So I finally decided to get one of those nice machined aluminum differential covers for my Diesel Silverado. I bring it to an old timers shop (and he is a friend of mine) in town to do the installation. Along with the diff cover, I bring 5 quarts of GM Synthetic "Grape Juice" 75W-90. I paid $160 for the oil at the local Chevy dealer. When I go to pick up the truck the owner Tony tells me, "The installation went fine. Only thing is you forgot to bring some limited slip friction modifier so I ordered some and put it in for you. And I knocked a half hour off the labor" My heart sank... I didn't have the heart to tell him the G80 in a pickup truck was not a limited slip but a locker and that the friction modifier absolutely did not belong in there. So I cringed as I said "Thanks, my friend" and drove directly to the Chevy dealer to buy $160 more worth of the "Grape Juice". I went to yet another friends shop and had him dump the 5 quarts of one day old very expensive axel oil and replace it with 5 quarts of new (with no friction modifier). My truck is in great shape now... but my wallet... not so much.
Sad sad G80 differential Story
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JHZR2
Staff member
Ummmm, what is the matter with having the FM in there on the locker?
Are you sure it is truly a locker?
5 quarts in the diff? Cripes!
Are you sure it is truly a locker?
5 quarts in the diff? Cripes!
Originally Posted By: JHZR2
Ummmm, what is the matter with having the FM in there on the locker?
Are you sure it is truly a locker?
5 quarts in the diff? Cripes!
"Originally Posted by General Motors
LOCKING DIFFERENTIAL LUBRICANT (SERVICE INFORMATION) #91-4-109
SUBJECT: LOCKING DIFFERENTIAL (G80) LUBRICANT - (SERVICE INFORMATION)
VEHICLES AFFECTED: ALL LIGHT TRUCKS EQUIPPED WITH G80 REAR AXLE ALL YEARS
Some light duty trucks equipped with locking rear axles (G80) may exhibit rear axle chatter, especially when turning a corner from a stop.
This condition of alternate engagement and disengagement of clutches in differential assembly is usually caused by contaminated axle lubricant.
To correct this condition, drain and refill the rear axle with SAE 80W-90 GL5 (P/N 10950849).
The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.
VEHICLES/COMPONENTS INVOLVED: ----------------------------- Some light duty trucks equipped with locking rear axles, RPO G80.
SERVICE PARTS INFORMATION:
Part Number Description ----------- ------------------ 10950849 Lubricant, Rear Axle (1 litre)
Parts are currently available through CANSPO.
WARRANTY INFORMATION:
As specified in Light Duty Truck Maintenance Schedules, locking rear axle fluid drain and refill is required owner maintenance at the first engine oil change. Failure to drain and refill the rear axle as specified may contribute to a later axle chatter condition. Refer to the appropriate Light Duty Truck Maintenance Schedule or service manual, section OB, for further details on change intervals.
General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.
© Copyright General Motors Corporation. All Rights Reserved."
Ummmm, what is the matter with having the FM in there on the locker?
Are you sure it is truly a locker?
5 quarts in the diff? Cripes!
"Originally Posted by General Motors
LOCKING DIFFERENTIAL LUBRICANT (SERVICE INFORMATION) #91-4-109
SUBJECT: LOCKING DIFFERENTIAL (G80) LUBRICANT - (SERVICE INFORMATION)
VEHICLES AFFECTED: ALL LIGHT TRUCKS EQUIPPED WITH G80 REAR AXLE ALL YEARS
Some light duty trucks equipped with locking rear axles (G80) may exhibit rear axle chatter, especially when turning a corner from a stop.
This condition of alternate engagement and disengagement of clutches in differential assembly is usually caused by contaminated axle lubricant.
To correct this condition, drain and refill the rear axle with SAE 80W-90 GL5 (P/N 10950849).
The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.
VEHICLES/COMPONENTS INVOLVED: ----------------------------- Some light duty trucks equipped with locking rear axles, RPO G80.
