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Discussion Starter #1 (Edited)
EDIT: See post #4.


I swear, who writes these service manuals (Haynes, in this case, but my brother's Chilton for his Nissan is even worse)!? So many things are incomplete, inaccurate, ambiguous, misleading, or just flat-out WRONG; it's just plain disgusting. It doesn't help that different GM truck models of slightly different model-years have a million and one changes from one to the next...

Anyway...the manual says that when refilling the oil in my truck's differential, I should leave the level of the fluid 5/8" to 1 5/8" below the bottom of the fill plug hole, IF I am using "synthetic axle lubricant". My problem is, I don't understand if that reference to "synthetic axle lubricant" refers specifically/exactly to what the manual calls for in GM quadrasteer (4 wheel steering) model applications (this: https://www.amazon.com/Genuine-88900401-75W-90-Synthetic-Lubricant/dp/B00BK7LXGM ), or if "if using synthetic axle lubricant" just refers to any old synthetic gear oil...as opposed to conventional/non-synthetic gear oil. In the "Recommended lubricants and fluids" section of the manual, there are two different specifications: one is "75W90 synthetic gear oil (for regular "Rear" differential), and the other is "GM synthetic axle lubricant" (for "Quadrasteer rear axle"). The issue is that, because of this uncertainty, I don't know if I should fill the differential all the way up until it drips out the fill plug hole, or if I should leave the level a little lower as indicated above ^^.

I am using Valvoline SynPower full synthetic limited slip SAE 75W90 GL5 gear oil. So, do I fill the diff to the fill plug level with this stuff, or leave the level lower?

I ASSUME that the whole "leave the level lower" thing is for the Quadrasteer systems only and that I should therefore fill the differential until the fluid drips out the fill plug hole, but I want to be completely sure: I hate acting on assumptions!
Thanks in advance if anyone happens to see this and could give me a definitive answer...
 

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Don't fill it to the hole when using synthetic, which is what you are supposed to use. Those numbers are not quadsteer specific. Supposedly, if you fill all the way, seals can start leaking because of the heat expansion and that it is synthetic. Use a Allen key or bend a hangar or something to check the level in the hole.
 

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Discussion Starter #3
Don't fill it to the hole when using synthetic, which is what you are supposed to use. Those numbers are not quadsteer specific. Supposedly, if you fill all the way, seals can start leaking because of the heat expansion and that it is synthetic. Use a Allen key or bend a hangar or something to check the level in the hole.

Hmm. That does seem to align with what I read in the owner's manual after posting this thread (was referencing only the service manual before). The owner's manual calls for
the specific GM "synthetic axle lubricant" regardless of which rear differential is in the truck (***???*** any synthetic GL5 gear oil of the proper viscosity should work... I don't actually need the specific GM part number for the GM "synthetic axle lubricant ***???*** right? I'm 90% sure this is correct, but again...assumptions...). The owner's manual also has different fill-level specifications for the regular 1500, HD1500/2500 and Quadrasteer axle models. It does call for 5/8" to 1 5/8" low fluid level for regular 1500 models, but I am still not 100 percent sure if that is the same for my aftermarket synthetic gear oil versus the GM specific fluid.


It seems fairly clear at this point that I need to not fill it all the way. I guess my only real uncertainty now is whether I can actually use the Valvoline Synpower or other synthetic GL5 gear oils of the proper viscosity, or do I actually need the specific GM fluid in there? Assuming...that I do not need the GM specific fluid? It seems like you were saying that it doesn't matter if I use the GM fluid versus any other synthetic fluid, but I'm not sure if I understood that right?
 

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Discussion Starter #4
Okay, I used the SynPower and filled to about 3/4 inch below the bottom of the fill plug (2 quarts plus 50 ML...won't add the 50 ML next time, just wanted to make sure it was actually on the Allen wrench I was using as a fluid dipstick...). Thanks for the response. Now on to trying to make sure my trans fluid level is correct after changing fluid and filter...
 

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here is my .02 .... Keep in mind i know nothing about the quardasteer trucks, i have never actually seen one in the real world so i only intend my comment for the standard truck...

