GM rear differential lube - synthetic 75W-90...
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Thanks to everyone for the good advice. I was beginning to think that I should just pay someone to change the fluid....but I do like doing these things myself.
I will take Mike's advice and buy a gasket at the dealership on Monday. I will drain and refill with the SVG.
Thanks again for the advice.
I will take Mike's advice and buy a gasket at the dealership on Monday. I will drain and refill with the SVG.
Thanks again for the advice.
One other thing, this is from GM TSB.
They are talking about GM gear lubes.
From 1999 up GM requires synthetic 75W-90 gear lube in the rear axles. No additive.
I have onwed many a GM truck with Locking rear axles dating back to the 70's. Only GM cars have limited slip, no trucks use it. DO NOT USE ANY ADDITIONAL ADDITIVE WITH A LOCKING REAR DIFFERENTIAL, GM says it can cause performance issues.
It will take a little less than 2-/2 qts to fill, You must replace the gasket, do not try to reuse it, IT WILL LEAK. Caution-GM does not use RTV on rear axle covers, only gaskets (they are less than $10 from dealer).
Amsoil has those pmups for the bottles, I have one here and it makes it a easy job, around $10.
Overfilling is as bad an underfilling.
Btw
I have the SVG in my 2004 GMC and 2005 CTS.
GMC has locker (no additive)
CTS has Limited slip (used additive, probably could have not but dod not want to have to redo it).
When using limited slip additive's, always mix them with the gear lube before adding to differential. This way the lube does not have a chance to settle and not mix with the lube. Also, it best to drive in a figure 8 to work the new lube into the differential. By doing the 8's, the locker and LSD engage and the fluid getting to coat all the internals and prevents chatters. This info is from GM TSB on rear axle noise's.
I have onwed many a GM truck with Locking rear axles dating back to the 70's. Only GM cars have limited slip, no trucks use it. DO NOT USE ANY ADDITIONAL ADDITIVE WITH A LOCKING REAR DIFFERENTIAL, GM says it can cause performance issues.
It will take a little less than 2-/2 qts to fill, You must replace the gasket, do not try to reuse it, IT WILL LEAK. Caution-GM does not use RTV on rear axle covers, only gaskets (they are less than $10 from dealer).
Amsoil has those pmups for the bottles, I have one here and it makes it a easy job, around $10.
Overfilling is as bad an underfilling.
Btw
I have the SVG in my 2004 GMC and 2005 CTS.
GMC has locker (no additive)
CTS has Limited slip (used additive, probably could have not but dod not want to have to redo it).
When using limited slip additive's, always mix them with the gear lube before adding to differential. This way the lube does not have a chance to settle and not mix with the lube. Also, it best to drive in a figure 8 to work the new lube into the differential. By doing the 8's, the locker and LSD engage and the fluid getting to coat all the internals and prevents chatters. This info is from GM TSB on rear axle noise's.
Mike -
That is great information. I will not add any additive or RTV. Thank you!!
That is great information. I will not add any additive or RTV. Thank you!!
and Define "overfilling."quote:
Originally posted by acewiza:
Measuring the additional drag from an extra 6-8 ounces of fluid in a rear would be like measuring the difference in the speed of light as it passes through the atmosphere as opposed to outer space.
One function of the fluid (besides lubrication) is cooling. The additional fluid will dissipate heat faster and help cool the diff better, NOT make it run hotter.
Where exactly does the fluid "dissipate heat" to? On every vehicle I've owned the differential is a closed system without a radiator or "cooler". The only heat dissipation is through the transfer of heat to the differential housing and the axle tubes. I doubt if the extra fluid dissipiates as much extra heat as it creates.
Overfilling is filling above the manufacturer's specification.
If you would be willing to take several IR temp readings with your differential overfilled and then drain it down to the proper level and take some more readings, you could report back your findings. I'm just not willing to do that with my vehicle.
Well ccfromc, I suppose if you had the experience to be aware how manufacturer's specifictions are defined and employed in the real world and understood the laws of physics as they apply to thermal efficiency in lubrication we wouldn't be having this conversation. But since you asked...
There are known examples of differentials and various gearsets being mismatched with axle housings, not to mention incorrectly manufactured housings that have the fill holes either too high or too low, virtually negating the "recommended" fill volume spec in some cases. That said, 6-8 ounces of fluid would not raise the level 1/4" in a Dana 70 or 80. "It's all relative," as they say.
