Friction modifier?

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ThisOldK10

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My rear diff likes to hum ever so slightly at all speeds and sometimes howl around 55 but only when I have my foot on the gas. I just changed the fluid (drained out some fluid that was as black as Satan's eyeballs, and way less than should have been in there), and put in the 80w 90 that it called for (non posi), but it now howls at 55 like I mentioned earlier. I have heard of using some additives or heavier weight oil to prolong its life until I can rebuild or replace whatever is dying. What do you guys use?
 

Honky Kong jr

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My rear diff likes to hum ever so slightly at all speeds and sometimes howl around 55 but only when I have my foot on the gas. I just changed the fluid (drained out some fluid that was as black as Satan's eyeballs, and way less than should have been in there), and put in the 80w 90 that it called for (non posi), but it now howls at 55 like I mentioned earlier. I have heard of using some additives or heavier weight oil to prolong its life until I can rebuild or replace whatever is dying. What do you guys use?
Put some sawdust in it..... jk it’s and old trick I’m sure ole @RustyPile could tell you all about it. Maybe @74 Shortbed too. Lol
 

ThisOldK10

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I keep hearing this lol.
 

hatzie

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Me-thinks it's done cap'n...
Time to start looking for a replacement or the parts to re-build it.
 

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I like to use synthetic 75w140 in my rear diffs. If it's not an open diff then I add limited slip additive.
 

DoubleDingo

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Did you get any pics while you had the cover off?
 

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Iirc, most of the gear lube on the market nowdays has the modifiers already added.
 

bucket

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If sawdust doesn't fix it, throw in a couple bananas too.


Halfway joking, lol
 

hatzie

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I like to use synthetic 75w140 in my rear diffs. If it's not an open diff then I add limited slip additive.

Be careful...
Limited slip additive is for Positrac diffs like you find in cars. It won't do a thing for a regular open diff.
If you have a truck with the G80 Gov Loc option this will get you in real trouble. GM released a TSB specifically stating that you do not use limited slip additive with the G80 optioned axles and why not to.

Iirc, most of the gear lube on the market nowdays has the modifiers already added.

That's why you carefully read the bottle... and be informed about what you need.

The below referenced GM 10950849 fluid is 1 Litre of GL-5 80W90 axle gear oil. Any GL-5 80W90 should do as long as it doesn't state it's for limited slip axles...
If it's a plain vanilla open diff you can use any GL-5 gear oil of the correct weight even if it does have the limited slip additive.

-------------------------------------------------------------------------------------------------

GM Service Bulletin 91-4-109

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. 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.
 
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hatzie

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They'll sing if the pinion depth or tooth engagement wasn't set right too...
If the bearings and gears look OK I'd take a whack at properly setting it up. Not an easy task. Very fussy precise work. You may want to find an old timer to help you out with the first one. I did...
 

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Bad part about gears is they will wear to each other and any further adjustment will make more noise and speed up the demise of your gear set. Best course of action for a daily would be to replace them.
 

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Put some sawdust in it..... jk it’s and old trick I’m sure ole @RustyPile could tell you all about it. Maybe @74 Shortbed too. Lol

The sawdust and banana "trick" has been around decades.. It's something used car lots did to quiet howling rearends. The theory is the sawdust particles and banana fibers form a cushion between the gears.. It sometimes works but most time it doesn't.. At best, it's a short term fix but not something I'd ever recommend..

The howling and/or whining the OP is experiencing is probably due to wear.. If the differential was recently overhauled and "set up" improperly, correcting pinion depth and gear backlash might eliminate the noise.. More than likely, the gear pattern has "taken a set" and the noise can't be eliminated without a major overhaul.. Personally, if the whine isn't very loud I wouldn't worry about it...

In my younger days I had a '55 Chevrolet that I did some drag racing with.. The differential was a 5:13 ratio.. The ring and pinion weren't a matched set but had a decent gear pattern.. The thing howled like a banshee during mating season.. Never gave me a bit of trouble..
 

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My Gov Lok does the same thing.
 

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