, I guess I'll get to R&R the xfr case, either rebuilding or replacing.

(if someone has a better alternative to the Ultra Black, feel free to chime in. I already have the UB, and it's suppose to have 'max oil resistance'). Saw a rebuild on YT where the guy used Permatex 'Great Stuff' for the case bead / gasket, saying it worked best.

While I was under there, took a look at the 4L80's pan. Looks like I'm going to have to bend a heat shield for the one side, and work around the trans mount for 2-3 bolts in the rear. Ugh. And there's fluid from the bellhousing to the rear. Should be fun tracking that leak down. Not.
 
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SpitShine_PL

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May 26, 2023
34
Poznan, Poland
I think I'm facing the same issue as @Reprise did.
There's a weep with a very slow loss of the fluid in droplets (my TC is the NP246) from around the three-four lower bolts at the casing parting line.
Thanks to ground clearance, I managed to wiggle myself and my busted lumbar (had an injury while lifting a load... Uncoordinated lifting of nearly 500 lbs, might I add) with a battery worklamp and couldn't find any evidence of pinholes from the pump rub.
This is how it looks (shown from the passenger side, to the left of the picture is the rear end of the transfer case).

IMG_20230607_073906.jpg

I figure the fluid stains reaching across the rear case half are just the weeping/droplets entrained by the air wash while driving. The TC responds to all settings (4auto, 2hi, 4hi, 2lo) without any issues.

@Reprise , how did that leak on your truck end? Did the Permatex fix work, or did you have to yoink the TC from the truck, reassemble, and reseal the sucker?
I'm kinda suspicious of this fix, as the sealing surfaces are inward of the bolt circle and I think, even with a sealant applied very carefully to penetrate this hair-thin line, the fluid might still weep from between the bolt threads and the case lugs the bolts sit in.
 
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Reprise

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@Reprise , how did that leak on your truck end? Did the Permatex fix work, or did you have to yoink the TC from the truck, reassemble, and reseal the sucker?
Looking at the date of my last post in this thread, it's from 2018, so you'll forgive me if my memory is a bit lacking with the fullness of time. :undecided: :laugh:

As I recall, it didn't end well. I wound up purchasing a new truck, and selling that one, after I got a (used) replacement t-case to install. I threw it in, along with the upgraded remediation plate, for the new owner.

I can't remember whether the leak(s) on the bottom of the case were successfully resealed -- I think (?) they were, because I had gotten the improved plate that fixes the 'pump rub' issue -- and I'm pretty sure that I did detect leakage from up top (which is where the OEM internals rub a hole in the magnesium case (great for weight saving, but not so good when you have steel parts constantly rubbing against the inside of it, by default).

That fix also requires a new top case half, if a hole has already been rubbed through in it, and for the price of that case half, I found a whole replacement t-case for not much more money. But right after that, I decided to cut my losses with truck #1, and bought the truck I have now (in my .sig)
 
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SpitShine_PL

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May 26, 2023
34
Poznan, Poland
Gah. Not too good. I'm gonna inspect the TC again, drain the fluid, look for metal debris, refill with fresh fluid, take brake cleaner to the case and check again for the source of the leak. It's definitely confined to somewhere on the case.
The worst case scenario is getting a new rear housing with the pump rub plate... Unless the internals are shot somehow. Not too many healthy used NP246's out here in Poland (found two, and for the money they are asking I can just as well take the truck to a garage and have the TC fully rebuilt, hah).
 

SpitShine_PL

Member
May 26, 2023
34
Poznan, Poland
Update: Inspected it on a lift. There's definitely a weep from the upper part of the rear housing... tho I don't know where from, exactly, despite looking at the thing in bright light and using a telescopic mirror. Similar to @Reprise 's woes, the case looks like it has been filled with regular Dextron, not Auto Trak II - the fluid is dark red, but it looks clean. The rear half is somewhat caked with dust stuck onto the coat of fluid that came out from whatever the origin of the leak is.
That said, I managed to drive it down to a garage I kinda trust without the TC internals grenading themselves from loss of fluid; I think it lost about one or two drops of fluid along the way (I had cleaned the case somewhat before heading out).
The work stalled there, as the filler bolt won't budge. I believe it's aluminium or whatever soft metal alloy that does not cause galvanic corrosion in contact with the magnesium alloy of the housing; despite careful tapping and freezing with a spray on a very warm transfer case, I only managed to shear the hex flats on the bolt head. :| GAH!
Left it sitting on a service pit with a low platform lift at my dad in law's garage; his boys are going to take ugga dugga gear to the pesky bolt (driving a 17 or 16 mm spline socket onto it to engage the head or trying the good old way of welding a large nut on to have any flats to grab on).
My plan is to have the boys clean the case with brake cleaner, change the fluid once they undo the seized filler bolt, inspect the case thoroughly for any pinhole I might have missed, and see where it might be coming from - Auto Track II is green in colour, AFAIR, unlike the reddish Dextron.
Surprisingly, the transfer case responds to the shifts commanded with the button panel and runs without any clunks or other suspicious noise. This bodes well for my budget, cuz import and duty included, a new rear housing + labor will make me 300 dollars poorer :tequila:
God damn, I wish I had a home with a pit in the garage!
 

Mooseman

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Dec 4, 2011
25,262
Ottawa, ON
Small correction. Auto Trak II is blue. We call it Smurf blood :biggrin:
 

SpitShine_PL

Member
May 26, 2023
34
Poznan, Poland
Yup, we're home with this issue. One hairline crack, about 1/4" long on that boss of the rear housing circumferential taper step beneath which the pump lives.
I diagnosed this by cleaning everything along the weep path with brake cleaner and driving it some around block with 4Hi engaged.
Cleaned it out again and slathered with a huge ass blob of red RTV in overkill mode (covering a radius of almost one inch in diameter, and making sure it's thick and won't peel off along the edges of the RTV patch).
Should last for coupla hundred miles. Ordered a new rear housing and a pump saver plate and called it in with a garage proficient in such job.

IMG_20230727_153927.jpg
 
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SpitShine_PL

Member
May 26, 2023
34
Poznan, Poland
A small update: had the new T/C rear housing installed with all the consumable parts included for replacement, plus a pump saver/rub plate. Bolted it all down, drove it some, no leaks; the T/C internals were very much mint, basically no metal debris from wear in the fluid, and the T/C works like it should without practicing any territorial pissing.
(Also decided to replace the oil pan gasket in the process... And the water pump, the weep hole started gushing out the coolant like mad. Ouch me wallet!)
 
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