On the subject of heat dissipation, I'll leave that for the physics majors, since it is not my intention to educate anyone around here on basic theory. This is an "automotive" forum, after all.
WRT actual testing and readings, I refer you to a series of sessions where I examined this subject quite as you describe, taking readings under different conditions, entitled "G360 heat study." This involves a commmonly known example where overfilling a transmission by up to two quarts over the spec is actually required if you expect the unit to last:
http://www.dieseltruckresource.com/dev/showthread.php?t=109531&highlight=g360+heat+study
Let me know if you still believe a bit of overfilling is such a bad thing.
[ September 18, 2006, 10:51 AM: Message edited by: acewiza ]
There are known examples of differentials and various gearsets being mismatched with axle housings, not to mention incorrectly manufactured housings that have the fill holes either too high or too low, virtually negating the "recommended" fill volume spec in some cases. That said, 6-8 ounces of fluid would not raise the level 1/4" in a Dana 70 or 80. "It's all relative," as they say.
On the subject of heat dissipation, I'll leave that for the physics majors, since it is not my intention to educate anyone around here on basic theory. This is an "automotive" forum, after all.
WRT actual testing and readings, I refer you to a series of sessions where I examined this subject quite as you describe, taking readings under different conditions, entitled "G360 heat study." This involves a commmonly known example where overfilling a transmission by up to two quarts over the spec is actually required if you expect the unit to last:
http://www.dieseltruckresource.com/dev/showthread.php?t=109531&highlight=g360+heat+study
Let me know if you still believe a bit of overfilling is such a bad thing.
[ September 18, 2006, 10:51 AM: Message edited by: acewiza ]
I called our local Chevrolet dealership this morning to order a gasket for the rear differential. They told me that the gasket is $72. I can't help but believe that this is a bit over-the-top. Has anyone else bought a gasket for this type of rear end? I would think that all of the trucks with the locking rear differential would be the same.
NewGuy, I wonder if they were looking at the wrong part. If the cover gasket is $72 I'd say that's way expensive.
Acewiza, I have a couple of questions:
Isn't the link you posted discussing a manual transmission (not a differential)? Is that transmission known to have overheating problems? The reason I ask is because I'm not familiar with that particular transmission. Is the factory fill spec lower than is optimal?
On my differential the spec allows a pretty wide range (5/8" to 1 5/8" below the fill hole) with 5/8" below the hole "full". I don't feel I need to go any higher than that and see no benefit to it. My differential does not have a known heat-related design problem (I'm not saying the g360 does but that was the impression I got from the links you provided).
If the factory spec is wrong or "not-optimal" then I believe we are talking apples-to-oranges (besides transmission vs. differential). I've not heard of any problems with the differential in my vehicle and I don't want to create any by overfilling it. I only took a couple physics courses between HS and College so I'm no expert but I did learn a little about fluid friction, energy and heat. The difference between a "properly filled" Tahoe differential and an overfilled differential in regards to heat generated may be 1 degree or 30, I don't know nor do I feel the value in testing that because I feel confident enough that a properly filled one will not have a fluid level related failure. If I knew that the factory spec was wrong or that my differential was prone to overheating, then I'd either start experimenting or I'd buy something else.
For my applications, yes, I believe that overfilling is a bad thing. For every application, well, maybe not, but your g360 may be the exception, not the rule. But we are talking about a Tahoe differential, the same diff as in my vehicle. To agree about disagreeing, I would just say, maybe it won't do harm but I see no reason to do it.
P.S. perhaps you were seeing lower temps because it will take longer to heat up the fluid since there is more of it. That does not mean that at some point the fluid temp would reach the same temp as previously experienced, or even go beyond that temp.
Acewiza, I have a couple of questions:
Isn't the link you posted discussing a manual transmission (not a differential)? Is that transmission known to have overheating problems? The reason I ask is because I'm not familiar with that particular transmission. Is the factory fill spec lower than is optimal?
On my differential the spec allows a pretty wide range (5/8" to 1 5/8" below the fill hole) with 5/8" below the hole "full". I don't feel I need to go any higher than that and see no benefit to it. My differential does not have a known heat-related design problem (I'm not saying the g360 does but that was the impression I got from the links you provided).
