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Discussion Starter · #21 ·
Changed transfer case fluid and front diff fluid – 215,181 kms

The old transfer case fluid that drained out looked very good and was still prettyy blue - either the transfer case is in excellent shape or the fluid has been changed somewhat recently. I suspect this is not the first fluid change, as the fill plug had evidence of wetness – either it was weeping very slow, or it never got wiped up completely from a previous change. I bought new fluid from the dealership - Autotrak-II fluid. They told me $17/bottle on the phone and when I got there they told me that was a error in their system it was supposed to be $27/bottle, but they honored their original price which was great.

The front diff fluid looked fine to me too, it was a dark brown, but far from black, and the magnet did not have much metal on it either. I am guessing the fluid has been changed previously. I have seen videos of the front diff fluid being silvery from all the metal in it, and the magnet being totally caked – not the case here. I refilled with Motomaster Synthetic 75W-90 from Canadian tire. I did find in some reading that there was a service bulletin to replace the factory fill of conventional fluid in the front diff with synthetic, and replace the breather at the same time due to incompatibility with synthetic fluid – I am assuming that this was done already based on the condition of the fluid and magnet, but will keep an eye on the breather. If it starts to weep and will address it then.

I wanted to change the rear diff fluid too. This diff has a handy drain plug so I don’t need to take the rear cover off, but the cover has a little weep that I would like to fix while I am at it. I didn’t have enough time to finish that, so I will do it another time. I assume this rear diff cover will have no gasket, just a sealant – can anyone confirm?
 

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2001 GMC Sierra 2500HD CCSB
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Your transfercase calls for the blue stuff?

I bought a replacement breather plug for my front diff because I changed to synthetic also, but the plug wouldn't fit. I'm pretty sure it was a GM or ACDELCO. The new replacement was just slightly to large and would not screw in. I'm not sure what the deal was. I put the original back In. Hasn't been a problem.

I might have something about that in my build log, but I can't remember. That's been a while ago.
 

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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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Unless someone did some kind of custom install of an AWD transfer case/front diff, you don't need Autotrak fluid, just plain old DexIII (and you want to find a Dex III, as Dex VI fluids may leak past the seals if they haven't been replaced).

DexIII is for if you have either a manual or electric shift 2-speed transfer case. Autotrak is only for transfer cases with the "Auto"/awd feature.

Edit: Most likely, that Autotrak fluid works just fine, it's just a lot more expensive than a DexIII fluid.

And IDK if you've done this already, but you might consider installed a pump rub kit in the transfer case. If you haven't/don't know if it has one installed, you at least want to regularly check for it (you have to crawl under and look at the rear of the transfer case to check), as it rubs a hole in a spot where the fluid only comes out whijle the truck is moving and normally doesn't leave a spot under the truck, so you only find out about the hole once the transfer case has failed due to running without fluid for some time.
 

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Discussion Starter · #24 ·
Your transfercase calls for the blue stuff?

I bought a replacement breather plug for my front diff because I changed to synthetic also, but the plug wouldn't fit. I'm pretty sure it was a GM or ACDELCO. The new replacement was just slightly to large and would not screw in. I'm not sure what the deal was. I put the original back In. Hasn't been a problem.

I might have something about that in my build log, but I can't remember. That's been a while ago.
Yep, it does call for the blue stuff!

And yes, i read that in your thread this am actually - It reminded me that I am supposed to change the breather - Thanks!

Unless someone did some kind of custom install of an AWD transfer case/front diff, you don't need Autotrak fluid, just plain old DexIII (and you want to find a Dex III, as Dex VI fluids may leak past the seals if they haven't been replaced).

DexIII is for if you have either a manual or electric shift 2-speed transfer case. Autotrak is only for transfer cases with the "Auto"/awd feature.

Edit: Most likely, that Autotrak fluid works just fine, it's just a lot more expensive than a DexIII fluid.

And IDK if you've done this already, but you might consider installed a pump rub kit in the transfer case. If you haven't/don't know if it has one installed, you at least want to regularly check for it (you have to crawl under and look at the rear of the transfer case to check), as it rubs a hole in a spot where the fluid only comes out whijle the truck is moving and normally doesn't leave a spot under the truck, so you only find out about the hole once the transfer case has failed due to running without fluid for some time.
Yes, I have the "auto" button for the transfer case so based on the info online I figured I needed the Autotrack-II fluid. But knowing that I gave the dealer my VIN and told them i needed transfer case fluid, and that is the fluid they sold me. I assume they would have sold me other ATF if i did not have the auto transfer case.

Good to know about the pump/rub issue, I hadn't heard of it before. The transfer case was dry, apart from the very light weepage around the fill port I mentioned, so it hasn't rubbed through yet. I will have to search this up and see what its about, Thanks for the heads up - does it apply to auto transfer cases, or manual only?
 

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2001 GMC Sierra 2500HD CCSB
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If you replace your breather fitting let me know what part number you used. I would be curious to know what fits. Maybe I just got a badly made part, that was a strange deal.

I didn't think the 2500HD came with a transfercase that had "Auto" function.

