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Discussion Starter · #1 · (Edited)
UPDATE (10-16-21) -- looks like it might not have been the diff at all, but in fact the leaf spring isolators. I pried apart the leaf spring at the isolators and hit with some white lithium during the diff eval. The noise kind of starting coming back the other day. So I pried apart the leafs again where the isolators live (front & back, 2 per side) and cut a rectangular piece (2"x3") of vinyl/rubber (that commercial grade base molding you see in office buildings, etc). Looked closer at the old isolators and they are there but pretty shot, I guess. Tucked in the new rectangular pieces and so far no noise.


LOW FLUID LEVEL

1994/2WD/4.3L with LOCKING REAR (7.5 or 7.625)

Finally got around to changing out the fluid in my rear-end in a further attempt to diagnose a rear-end clunking problem that has been going on for months. Previously had tried all manner of jounce tests, bounce test and side-to-side tests. The noise what 100% coming from the right side (passenger) axle IMO. The clunking was only at 1-5 mph and usually while turning but sometimes when straight.

Axial end play (horizontal axle in & out) is basically zero on the driver side, but was noticeable on the passenger side, approx 0.0234" which I think is at the high end of spec for the locking rear. There is zero axle bearing play on either side.

I had purchased Dorman p/n 81050 to replace the axle shaft lock (c-lock), but 81050 if for the larger ring gear diffs (8.5 In., 8.625 In., 8.75 In., or 8.875 In ) I have the a 7.5" or 7.625" locking diff. Didn't matter though, because when i got the c-lock out of the passenger side (note: I did not inspect the driver side) it mic'd at 0.0150" thickness RIGHT ON THE MONEY...right at spec. The only thing I noticed was you might be able to say that the wheel facing side of the c-lock was polished smooth vs the diff side which was not Hubble Telescope smooth. I put it back in, but flipped it so the smooth side was to the diff -- shouldn't do anything, but figured why not.

Buttoned it back up with a new FelPro RDS55072 and proceeded to fill with 4.2 pints of Wal-Mart's finest SuperTech 80W90 GL-5. I have given up on everything by Supertech at this point...as have yet to find a study that shows it to be any worse than most of the brand name stuff out there at 3-5X the cost. It usually performs right at the median or top third. Problem was, it started to spill out at 2QT as marked on the jug (I was pumping out of a 1 gallon jug). I think the 4.2 pint spec is for the larger ring gear rear, Regardless, it took right at 4 pints, or 2 quarts to get it "level with the bottom of the fill plug.

And this is where I think I found the issue.

I have traditionally kept my diff fluid levels to just shy of the fill plug hole, and as long as I can poke my finger in and touch it I called it good. Spec though per the manual is to fill til it starts to pour out or "fill to bottom of fill hole" to use their words. Fortunately I kept the old fluid because I wanted to measure how much was taken out. Low and behold I was a full "cup" low...if spec is 4 pints. i.e, 8 cups, then a cup is a fair amount (?), although I would have thought, meh what's 8oz.

Well, anywho....after a couple of test drives, the clunking noise is GONE. We'll see if it stays that way. I have read elsewhere that low fluid levels on locker rears can causes noises, specifically clunking...but just 8oz low? Oh well lesson learned.

A final note, in my research I did find out that locker rears (in my case) have to have more axial play on one side (usually the driver side?) in order to operate properly -- too tight causes issues. I haven't yet rechecked the end play on the passenger side where I flipped the c-lock but I would guess that it's still running the same as before -- but I do need to verify.

Final Notes/Disclaimers:
[1] I still need to adjust the rear back shoes, so right now the rear drum brake are "running loose" as oppose to a few clicks of the star wheel tighter once I adjust them
[2] I also "lubed" the leaf spring isolators (qty=4) by prying between the leaves and shooting in some white lith grease
[3] I also centered the passenger side rear shock on it's lower mount pin as it was right up against the u-bolt plate and might have been also causing the noise (I'll never know)
 

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Discussion Starter · #2 ·
Update: was able to track down and confirm proper capacity for 7.500"/7.625" locking rear diff = 4.0 pts (as opposed to 4.2 for the larger ring gear diffs).
 

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Discussion Starter · #4 · (Edited)
brand new fluid might have been the problem solver as well. how old was the old fluid? last time it was flushed?
Great point...and I forgot to mention two other points:

[1] While there are "no leaks" ...there is "vapor seepage" at the pinion area, meaning that there may be a slight amount getting by or it could be from the breather hose that is above the same general area. That is to say, in 11 years since the last change (see next point below) I have only "checked" and never added any fluid. So some of my 8oz shortage noted above was from under filling (probably most of the issue, in fact) and "evaporation" or weeping would be the cause of some small remainder.

[2] But back to the your point regarding the timeframe. Last change was 2010 -- 11 years ago, but only 18,000 miles -- the truck gets driven about 1,500 miles per year. But yes, I think the age absolutely could have been a factor.
 
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