Transfer Case & Rear Differential Issues

PeterJr · September 4, 2017

Hello Everyone,

I have a 2005 Silverado 2500HD 6.0L. I have recently taken it to the mechanic with a few issues but of the major ones they are a rear differential whining and a transfer case leak that is pretty serious. I am filling the T-Case with about a quart of fluid every week.

Anyways, the mechanic has suggested replacing the entire rear axle assembly as well as the transfer case.

I have a rear differential picked out from a salvage yard and am in the process of looking for a transfer case but I cannot seem to find one that will fit my vehicle. Does anyone know where I am to start in figuring out what type I need? I have a floor mounted shifter and an "MT1" RPO code in the glove box but when I look at an RPO code sheet the MT1 is deciphered as,

"MT1 : TRANSMISSION, 4 SPD, AUTO W/ELECT CONTROLS H.D. (HYDRA-MATIC 4L80-E) "

I do not have any electronic controls so I am a bit confused here.

Also, does anyone have any experience ripping apart the T-Case and rebuilding it? I am leaning toward doing this because it could save me probably about $800 just in parts.

Does anyone know what is going on? Thanks a lot for your help,

Peter

Formulabruce · September 4, 2017

HI Peter,

1. There is a kit for the transfer case leak to fix it, its cheap, and on ebay. the case has a worn hole and there is a guard in the kit you put in it after cleaning and sealing with JB weld. There are many videos on this. If yours works, dont replace it, repair it ( you have a "work truck" style T -case)

2. rear end. Check your glove box tag for the ratio should say GT and a number., see if it says G80 as well ( limited slip)

Howling could mean a worn pinion, and might be cherper to grab the right rear.

You need to know the ratio ( GT??) and disc or drum

you may be able to sell your old rear as many rebuild them. Good luck

davester · September 4, 2017

First, either you picked a really expensive mechanic (so, the cheapest option is always to just replace X) or they aren't a very good/experienced one. It should be cheaper to fix the TC than to replace it with a used one. And did the mechanic say what was wrong with the rear end, as it's unusual for them to need a lot of expensive work unless you are doing a lot of burnouts...

Yeah, it is likely that your TC needs something called a "pump rub fix". Cheapest fix is a pressed metal plate, then more expensive is a billet oil pump, and then most expensive is a new case half made of something else besides magnesium. Depending on how long you've let it go, you may need to buy a case half anyway. Doing this is really straightforward, and doesn't need a lot of special tools (c-clip spreader, things of the right diameter to pound in the seals if you do them).

When I installed 4wd in my truck, I rebuilt the NP261HD with new bearings, seals, output shaft bushing, synchro pad, and pump rub plate. Bearings didn't really need replacing, seals do if you disassemble the TC (ie, remove the shafts and take everything apart not just split it to install the pump rub plate).

I would suggest while it's open, you also check out the shift forks, as the pads on them wear, and replace the rear output shaft bushing. Check the bearings and gears if they have play, it's difficult to check if the chain needs replacing as you can't easily tell if it's stretched (as the case needs to be assembled for the two shafts to be held the proper distance apart, and then you can't see the chain), the synchro also has a cone-shaped friction plate in it (at least the NP261/263 ones do)....

And while you have a shift lever in your truck, the 4l80e is considered an electronic transmission, as there is a computer (the TCM) that controls when it shifts.

As for the rear axle, no idea what could be wrong, low on fluid, bearings, or somethings up with the ring/pinion. A mechanic should be able to tell you what the problem is, more than just suggesting you replace it.

If you do replace the rear axle, make sure it's got the same gear ratio as the one you are removing, and find out if it's got a G80 locker or not (so you know how it should work and what fluid to fill it with). And you also want an axle from a SRW, as a dually axle is a different width (regular dually is wider, cab&chassis is narrower).

PeterJr · September 4, 2017

_{Hi Guys,}

_{Thanks for the replies.}

_{From what I remember, the leak is coming from the gasket / seal on the right side of the TC if you are looking at it from the rear diff. As far as I know this is the only thing wrong with it. It is just losing fluid. He said he checked for cracks / holes and didn't find any and said it was leaking from this spot. As it sits in my driveway you can see where it is leaking from.}

_{It is the "MT1" Transfer Case NP261, YX Build Code but when I look for it on salvage yard sites it is only showing a "MW3", I guess that is where I am confused.}

_{The rear differential is a "GT5" option, full floating 10.5 ring gear & 4.10 gear ratio. I think the rear axle I am just going to replace because I can get one for $600 with 43k on it, but the TC I am going to rebuild with a friend of mine who has done it before.}

_{I just want to make sure I am getting what I need to rebuild this thing. As far as I know the only issue with the TC is the leak. I am putting about 1 quart of fluid in there every week. I am having trouble figuring out exactly what kind of TC I have though. So far for sure I know I have: NP261 YX Build Code. I do not know if it is NP261HD or not. I would think with a HD truck it is a HD transfer case. Can anyone shed some light on this? Or what kind of rebuild kit / parts I would need to get or what could be wrong besides the leak? Thanks a lot,}

_Peter

davester · September 4, 2017

You likely have a NP261HD, as that's what was in my donor '06 Sierra 2500HD with the MT1 option. There's a round tag on the outside, which generally needs to be cleaned really good to see the full part number.

Never see that mechanic again, if he said it was just leaking, and still recommended you replace it.

It's relatively unusual for the case to just start leaking where the two halves join. Put the truck up on jackstands, then clean the TC as best you can, to figure out exactly where it is leaking. In particular, google for "gm pump rub fix", go through the results and look at the TC images showing where it makes the hole, and clean that area of the case really well to make sure there's no hole there.

Also, check the fluid level of your transmission, as sometimes the input shaft seal will fail, and then fluid goes forward into the transmission and some leaks out along the edges of the adapter between the transmission and the TC.

If you don't know if a pump rub fix has been installed, you DEFINITELY want to install one (either in your TC or in the replacement unit).

markwilks · September 26, 2017

Check the top of the transfer case to make sure a hole hasn't already been rubbed through. If it has you can buy a case half and the pump rub kit. I had the kit installed as a preventive measure. Luckily when they split my case open they said the inside didn't show Rub damage but I'd rather be safe than sorry. It's a huge issue GM never took responsibility for and I was told it probably was a matter of time before a hole would rub through. My truck is a 03 Sierra HD with 6.0 and 129000 miles.