Check out my rear end

What were you hoping for with that title you perverts? Ha, so I am having a vibration at around 67-72mph, and again way worse at about 85. It does it on the dyno so obviously it's drive line related. Wheels have been balanced, no change. The driveshaft was re-balanced with new u-joints when I was building the truck, that was about 6 years ago. I've only now started driving it, have about 1000 miles on it. The driveshaft could still be suspect, I will take that to get it checked. In the meantime, my 12 bolt posi 3.73 rear end is chattering on turns from a stop, especially when hot. I changed the oil again with Valvoline 80w90 and AC delco posi additive. No change. I am wondering if the chattering on turns and the highway speed vibration could be related. Clearly the posi clutches are sticking. I did the whole parking lot u-turn thing too, no change. Just curious if anything looks weird from the photos? Is it normal to have pitting on the carrier? I had this rear end built for me 30 years ago, it has about 30,000 miles on it, but sat for about 20 years. Maybe time for a rebuild?

That is not a GM limited slip differential. The GM govern lock differential does not require posi additive. That locker may require additive, what brand is it?

Is the pinion loose, can you wiggle it? Unless the axles are bent, nothing else in the rear will give a vibration at speed. You stated wheels and tires are good.

You said the drive shaft has been balanced, did a weight fall/rust off? Is the drive shaft in phase?

Turbo4whl said:

That is not a GM limited slip differential. The GM govern lock differential does not require posi additive. That locker may require additive, what brand is it?

Is the pinion loose, can you wiggle it? Unless the axles are bent, nothing else in the rear will give a vibration at speed. You stated wheels and tires are good.

You said the drive shaft has been balanced, did a weight fall/rust off? Is the drive shaft in phase?

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Well that is interesting, I always assumed it was a GM unit. I have no idea what brand it is. I had it built by a guy who had an ad in Hemmings Motor News in about 1993, good ol' pre internet days. I was 15 years old. Took 8 months for it to show up in a crate from Pennsylvania, I was convinced I had been ripped off and it would never arrive. Of all the receipts and paperwork I have from back then, I can't find anything on this. The pinion doesn't seem loose. You may be onto something with the drive shaft, I will see if I can determine if a weight fell off.

Looks to be a normal 1970s Eaton GM 12 bolt truck posi carrier. I’ve owned several, may even still have one or two.

vr1967 said:

Looks to be a normal 1970s Eaton GM 12 bolt truck posi carrier. I’ve owned several, may even still have one or two.

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Yes it could be a much earlier rear axle/parts. Not something from GM available in the 1980's that I know of.

Turbo4whl said:

Yes it could be a much earlier rear axle/parts. Not something from GM available in the 1980's that I know of.

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I always thought that every truck in the 80's got either a Gov Loc or an open diff, but @89Suburban had a rig with a factory installed limited slip.

Bloodhound1981 said:

I always assumed it was a GM unit.

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With Andy's info, then in 1980 GM did still have their posi units in some trucks.

bucket said:

I always thought that every truck in the 80's got either a Gov Loc or an open diff, but @89Suburban had a rig with a factory installed limited slip.

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The RPO code, would that be different than G80?

Hey Paw Paw, @89Suburban any info you can share on your Suburban?

bucket said:

I always thought that every truck in the 80's got either a Gov Loc or an open diff, but @89Suburban had a rig with a factory installed limited slip.

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Just to be clear, this was a custom built rear. The factory one was an open diff 12 bolt.

Don’t have much to say on the vibration front,

However, regarding the chattering issue, the original Dana 70 under my Dodge (factory power-loc limited slip), the pinion had some play (360k miles), when taking slow corners like in a drive thru or parking lot it would “chatter.” Best way I can describe it is like waking up the stairs and continuously putting one foot on top of the other or getting caught on the stair tread. Before I discovered the pinion was loose, I suspected it was the clutches were bad. When in the whole reconstruction project, I swapped that limited slip into the new axle, that the pinion is tight, the chatter disappeared.

Just a data point, something to check, give the yoke a wiggle.

Probably not necessary to comment but if the driveshaft is suspect did you check that the transmission yoke and pinion angles are equal? They should be within a degree or two of each other.

Yes many had factory spring/clutch limited slip.
So you have 2 problems.
Might be time for new clutches in the diff. Generally, change fluid, add additive, find big parking lot, drive in circles, al lot, then see if chatter gets better.
As far as driveline,are you sure is from rear? Either of the rear tires out of round, Truck have a lift on it?

Turbo4whl said:

With Andy's info, then in 1980 GM did still have their posi units in some trucks.


The RPO code, would that be different than G80?

Hey Paw Paw, @89Suburban any info you can share on your Suburban?

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I don't recall about the RPO code, but I do know that the G80 cade was also used in 80's cars that had a limited slip.

PawPaw's Burbs were '89 and '90, but I don't remember which one had the limited slip and not a gov-loc.

@89Suburban

Generalmoto said:

Probably not necessary to comment but if the driveshaft is suspect did you check that the transmission yoke and pinion angles are equal? They should be within a degree or two of each other.

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Yes they are good. Before I re-rebuilt the truck, I was using this transmission and this rear end with no issues.

Robert Bare said:

Yes many had factory spring/clutch limited slip.
So you have 2 problems.
Might be time for new clutches in the diff. Generally, change fluid, add additive, find big parking lot, drive in circles, al lot, then see if chatter gets better.
As far as driveline,are you sure is from rear? Either of the rear tires out of round, Truck have a lift on it?

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I think new clutches are in order. I did do the fluid change, additive, and lots of u-turns in a parking lot. Definitely helped a bit, but still an issue. The only reason I can be certain it's somewhere in the driveline is it did it on the dyno the same way it did on the highway. I rebalanced all tires first. I swapped one of the rear wheels out with a spare, no change.

I guess it's driveshaft shop first, make sure that checks out, then rear end shop.

Before you go rebuilding the posi, if it still does it after the 'shaft gets rebalanced, pull the shaft out and rotate it 180* and try again.

TotalyHucked said:

Before you go rebuilding the posi, if it still does it after the 'shaft gets rebalanced, pull the shaft out and rotate it 180* and try again.

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I'll give that a try thanks!