That differential is a GM 10 bolt and its really no different than any other GM 10 bolt that they have been using in trucks for decades, a lot of the parts are still interchangeable from old to newer. I personally would not hesitate to just fill it to the fill hole and be done with it. I personally don't buy for a second that heat expansion will be a problem, these diffs are vented so pressure will not be a problem and there is more than enough room in there for expansion. If that was truly an issue my old 2006 with 200k+ and counting on the clock would have had differential issues a long time ago but it has not. At 1000 miles the factory fill rear end oil got swapped out for Amsoil severe gear 80/90 and it was filled right to the hole same with every other time i changed it approx every 50 to 60k after that. That truck has never had an issue with seals or anything else and unlike a lot of other trucks i have seen of its vintage and mileage its not sloppy and zero gear whine. Not a single part has ever been replaced on that rear diff.

In my opinion the reason for the lower fill point is fuel economy, with less oil in there the rear end will turn slightly easier and it can lead to better mileage. Low fill was and may still be an old drag racing truck to try reduce drag on the driveline.

GM did the same thing on my 2014 and went with a low fill point and also went with a lighter 75/85 oil. That truck has a 12 bolt and just like the 2006 the factory stuff wasn't in there long and in went the Severe gear 80/90. I also swapped the cover for one that holds about an extra 2 quarts of oil. That truck has 75k on it and again problem free and its not sloppy at all.
 

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Discussion Starter #6 (Edited)
jb882: You may be right. I figured it couldn't hurt to fill it lower as specified (and it was specified in both owner's and service manual), so I went with that. As for the expansion being an issue, perhaps if that vent gets plugged... On mine, the hose was broken about two inches from the vent, and it was pretty dirty. I just cut a new piece of hose and put it on there.




Some of the stuff on this truck makes me really scratch my head concerning the engineers responsible (and maybe a few thoughts of hanging them in the town square after public humiliation and flogging). No drain plug on my transmission pan, shift linkage bracket has to be removed in order to drop the drain pan (via bolts that are nearly impossible to see or access...and the manual just says to "remove the bolts"; so I had to figure out that they are Torx T-40 bit bolt heads just by poking around in the dark with my wrists all twisted around akwardly...LOL, and one of them was filled with dirt so I had to clean it out with a screwdriver first to help figure that out), trans filter seal is pressed up into the valve body where the filter seats, and you have to crimp the rim of the seal inward and then pull on it with pliers to remove it...risking damage to the aluminum), no drain plug on the rear differential (yet the front has a drain plug and a fill plug...meaning 30 minutes to an hour painstakingly scraping away at the old gasket), transfer case fill plug is perfectly horizontal (why the hell couldn't they have just angled it up to accept a funnel without needing a hose attached to the funnel!?)...

LOL, but at least I am getting it done and learning as I go. Next time around, all of this will be a lot easier and faster, as I will already know which components I have (e.g. ring gear, transmission, which transfer case...took me a while to figure out there was a tag on the t-case itself). I guess this is basically the process with any "new" vehicle until it's been completely gone through; just seems like it's been a little more painful with this one haha.
 

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The reason for leaving the level a little low is so that when you add the limited slip differential additive to the existing oil, it isn't overfilled. If the differential is equipped with a limited slip carrier, the clutch plates that limit the free movement of the axles independently require a specific additive that when added will occupy some of the space in the differential. The additive is a friction modifier that helps control the noise that a limited slip carrier would otherwise make without the additive. Your first task in tackling your job will be to positively identify whether you actually have a limited slip differential. This can be done by deciphering the coded numbers stamped somewhere on the axle. (I haven't kept up with the locations of those numbers over the years for all axles, sorry.) Or, you can simply pull the inspection cover off the differential and look at it. That also is a quick way to completely drain it if you were wanting to do a full fluid change. If you do have an LSD, not installing this friction modifier shouldn't cause any major catastrophic failures to your rear end, but, over time you will begin to notice it's not in there. The symptoms include chattering, low frequency popping, creaking and such as the plates slide over each other while turning. The sounds will mimic the sound disc brakes make when barely starting to move with them applied heavily... such as when a Jeep is going down a ledge really slowly. Some discussion on the topic. https://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=510512