SERVICE PARTS INFORMATION:
Part Number Description ----------- ------------------ 10950849 Lubricant, Rear Axle (1 litre)
Parts are currently available through CANSPO.
WARRANTY INFORMATION:
As specified in Light Duty Truck Maintenance Schedules, locking rear axle fluid drain and refill is required owner maintenance at the first engine oil change. Failure to drain and refill the rear axle as specified may contribute to a later axle chatter condition. Refer to the appropriate Light Duty Truck Maintenance Schedule or service manual, section OB, for further details on change intervals.
General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.
© Copyright General Motors Corporation. All Rights Reserved."
I thought it was another govbomb story. A lot of syn gear oils come with FM and you see alot of these guys with this rear end run M1 or valvoline with no ill effects.
Because of the above TSB I am running Eneos 75W90 in my govbomb. Only syn OTC gear oil in my area with no FM
Because of the above TSB I am running Eneos 75W90 in my govbomb. Only syn OTC gear oil in my area with no FM
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Originally Posted By: fdcg27
The LSD FM would not have hurt a mechnically locking diff in any way at all, nor would it have hurt an open diff.
The FM is for the clutches in an LSD. It won't do any harm to the ring and pinion nor the seals.
The FM causes the locker to delay engagement. It can result in decreased axel life.
The LSD FM would not have hurt a mechnically locking diff in any way at all, nor would it have hurt an open diff.
The FM is for the clutches in an LSD. It won't do any harm to the ring and pinion nor the seals.
The FM causes the locker to delay engagement. It can result in decreased axel life.
Originally Posted By: wirelessF
I thought it was another govbomb story. A lot of syn gear oils come with FM and you see alot of these guys with this rear end run M1 or valvoline with no ill effects.
Because of the above TSB I am running Eneos 75W90 in my govbomb. Only syn OTC gear oil in my area with no FM
Although M1 and Valvoline do contain some FM's, I think they contain far less than if you buy a slug of it and dump it in. But agreed. This is why I use the uber expensive grape juice.
I thought it was another govbomb story. A lot of syn gear oils come with FM and you see alot of these guys with this rear end run M1 or valvoline with no ill effects.
Because of the above TSB I am running Eneos 75W90 in my govbomb. Only syn OTC gear oil in my area with no FM
Although M1 and Valvoline do contain some FM's, I think they contain far less than if you buy a slug of it and dump it in. But agreed. This is why I use the uber expensive grape juice.
ouch, exactly why I do as much work on my own cars as possible.
Just my opinion, but there is no G-80 in the world that works good enough to spend that much on grape juice gear oil. I'm pretty meticulous with my truck and I wouldn't spend more than M1 gear oil costs at best.
They are a marginal "locker" when new. I've owned 3 now and not one of them worked that well.
They are a marginal "locker" when new. I've owned 3 now and not one of them worked that well.
Did you save the oil for something else ?
Originally Posted By: fdcg27
The LSD FM would not have hurt a mechnically locking diff in any way at all, nor would it have hurt an open diff.
The FM is for the clutches in an LSD. It won't do any harm to the ring and pinion nor the seals.
Agreed. Have had 2 Silverados w/ G80 and ran RP MaxGear in them which has FM already in it. No problems with it at all.
The LSD FM would not have hurt a mechnically locking diff in any way at all, nor would it have hurt an open diff.
The FM is for the clutches in an LSD. It won't do any harm to the ring and pinion nor the seals.
Agreed. Have had 2 Silverados w/ G80 and ran RP MaxGear in them which has FM already in it. No problems with it at all.
Originally Posted By: andyd
Did you save the oil for something else ?
No, I sure didn't. I've sort of given up on trying to convince people of the value of "questionable things" like 1 day old gear oil. Most people are too suspicious (even when the offering is free) and I am low on bandwidth.
Did you save the oil for something else ?
No, I sure didn't. I've sort of given up on trying to convince people of the value of "questionable things" like 1 day old gear oil. Most people are too suspicious (even when the offering is free) and I am low on bandwidth.
