Originally Posted By: 390pi
Google GM "pump rub."
These TCs wear through the case due to the oil pump.
That may explain the high magnesium.
Well - I hadn't thought of that. Because this is a Tahoe, and I'm not very familiar with the drivetrain, I wasn't thinking in that mode.
It is very probable that the Mg in this report is telling the OP that pump-rub onset has begun! DANGER, WILL ROBINSON, DANGER!
What is pump rub? In short, the t-case eats itself a hole low in the case, and your fluid will drain out without your knowledge, and your case gears/chain will grind to an ugly halt.
If it has (and I suspect it has) there is no way to tell how bad the condition is unless you take the t-case apart. And by the time you go to that effort, you might as well do the fix at the same time. Read on ...
If this Tahoe has a t-case that is affected by pump rub, then I HIGHLY suggest it be addressed before pump rub eats a hole right through the case. Once the magnesium case gets a hole in it, you're into a VERY expensive repair or replacement.
Essentially, the t-case has a fluid pump body made of aluminum. Al is pretty soft, but unfortunately, the case halves are made from magnesium, which is actually softer. So as the pump body tangs bear against the internal case recesses, one of them grossly wears into the magnesium, and eventually rubs a hole right though the rear case half! (Hence the term "pump rub"). They discovered this issue early in design, and GM (actually New Venture) put a small metal clip in-between the pump and case, but it does not take long for that clip to:
1) break
2) fall out of position
3) stay in position, but bend out of shape, and not offer protection
Pump-rub can occur as early as 50k miles, or as late as 150k miles, but the general concensus is that it WILL happen at some point.
You can google GM pump rub for some videos. There are some vendors out there that sell kits to avert this disaster. There is one at Merchant Automotive that has a great detailed pump rub kit and fantastic DIY article. I didn't use their kit, but I did use their DIY for the "how to" remove/repair/replace.
http://www.merchant-automotive.com/transfer-case-pump-upgrade-kit.aspx
Here is the tech article:
http://www.merchant-automotive.com/tech/tcp.pdf
There is a lot of good info on that page. I am not pushing this brand over another; it's just that they have a great video and great write-up for the DIY. Also, their kit cost has come WAY down; it used to be around $175, but it's only $75 now!
The upside to the fix kits is that even if pump rub has started, as long as you catch and fix the problem before it eats all the way through, the "fix" will stop the continuation of the problem. So if you have an indentation started, but it's not rubbed all the way through, you can salvage the rear case half rather than buy a new one. Some guys have even gone so far as to use JB Weld epoxy to fill an existing hole, and it's held up for many tens-of-thousands of miles. I know of one such person over on another site.
I have a 3500 GM truck, and it has one of the very t-cases that is susceptible to "pump rub". I was so concerned about it that I did a pre-emptive install of a "pump rub elimination" kit at 10k miles; I'll never have to worry about the problem now. I used a "Riverside" kit; only $50 at the time. You can get the plate only for $50, or you can get a kit for $100. Choose product as your needs would dictate.
http://www.riversidegear.com/transfer-ca...ra-all-wit.html
Here is one from Kennedy:
http://www.kennedydiesel.com/categoryres...ubCategory2=135
This site offers just the plate (lower down on the page) or a full repair kit including the rear case half (near top of page).
The Merchant Auto kit actually replaces the back half of the pump where the others are simply a plate that sits behind the pump. Personally, I think they all accomplish the task at hand. The "plate" style is a bit easier to install and requires just a bit less disassembly. Other than that, I see these all doing a great job equally well.
If you have pump rub and it's early in the progression (small indentation) I'd say just get the cheapest plate, install, and you'll never have to worry.
If you have pump rub and the progression is heavy (large indentation, but not rubbed completely through) then I'd say perhaps put in some epoxy to fill the void, and do a cheap plate.
If you have a full hole rubbed through (which means you'll have no oil left in the t-case) I'd suggest the full repair kit with case half.
Regardless, do it soon before it's "too late".
I credit 390pi with catching this tell-tale sign. And this is a perfect example of how a UOA can find problems BEFORE they competely destroy a piece of equipment. While not a 100% assurance that pump-rub has begun, I'd be willing to stake a small wager on it, as your Mg is WAY above average, and there is only one logical cause for the excess metal count.