The actual product. https://www.amazon.com/Genuine-GM-88900330-Lubricant-Additive/dp/B0028CMPIS/ref=sr_1_3?ie=UTF8&qid=1534419952&sr=8-3&keywords=GM+limited-slip+differential
 

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I don't think that the limited slip additive is the reason either, when you do a service if its needed that would normally go in first and then you fill to the correct level. At least that is how i learned to do it. I also don't know of any modern GM trucks that actually have a limited slip. They normally come with either an open differential or a Eaton mechanical locker from what i remember. If you have RPO code G80 in your glove box its a locker not limited slip. My last 4 Gm trucks have had the locker going back to the early 90's. The Eaton locker does not require a friction modifier to work and GM makes no note of its use in the owners manual.


I agree that the no plug on the transmission pan is dumb, GM has not had one in decades. But they are also not the only one that omitted the plug. I know my buddies Ram doesn't have one, neither did the 2 he has owned before that and i'm pretty sure Ford doesn't put them on either. None of the manufacturers expect that these vehicles will be self maintained( if at all). A friend of mine has a 2014 Ford Explorer that does not even have a fill tube for the transmission or a dipstick to check the level. The owners manual says to bring it to the dealer.....

Which shift linkage did you have to remove the transmission or the transfer case? On my 06 i have the autotrac case and i never had to remove anything to get the transmission pan down. My 2014 GM decided to make it extra fun by putting the exhaust so close that the only way to drop the pan is to also drop the y pipe and that requires disconnecting them at the manifold.

As far as filing the transfer case goes i have a fluid pump that screws on to the bottle and has a long hose for that. Works great for the front and and rear end too. No having to juggle a funnel under the vehicle.

One upgrade i have done to my trucks for a long time now is a B&M transmission pan. It has a drain plug and it makes the whole job sooooo much easier. It also gives a couple quarts of extra capacity and thanks to the aluminum material dispersing heat the transmission runs cooler. My 2014 for instance shows 10deg cooler on average and over 20deg cooler when towing. Cooler is better when it comes to an automatic.

Here is a pic of it.



I also put a new cover on the rear diff. looks like this. It also has a drain plug :)
 

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Discussion Starter #9
The reason for leaving the level a little low is so that when you add the limited slip differential additive to the existing oil, it isn't overfilled. If the differential is equipped with a limited slip carrier, the clutch plates that limit the free movement of the axles independently require a specific additive that when added will occupy some of the space in the differential. The additive is a friction modifier that helps control the noise that a limited slip carrier would otherwise make without the additive. Your first task in tackling your job will be to positively identify whether you actually have a limited slip differential. This can be done by deciphering the coded numbers stamped somewhere on the axle. (I haven't kept up with the locations of those numbers over the years for all axles, sorry.) Or, you can simply pull the inspection cover off the differential and look at it. That also is a quick way to completely drain it if you were wanting to do a full fluid change. If you do have an LSD, not installing this friction modifier shouldn't cause any major catastrophic failures to your rear end, but, over time you will begin to notice it's not in there. The symptoms include chattering, low frequency popping, creaking and such as the plates slide over each other while turning. The sounds will mimic the sound disc brakes make when barely starting to move with them applied heavily... such as when a Jeep is going down a ledge really slowly. Some discussion on the topic. https://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=510512

The actual product. https://www.amazon.com/Genuine-GM-88900330-Lubricant-Additive/dp/B0028CMPIS/ref=sr_1_3?ie=UTF8&qid=1534419952&sr=8-3&keywords=GM+limited-slip+differential

I haven't checked to see if I have an LSD or not, yet, although I'm 95% sure that I don't. I used synthetic limited slip gear oil, though, so it should be fine either way (assuming I wouldn't need that GM specific LSD fluid if I did have an LSD...).
 

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Discussion Starter #10 (Edited)
jb882: I had to remove the transmission shift linkage bracket from the driver's side of the transmission oil pan. At first I was prying on it like an idiot because of some YouTube jackass' advice LOL...no harm done: I didn't pry too hard...there was no way prying would have ever created enough space.