Last edited:
Originally Posted By: NHHEMI
Originally Posted By: fdcg27
The LSD FM would not have hurt a mechnically locking diff in any way at all, nor would it have hurt an open diff.
The FM is for the clutches in an LSD. It won't do any harm to the ring and pinion nor the seals.
Agreed. Have had 2 Silverados w/ G80 and ran RP MaxGear in them which has FM already in it. No problems with it at all.
As I mentioned that amount of FM's that are already in oils that many have used with complete success in the Gov Loc (Mobil 1 does come to mind)... pales in comparison to what you get when you go out and buy a bottle of FM and dump it in there. GM says don't put it in there. It will decrease axel life. I bought the expensive machined cover because I want maximum axel life. So as far as I could see, that was the only course of action compatable with my objectives.
Originally Posted By: fdcg27
The LSD FM would not have hurt a mechnically locking diff in any way at all, nor would it have hurt an open diff.
The FM is for the clutches in an LSD. It won't do any harm to the ring and pinion nor the seals.
Agreed. Have had 2 Silverados w/ G80 and ran RP MaxGear in them which has FM already in it. No problems with it at all.
As I mentioned that amount of FM's that are already in oils that many have used with complete success in the Gov Loc (Mobil 1 does come to mind)... pales in comparison to what you get when you go out and buy a bottle of FM and dump it in there. GM says don't put it in there. It will decrease axel life. I bought the expensive machined cover because I want maximum axel life. So as far as I could see, that was the only course of action compatable with my objectives.
Last edited:
dnewton3
Staff member
Allow me to provide some better understanding here of the G-80 from GM. Seems that about every year or two, this comes up, as well as what is "best" for the AAM axles.
It's not a "locker" in the sense we all think of, such as an e-locker or air-locker, where they are linked solidy via a temporary direct mechanical attachment.
The G-80 operates with clutches, believe it or not. It's an "open" differential until there is enough speed differential between the two axles, and then when a preset delta is achieved, the flyweight activates a pawl to "lock" against the clutch pack, thereby engaging the LSD clutches. There is also an over-speed function that will disengage the LSD once a preset speed is attained (generally around 25mph). There most certainly are clutches in a G-80 diff. It is, essentially, a glorified LSD that operates only part time, versus a traditional LSD that is in constant engagement. If I have a bit of this wrong, Jim will correct me, as he's the one that clarified it for me. But I believe I have it right, or close enough to make it understood. If you treat the diff right, it operates well and can last a long time. What it does not appreciate is the massive torque changes that cause it to go from unlocked at low rpm differential, to locking, and then unlocking upon a high axle speed (like doing burn-outs, sled pulling, drag racing, etc). It was not designed or meant for this. If you approach the delta speed changes slowly, the system will properly lock and unlock as intended, and it can last a reasonably long time. Abuse it, and you'll grenade it in short order.
As to why GM says to not add FM to their "grapejuice", it is because the over-the-counter bottle of their product is already pre-mixed with FM. Part number 9986115 is simply a syn 75w-90 GL-5 with some FM added into it for parts counter sales. It is grossly over-priced (but that's true of most all OEM lubes). It works well, but it's nothing worthy of braging about.
Amsoil has a nice write up about the basic understanding of the fluid and the FM notation from GM:
http://www.amsoil.com/techservicesbullet...%20revision.pdf
Here is a quote that's been around for quite some time now; it's from an Eaton engineer (Eaton makes the G-80 diff for the AAM axles):
Automatic Locking Differential Lubricants. [Note from Michael Asmussen, Torque Control Products Division of Eaton Corp.] We recommend the following lubrications for our locking differentials:
1)Texaco 2276; Synthetic 75 W90; Gm Part # 9986115
2)Texaco 9622; Mineral based 80W90; GM Part # 9985290
3)Texaco 2080; Synthetic 75W140 (heavy duty applications); GM part # 9985991
Note - All of the above lubes are preblended with friction modifier. No additional modifiers are necessary or recommended. As far as other lubes are concerned, any standard GL 5 lube will work, but the units perform optimally with the three listed above.
