Fluid Change Questions (Diffs + Transfer case)

[ejj]

My truck as 80k miles, and I don't know the maintenance history. I would like to change all the fluids in the differentials and transfer case. I have a few questions:

Diffs:
* My manual says that the front diff uses 80w-90 for "Four-Wheel Drive" models, and 75w-90 for "All Wheel drive" models. Since mine is electronically selectable (Auto, 2H, 4H, 4L), do I assume that's the Four-Wheel drive model?
* What is the approx capacity of each diff? I can't find this listed anywhere, and I'm not sure how much fluid I'll need for each.

Transfer case:
* Again, Auto vs. Four-Wheel. For Four-Wheel drive, the listed fluid is AUTO-TRAK II. Is this available at any part store, or a GM only part?
* My Haynes manual lists close to a dozen Transfer Case models. How do I identify mine? Is there a tag on the case?

Thanks!

[Z07]

Look for a fluid service tag on the fill plugs. It is a stamped piece of aluminum about 1.5 inches long. It will list the fluid needed.

Your RPO codes on your option sticker will tell you what diffs and t-case you have too.

If you can't find a tag and can't decode the RPO then post pics of the diffs and t-case.

Always remove the fill plug BEFORE you remove the drain because if the drain plug comes out freely and the fill plug is seized then you have a vehicle that can no longer move under its own power until the fill plug is sorted out.

I follow the recommendations from Dana/Spicer reguarding the use of LSD additive. Some don't use it and have no ill affects some are not so lucky.

[ejj]

Look for a fluid service tag on the fill plugs. It is a stamped piece of aluminum about 1.5 inches long. It will list the fluid needed.

Your RPO codes on your option sticker will tell you what diffs and t-case you have too.

If you can't find a tag and can't decode the RPO then post pics of the diffs and t-case.

Always remove the fill plug BEFORE you remove the drain because if the drain plug comes out freely and the fill plug is seized then you have a vehicle that can no longer move under its own power until the fill plug is sorted out.

I follow the recommendations from Dana/Spicer reguarding the use of LSD additive. Some don't use it and have no ill affects some are not so lucky.

Couldn't locate a tag on the rear diff, but that doesn't have options. The Transfer case is a model #246, which uses the GM AUTO-TRAK II. Do I have to get that through the dealer. According to the Haynes manual, I need 2 quarts.

Couldn't locate a tag on the front diff either, but based on the transfer case, I assume I need 80w-90. Can the dealer confirm this for me? I need to stop there for parts anyways...

I hadn't heard of LSD additives used in the diffs...should they be?

Any input on the capacity of both diffs?

[MrShorty]

Unless you have an aftermarket clutch type LS, you don't need any kind of LS additive. The factory diffs (open or G80) don't need the friction modifier additive.

80/90 gear lube should be fine for the diffs

You do need the blue ATII fluid for the transfer case. I haven't yet seen available aftermarket, so I think you still need to get it from the dealer (be careful asking aftermarket places. I had some try to tell me that AT II and DEX III are close enough).

And I will second removing the fill plugs first. There's isn't much worse than getting the fluid drained only to find out that you can't fill it. I don't know if they changed it by 2005, but the fill plug on my '98 NV246 takes a 5/16" square head driver which I had a hard time buying off the shelf. Ended up buying a 1/4"Fx3/8"M adapter and grinding down the 3/8" to 5/16".

[ejj]

Unless you have an aftermarket clutch type LS, you don't need any kind of LS additive. The factory diffs (open or G80) don't need the friction modifier additive.

80/90 gear lube should be fine for the diffs

You do need the blue ATII fluid for the transfer case. I haven't yet seen available aftermarket, so I think you still need to get it from the dealer (be careful asking aftermarket places. I had some try to tell me that AT II and DEX III are close enough).

And I will second removing the fill plugs first. There's isn't much worse than getting the fluid drained only to find out that you can't fill it. I don't know if they changed it by 2005, but the fill plug on my '98 NV246 takes a 5/16" square head driver which I had a hard time buying off the shelf. Ended up buying a 1/4"Fx3/8"M adapter and grinding down the 3/8" to 5/16".

I picked up 4 quarts of ATII at the dealer. Is that enough for a drain and fill?

Anyone have any input on how much gear oil is needed for each diff?

Thanks!

[1st Synthetics]

I would say 1.5 quarts in the front and 2.75 in the rear, but that is a guess.

[Browntowns20s]

just take it in to a service shop, we do both difs and the transfer case for 250-

[ejj]

just take it in to a service shop, we do both difs and the transfer case for 250-

Now that's no fun. I prefer to do it myself.

[grimreapersshadow]

your rear should take about 2 1/2 quarts and the front maby 2 quarts best way to know is fill em to the plug and it dribbles out a bit then your done

[1st Synthetics]

Your transfer case should only be 2 quarts of Auto-Trak II.