Transmission fluid/filter/seal, transfer case fluid, rear differential oil, and engine oil and filter done; front diff, accessory drive belts/pulleys, fuel filter, spark plugs and wires, air filter replacement, and drive shaft/chassis lubrication to be done, as well as fixing some little things like driver's side window/lock controls bracket and camper shell mounting brackets. Then I'm going to drive the crap out of it for 6 to 10 years :)

EDIT: I don't have a serviceable fuel filter in this truck, even though my garbage Haynes manual says to change it...sigh.
 

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Huh, i don't remember having to ever move the bracket on my 2006 or my 2000 and those have the same transmission as yours. I do know when i swapped the pans out for ones with drains i had to reposition the linkage with a special bracket but never needed to remove it to get the factory pan down.

As far as your rear diff goes, look in the glove compartment at the option codes and look for G80. If that code is there you have an Eaton mechanical locker otherwise its open diff if the code is missing. Either way you should be fine with what you put in there.

When you lube the chassis don't overlook the idler arm on the passenger side. The bottom fitting where it attaches to the steering is a real pain to get to so some people ignore it. I ended up swapping mine for a 90deg fitting and pointed it to the wheelwell side to make it easier to get to. The top fitting is much easier to get to from the engine compartment than it is from underneath if you don't have a lift. The pitman arm fitting is a challenge too but seemingly easier than the idler arm.

FWIW i used Lucas #2 red and tacky on the front end of my 06 from day 1 and the front end parts are still original in that truck 12 years and 200k+ later, they are still tight today. Given the condition of the roads here in New England 200k out of a set of ball joints and rod ends is nothing short of amazing. My 2000 i used something else in them and they didn't last 100k and they are the same parts that are in the 06, same basic truck.... My 2014 on the other hand has that lubed for life junk on the ball joints and the ball joints have been replaced twice already under warranty in 75k. When my warranty is up it will be getting new Moog joints that have a grease fitting.....
 

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Discussion Starter #13 (Edited)
Huh, i don't remember having to ever move the bracket on my 2006 or my 2000 and those have the same transmission as yours. I do know when i swapped the pans out for ones with drains i had to reposition the linkage with a special bracket but never needed to remove it to get the factory pan down.

As far as your rear diff goes, look in the glove compartment at the option codes and look for G80. If that code is there you have an Eaton mechanical locker otherwise its open diff if the code is missing. Either way you should be fine with what you put in there.

When you lube the chassis don't overlook the idler arm on the passenger side. The bottom fitting where it attaches to the steering is a real pain to get to so some people ignore it. I ended up swapping mine for a 90deg fitting and pointed it to the wheelwell side to make it easier to get to. The top fitting is much easier to get to from the engine compartment than it is from underneath if you don't have a lift. The pitman arm fitting is a challenge too but seemingly easier than the idler arm.

FWIW i used Lucas #2 red and tacky on the front end of my 06 from day 1 and the front end parts are still original in that truck 12 years and 200k+ later, they are still tight today. Given the condition of the roads here in New England 200k out of a set of ball joints and rod ends is nothing short of amazing. My 2000 i used something else in them and they didn't last 100k and they are the same parts that are in the 06, same basic truck.... My 2014 on the other hand has that lubed for life junk on the ball joints and the ball joints have been replaced twice already under warranty in 75k. When my warranty is up it will be getting new Moog joints that have a grease fitting.....

That trans pan definitely was not coming down without removing the linkage bracket. I know GM likes to do a million things differently even on the same model/year trucks LOL, so...maybe the pans are different or something.

I definitely don't have a locker (I wish, and might buy one some time, maybe).

As for grease, I use Bel Ray Waterproof grease -- very high quality stuff. I just ordered two tubes and a new grease gun for it.

According to the service history that came with the truck, these tie rides/ball joints have never been replaced. The truck drives fine, and as long as it does, I'll probably leave all of that alone. It did get new wheel bearings very recently. I'll do tie rods/ball joints probably after I notice some front end issues, or maybe preventatively at 200k (163k now).
 
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