Neglected rear diff

Status
Not open for further replies.
dlundblad

dlundblad

Joined
Sep 30, 2013
Messages
16,269
Location
Indiana
Vehicle: 1998 Blazer 75k

The plan was to help my brother change the rear diff fluid in his blazer today since it is already on stands from the shock swap on Sunday, but after taking the diff cover off, it had no magnet. You can see where it once was though which is a plus I guess.. (I'll have to look in the folder the PO gave my brother to see when it was changed last..)

We had some Castrol synthetic 75w90 laying around that I was going to put in there, but now I am not so sure we should just yet assuming the internals could be a bit dirty even though visually they appear clean.

Walmart has a gallon of ST 80w90 for $12.xx. Would it be beneficial to run a few short intervals of this? By short intervals, I mean drive it 100 miles on the highway and do a drain and fill. I am hoping to be able to just siphon the fluid through the fill hole so I wouldn't have to mess with remaking the gasket(s).
21.gif


(Not knocking ST branded products, but the the gear oil seems to leave something to be desired by many here on BITOG. Apparently a while back, it wasnt meeting the current gear oil spec??)

We ordered another magnet from the dealer btw. $12.
 
Last edited:
I do not know but it sounds good to me that you know it was at least changed at one time. How many miles are on the vehicle?
 
Originally Posted By: mcrn
I do not know but it sounds good to me that you know it was at least changed at one time. How many miles are on the vehicle?


I knew i was forgetting something!

75k
 
Is this the kind of differential that has a removable pan? When I deal with that kind of differential, I like to blast it with brake cleaner until I no longer see any gray slime. Afterwards, I fill with premium fluid.

I use the method you mentioned on differentials that don't have a removable pan. That is what I have done to my own car. I used Valvoline conventional because I found no less expensive oil that contained LS additives.

Do you know if this truck has limited slip or not? That would be a factor in which gear oil you choose.
 
I wouldnt worry about a cheapo fluid if Im changing it again soon and just using it as a carrier/flush. Depending upon how I plan to use the vehicle between now and when the good stuff goes in, I might be more considerate, but likely not.
 
Originally Posted By: artificialist
Is this the kind of differential that has a removable pan? When I deal with that kind of differential, I like to blast it with brake cleaner until I no longer see any gray slime. Afterwards, I fill with premium fluid.

I use the method you mentioned on differentials that don't have a removable pan. That is what I have done to my own car. I used Valvoline conventional because I found no less expensive oil that contained LS additives.

Do you know if this truck has limited slip or not? That would be a factor in which gear oil you choose.


When you say removable pan, do you mean a differential cover? This has a removable pan. I shot a good dose of brake cleaner into it and a lot of chunky oil clumps dripped out.

I still have it dripping now which is best I suppose. I might hit it again tomorrow with some more brake cleaner.

*I dont think it has LSD. We supported the rear end while one wheel went forward and the other went in the opposite direction.
 
Last edited:
"Flushing" a diff is more effort than it's worth and won't do much. Just change it and call it good. Most of the junk will come out with the fluid, and what settles on the bottom you can scrape out if you have time to let it drip. Some people spray the gears and housing down with brake cleaner, but if you do that avoid the axle tubes and bearings and let it air dry for a while so the brake cleaner fully evaporates out.

Diffs break in and dirty up the fluid quick. I think a good rule of thumb is to change at 15K with a synthetic fluid that meets specs and monitor after that. In most vehicles you probably don't need to change the diff fluid more than once. My trucks have always had nasty fluid on the first change around 30-50K mi, but super clean fluid always after that. All of my trucks have only had a magnet on the fill plug...no other magnets in the housing.

If the fluid has ever been changed, it's better off than 90% of Blazers. If it were mine, I'd let it drip out as much as possible, seal it up, fill it, and call it good.
 
Originally Posted By: dlundblad
Originally Posted By: artificialist
Is this the kind of differential that has a removable pan? When I deal with that kind of differential, I like to blast it with brake cleaner until I no longer see any gray slime. Afterwards, I fill with premium fluid.

I use the method you mentioned on differentials that don't have a removable pan. That is what I have done to my own car. I used Valvoline conventional because I found no less expensive oil that contained LS additives.

Do you know if this truck has limited slip or not? That would be a factor in which gear oil you choose.