If the factory spec is wrong or "not-optimal" then I believe we are talking apples-to-oranges (besides transmission vs. differential). I've not heard of any problems with the differential in my vehicle and I don't want to create any by overfilling it. I only took a couple physics courses between HS and College so I'm no expert but I did learn a little about fluid friction, energy and heat. The difference between a "properly filled" Tahoe differential and an overfilled differential in regards to heat generated may be 1 degree or 30, I don't know nor do I feel the value in testing that because I feel confident enough that a properly filled one will not have a fluid level related failure. If I knew that the factory spec was wrong or that my differential was prone to overheating, then I'd either start experimenting or I'd buy something else.
For my applications, yes, I believe that overfilling is a bad thing. For every application, well, maybe not, but your g360 may be the exception, not the rule. But we are talking about a Tahoe differential, the same diff as in my vehicle. To agree about disagreeing, I would just say, maybe it won't do harm but I see no reason to do it.
P.S. perhaps you were seeing lower temps because it will take longer to heat up the fluid since there is more of it. That does not mean that at some point the fluid temp would reach the same temp as previously experienced, or even go beyond that temp.
New Guy,
Are you sure there's no drain plug? My 2001 pickup has one on the very bottom, flush with the housing. 2001 automatics had a trans drain plug, too, that was discontinued not long after. I guess it would stand to reason that GM took out the axle drain plugs in later years as well.
Are you sure there's no drain plug? My 2001 pickup has one on the very bottom, flush with the housing. 2001 automatics had a trans drain plug, too, that was discontinued not long after. I guess it would stand to reason that GM took out the axle drain plugs in later years as well.
I looked all over both sides of the housing, but didn't find a drain plug. A friend of mine changed his rear diff several weeks ago and that is what got me to thinking about changing mine. His is a 2001 Tahoe and had a drain plug. He didn't have to go through buying a $72 gasket....just unplug....drain...and refill. He got out pretty cheap.
I wonder why GM removed that drain plug? Maybe it was to sell more $72 gaskets!!
I wonder why GM removed that drain plug? Maybe it was to sell more $72 gaskets!!
Oddly enough they are the same people Mike says their synthetic lube "will attack RTV sealant." That may well be, I suppose if they add some RTV-specific attack formulation in order to sell more $72 gaskets. I have personally been running two different brands of syntheric gear lubes in different vehicles and never use anyting but RTV to seal up the rears on my diffs. No problems so far...
I used SF's synthetic 75W-90 with Permatex(?)Blue RTV and after 6+ months it developed a leak which got worse as time went on. I had Ford "fix" the leak. They used their VERY EXPENSIVE rtv, $17 I believe, but I don't have anymore leaking. After this experience I DO believe that some rtv gasket material can react with synthetic diff fluids to cause eventual leaking.
Whimsey
cfromc, I think you are beginning to see my point(s). There are potentially many factors having a bearing on "optimal" in this context. Design, manufacture, usage and type of fluid are the big ones, any one of which if misapplied in combintaion with the other three, can spell disaster in a gearset, be it a manual trans of diff. Point 1) Never say never, and avoid making blanket statements in advice that may not apply to every possible scenario. And point 2) Overfiling most certainly produces a lower operating temperature in at least one verified case I can account for.
In this particular case, the G360 suffered a combination of misfortunes, smallish fluid capacity and manufacturing tolerances being the two primary culprits. Some large runs of these boxes came out of the factory with incorrect bearing preload, causing them to run hot. If they were used to tow in the early part of their lives they failed, some reportedly smoking red-hot! Those that lasted beyond the warranty did so because their owners either did not use them heavily, or learned to overfill them by as much as two full quarts. This enabled the additional fluid and fluid-to-case contact to dissipate enough heat to avert failure. I confirmed this behavior in my experiment, through temperature readings over the same course in the same conditions, changing first the amount of fluid, then later with additional sump extension coolers and even more fluid.