I have a write up for pump rub in my thread, pictures of tear down and parts, etc. My pump rub fix and documentation
If you search you should find some threads about it here, and quite a few around the rest of the internet. It seems to have mainly been a problem with the HD trucks, gas or diesel with NP261/263 type transfercase, but the 1/2 trucks may be susceptible as well. I never read anyone complaining about it for 1/2 trucks though.
 

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Yeah, it has to be some kind of light duty thing, the regular 2500HD's never had them.

Edit: And I'm not sure the GM pump rub problem also applies to the TC's with the auto funciton, it does apply to the full range of 2-speed TC's used in the GMT800's (all the variants of NP261 and NP263 TCs).
 

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Discussion Starter · #27 ·
Car Vehicle Motor vehicle Steering wheel Steering part


I tried the auto function breifly this morning, I didn't notice anything different in operation (no surprise as there was no tire slip) but I wanted to see the button light up, and it did.

If you replace your breather fitting let me know what part number you used. I would be curious to know what fits. Maybe I just got a badly made part, that was a strange deal.

I didn't think the 2500HD came with a transfercase that had "Auto" function.

I have a write up for pump rub in my thread, pictures of tear down and parts, etc. My pump rub fix and documentation
If you search you should find some threads about it here, and quite a few around the rest of the internet. It seems to have mainly been a problem with the HD trucks, gas or diesel with NP261/263 type transfercase, but the 1/2 trucks may be susceptible as well. I never read anyone complaining about it for 1/2 trucks though.
I will add any info about breathers to this thread, but I won't be dealing with it unless i develop a leak there. Thanks for that write up and link on the pump rub fix, very helpful! There is lots of helpful stuff in your thread.

Yeah, it has to be some kind of light duty thing, the regular 2500HD's never had them.

Edit: And I'm not sure the GM pump rub problem also applies to the TC's with the auto funciton, it does apply to the full range of 2-speed TC's used in the GMT800's (all the variants of NP261 and NP263 TCs).
I thought it was odd when I first noticed the "auto" button, was kind of surprised a 3/4 ton would have it. Never thought much more of it after that.

I looked at the transfer case tag when I was changing the fluid but I don't remember the whole number as the numbers dont mean much to me just yet. I think it ended in "1" but should have taken a picture. Next time I'm under there I will try grab that number.

Thanks again for the heads up, knowing this now I will investigate any drips asap, and have a look at the transfer case every time in under there if I'm not able to get a fix installed.
 

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2015 High Country Duramax
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Being a 2500 non HD, it is the same as the later 1500hd trucks. In that its a mix up between the 1500 and the 2500hd series trucks.

You get the 14 bolt semi float rear axle, 8 lug wheels, 6.0/4l80 combo, but the autotrac tranfer case out of the 1500s. Lighter frame than the HD trucks, but heavier than the 1500s.

Think of it as a heavy half ton. Because that's basically what it is. Sorta like the old F44 option on the gmt400 trucks. My brother's truck is a 06 1500hd and has the same setup as yours. I want to say 03 is when they started the 1500hd instead of the 2500

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2001 GMC Sierra 2500HD CCSB
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Thanks for info @19trax95. I thought the 2500 non HD was only for a few early years before they transitioned everything to 1500 or 2500HD (and then 1500HD later), and not going as late as 04+. I had initially wanted a 1500HD, but glad I ended up with the full 3/4 ton after learning more about them.
 

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Discussion Starter · #30 ·
Thanks 19trax95, I was a bit confused about what the 2500 vs 2500HD was all about, now I at least know what I have. Although it is kind of disappointing that is isn't a "real" 3/4 ton, it is serving its purpose for us and we didn't buy with the intent of it being our forever truck anyways. I suppose the only real scenario we will see that having a "heavy half ton" will not be as good as a real 3/4 ton is if we hook up our boat behind our fifth wheel - . A heavy half ton is sufficient, or maybe even suits us better for every other scenario - daily driving, towing a small fifth wheel, or a small ski boat, maybe some hunting etc.

So just so I am clear, do I have the same rear axle as the HD trucks - the 14 bolt semi float axle? or do the HD trucks have a heavier axle? Forgive my ignorance, I'm learning here.
 

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So the gmt800 trucks, in the 1500hd/2500 and the 2500/3500HD trucks have 3 different axles for the rear. All of them are 14 bolt covers.

The 1500hd/2500 use the 14 bolt semi floating axle. Stronger than the 10 bolts that are in the 1500s, but not as good as the full floaters in the HD trucks. It has a 9.5" ring gear

The 2500/3500 gas trucks got the 14 bolt full floating axle. Tons of aftermarket support and they are super common for offroad use as well. It has a 10.5" ring gear

The diesel trucks (2500 or 3500) got the 14 bolt AAM full float axle. This is a beast of an axle. It has an 11.5" ring gear.

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No problem. It's interesting all the various combos that GM has had over the years.

A word of caution though, with the 1500hd/2500 trucks, watch out when getting parts. Mainly chassis and driveline parts. Some places don't call out or even list the 1500hd or 2500 non HD trucks.
 

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Discussion Starter · #35 ·
Yes, I noticed that going through Rockauto previously - I saw in the axle section they had parts for axles with 9.5", 10.5" and 11.5" ring gear. Now I know what that is all about and most importantly I know which one applies to my truck.
 
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