When you say removable pan, do you mean a differential cover? This has a removable pan. I shot a good dose of brake cleaner into it and a lot of chunky oil clumps dripped out.

I still have it dripping now which is best I suppose. I might hit it again tomorrow with some more brake cleaner.

*I dont think it has LSD. We supported the rear end while one wheel went forward and the other went in the opposite direction.

Yes, when I say removable pan, I'm also implying removable cover.

If you can't get it clean with just brake cleaner, maybe running cheap oil for 100 miles is a good idea.
 
If the rear diff. has been neglected that bad, I would install a diff. cover with a drain plug,and do some short runs with the Castrol you already have. After a couple runs, drain and refill it with something like RP Max Gear. The short runs will clean out any of the gunk left over from the old lube and the RP will offer superior protection, keeping that diff. moving on down the road for years to come. Just my
49.gif
 
Originally Posted By: aquariuscsm
Originally Posted By: bama7x57
Does rear diff fluid go bad?


I've wondered about that myself.


All additives run out eventually. Isn't that why we pay the extra couple bucks to Blackstone to get a TBN? Arguably, diff. oil needs to be changed every 30-100k miles depending on the severity of service.
 
Originally Posted By: jk_636
Originally Posted By: aquariuscsm
Originally Posted By: bama7x57
Does rear diff fluid go bad?


I've wondered about that myself.


All additives run out eventually. Isn't that why we pay the extra couple bucks to Blackstone to get a TBN? Arguably, diff. oil needs to be changed every 30-100k miles depending on the severity of service.


I just always wondered if rear diff oils suffered shearing during service. I'm guessing contamination (like oil in a combustion engine) doesn't take place since differentials are sealed and there's no combustion contamination taking place.
 
So the Diff has 75k on it and the oil may have been changed at least once.

How is this neglected?

Heck, I have put many times this mileage in a Diff or Gearbox, on Factory fill, and not changed the oil.
 
Just put the 75w90 in it and be done with it, it's not neglected like you think, if it was neglected you would see evidence visually with large metal particles or some bad colored fluid.
 
Hi,
expat - I agree

Originally Posted By: expat
So the Diff has 75k on it and the oil may have been changed at least once.

How is this neglected?

Heck, I have put many times this mileage in a Diff or Gearbox, on Factory fill, and not changed the oil.


I used a "life time" OCI programme for the transmissions and diff in my Heavy Trucks - based on UOAs this translated into around 1m kms on average

The biggest danger is from water contamination from crossing creeks and etc
 
Last edited:
Originally Posted By: dlundblad
Originally Posted By: artificialist
Is this the kind of differential that has a removable pan? When I deal with that kind of differential, I like to blast it with brake cleaner until I no longer see any gray slime. Afterwards, I fill with premium fluid.

I use the method you mentioned on differentials that don't have a removable pan. That is what I have done to my own car. I used Valvoline conventional because I found no less expensive oil that contained LS additives.

Do you know if this truck has limited slip or not? That would be a factor in which gear oil you choose.


When you say removable pan, do you mean a differential cover? This has a removable pan. I shot a good dose of brake cleaner into it and a lot of chunky oil clumps dripped out.

I still have it dripping now which is best I suppose. I might hit it again tomorrow with some more brake cleaner.

*I dont think it has LSD. We supported the rear end while one wheel went forward and the other went in the opposite direction.


IS there a way to check for posi-trac, aka 2 tires turning, without lighting them up? You see, my rears are almost bald and im in no shape for new ones just yet.
 
Just changed the 15 years and 205k old factory diff oil on the Falcon last week! Was very black (must be all that blow by! :p) and filled it up with some dino Nulon 80w90. It was fine and silent before and still is now, and isn't leaking or anything so I wouldn't worry about it
 
Status
Not open for further replies.

Similar threads

alcyon
Diff oil to quiet diff whining
Replies
18
Views
806
bonjo
B
Amsoil Severe Gear+Valvoline Synpower 75w90
Replies
1
Views
337
Bowtieluvr
K
80W90 in transfer case and GL5 75W90 in rear diff, or use GL5 75W90 in both?
Replies
3
Views
1K
V70tdi
EMOTO
2017 JKUR (Wrangler Rubicon) Locking Diff Fluids?
Replies
1
Views
193
Rand
W
Cross contaminated my atf with gear oil.
Replies
19
Views
1K
Vitamin Yoda
Back
Top