Additional fluid was still not enough to overcome other issues in the stock trans like various leaks, etc. which all combined to give this actually quite good transmission a deservedly abysmal overall reputation. PTO sump extension coolers, an additional two quarts + overfill = a total of around seven quarts in a box that is spec'd to hold 3 and a half and an external filter/pump makes mine run cool, tight bearings and all. This whole episode from the transmission debacle to our exchange here, to me, only serves to reinforce my opinion that the terms "stock" and "manufacturer's specifications" often leave much to be desired. Here is how to make a stock Getrag 360 live up to the manufacturer's specifications:
http://66.17.171.114/PhotoAlbum/Cars/Pics/CTD_31.jpg
http://66.17.171.114/PhotoAlbum/Cars/Pics/CTD_32.jpg
http://66.17.171.114/PhotoAlbum/Cars/Pics/CTD_33.jpg
http://66.17.171.114/PhotoAlbum/Cars/Pics/CTD_61.jpg
http://66.17.171.114/PhotoAlbum/Cars/Pics/CTD_62.jpg
In this particular case, the G360 suffered a combination of misfortunes, smallish fluid capacity and manufacturing tolerances being the two primary culprits. Some large runs of these boxes came out of the factory with incorrect bearing preload, causing them to run hot. If they were used to tow in the early part of their lives they failed, some reportedly smoking red-hot! Those that lasted beyond the warranty did so because their owners either did not use them heavily, or learned to overfill them by as much as two full quarts. This enabled the additional fluid and fluid-to-case contact to dissipate enough heat to avert failure. I confirmed this behavior in my experiment, through temperature readings over the same course in the same conditions, changing first the amount of fluid, then later with additional sump extension coolers and even more fluid.
Additional fluid was still not enough to overcome other issues in the stock trans like various leaks, etc. which all combined to give this actually quite good transmission a deservedly abysmal overall reputation. PTO sump extension coolers, an additional two quarts + overfill = a total of around seven quarts in a box that is spec'd to hold 3 and a half and an external filter/pump makes mine run cool, tight bearings and all. This whole episode from the transmission debacle to our exchange here, to me, only serves to reinforce my opinion that the terms "stock" and "manufacturer's specifications" often leave much to be desired. Here is how to make a stock Getrag 360 live up to the manufacturer's specifications:
http://66.17.171.114/PhotoAlbum/Cars/Pics/CTD_31.jpg
http://66.17.171.114/PhotoAlbum/Cars/Pics/CTD_32.jpg
http://66.17.171.114/PhotoAlbum/Cars/Pics/CTD_33.jpg
http://66.17.171.114/PhotoAlbum/Cars/Pics/CTD_61.jpg
http://66.17.171.114/PhotoAlbum/Cars/Pics/CTD_62.jpg
That fine, but this thread is not titled "should I overfill my G360 transmission?". Is was started by a guy who sounds like he hasn't changed differential fluid in the past. For someone new to the DIY automotive maintenance world, I would suggest sticking with the manufacturer's recommendations. Your transmission sounds like it has design and manufacturering flaws and is not what I would consider "normal" or even common. Per your post: "In this particular case, the G360 suffered a combination of misfortunes, smallish fluid capacity and manufacturing tolerances being the two primary culprits. Some large runs of these boxes came out of the factory with incorrect bearing preload, causing them to run hot."
To state or imply that it is "OK" to overfill (all) differentials or transmissions is just not right and may give people at the beginner or intermediate level the impression that more is better, it is not in most cases. By your own post, the transmission in your vehicle suffered from a combination of flaws that led to the need to add additional fluid-most transmissions and differentials don't.
Please understand, I now know why you've overfilled your transmission. I'm just saying, you have an exception; overfilled should not be the rule.
To state or imply that it is "OK" to overfill (all) differentials or transmissions is just not right and may give people at the beginner or intermediate level the impression that more is better, it is not in most cases. By your own post, the transmission in your vehicle suffered from a combination of flaws that led to the need to add additional fluid-most transmissions and differentials don't.
Please understand, I now know why you've overfilled your transmission. I'm just saying, you have an exception; overfilled should not be the rule.
I wonder if the gasket is like the one on my trailblazer. Somethin' tells me they are pricey buggers. Check my post here: http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=16;t=003326
G/luck
Joel
cfromc, I guess you missed my point. It was not about any specific equipment combo. It was to refute your claim that overfilling increases heat. Re-read the post and stop trying to avert your mistake regarding what you believe as opposed to what you know to be fact.
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