Too little FM and the clutches will chatter and occasionally lock up as a nuisance. Too much FM will effect the ability of the clutches to transmit adequate torque balance. Kind of like Goldie-Locks, it needs to be "just right". Products that already have FM (SVG, M1, etc) are suitable for use in the G-80. If you select a GL-5 that does not include FM, then you'll need to add some a bit at a time until the chatter goes away.
All that being said, when your "old timers shop" added in more FM to the "grapejuice", they may have not done you any favors. This is the risk when presuming someone knows what they are doing; clearly he was not "in the loop" when it comes to the products you supplied. I would go back and educate him, so that he does not make the same mistake, and I'd at least politely ask to be reimbursed for the second round of fluid you had to buy. It was his mistake, not yours. I'd offer to pay him that 1/2 hour labor, and then request my $160!
It's not a "locker" in the sense we all think of, such as an e-locker or air-locker, where they are linked solidy via a temporary direct mechanical attachment.
The G-80 operates with clutches, believe it or not. It's an "open" differential until there is enough speed differential between the two axles, and then when a preset delta is achieved, the flyweight activates a pawl to "lock" against the clutch pack, thereby engaging the LSD clutches. There is also an over-speed function that will disengage the LSD once a preset speed is attained (generally around 25mph). There most certainly are clutches in a G-80 diff. It is, essentially, a glorified LSD that operates only part time, versus a traditional LSD that is in constant engagement. If I have a bit of this wrong, Jim will correct me, as he's the one that clarified it for me. But I believe I have it right, or close enough to make it understood. If you treat the diff right, it operates well and can last a long time. What it does not appreciate is the massive torque changes that cause it to go from unlocked at low rpm differential, to locking, and then unlocking upon a high axle speed (like doing burn-outs, sled pulling, drag racing, etc). It was not designed or meant for this. If you approach the delta speed changes slowly, the system will properly lock and unlock as intended, and it can last a reasonably long time. Abuse it, and you'll grenade it in short order.
As to why GM says to not add FM to their "grapejuice", it is because the over-the-counter bottle of their product is already pre-mixed with FM. Part number 9986115 is simply a syn 75w-90 GL-5 with some FM added into it for parts counter sales. It is grossly over-priced (but that's true of most all OEM lubes). It works well, but it's nothing worthy of braging about.
Amsoil has a nice write up about the basic understanding of the fluid and the FM notation from GM:
http://www.amsoil.com/techservicesbullet...%20revision.pdf
Here is a quote that's been around for quite some time now; it's from an Eaton engineer (Eaton makes the G-80 diff for the AAM axles):
Automatic Locking Differential Lubricants. [Note from Michael Asmussen, Torque Control Products Division of Eaton Corp.] We recommend the following lubrications for our locking differentials:
1)Texaco 2276; Synthetic 75 W90; Gm Part # 9986115
2)Texaco 9622; Mineral based 80W90; GM Part # 9985290
3)Texaco 2080; Synthetic 75W140 (heavy duty applications); GM part # 9985991
Note - All of the above lubes are preblended with friction modifier. No additional modifiers are necessary or recommended. As far as other lubes are concerned, any standard GL 5 lube will work, but the units perform optimally with the three listed above.
Too little FM and the clutches will chatter and occasionally lock up as a nuisance. Too much FM will effect the ability of the clutches to transmit adequate torque balance. Kind of like Goldie-Locks, it needs to be "just right". Products that already have FM (SVG, M1, etc) are suitable for use in the G-80. If you select a GL-5 that does not include FM, then you'll need to add some a bit at a time until the chatter goes away.
All that being said, when your "old timers shop" added in more FM to the "grapejuice", they may have not done you any favors. This is the risk when presuming someone knows what they are doing; clearly he was not "in the loop" when it comes to the products you supplied. I would go back and educate him, so that he does not make the same mistake, and I'd at least politely ask to be reimbursed for the second round of fluid you had to buy. It was his mistake, not yours. I'd offer to pay him that 1/2 hour labor, and then request my $160!
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