1964 Skylark Posi-Rear Chatter

[Machine Gun]

Roseann Roseannadanna's spirit came back to haunt me. "It's always something."

 

I picked up the car from the mechanic last week after he installed new rear axle bearings. He told me that he was surprised that I was able to drive the car the way it was without hearing the bearings or the rear end screaming. The rear was so low on oil that nothing had reached the axle bearings in quite a long time (totally my bad...I had the car for nine years and never bothered to check the oil in the diff.). The driver side bearing appeared to be the original one. It was badly rusted and was the source of most of the subtle sound I was hearing. Anyway, he replaced the bearings, flushed out the rear and replaced the oil with the proper gear oil with positraction additive.

 

I drove the half-hour trip home and was treated to a smooth ride with no noise whatsoever. Then it happened. I made a hard turn to pull into my driveway and it felt like something was grabbing in one of the brake drums. I straightened out the car and the noise disappeared. Over the next few days the noise would come and go, never at road speed and never on a straightaway or curves, only on full turns. On Sunday I put the car on my QuickJack, pulled the drums, turned the rear wheels, checked the U-joints, nothing. Just for yuks I checked the rear oil level...full. I began to worry that the clutches in the rear end were chattering. I brought the car back to the shop yesterday and demonstrated the issue without saying anything to the mechanic about my suspicions. Diagnosis: clutch chatter. Crap. I left the car there. He won't be able to open it up until next week to have a look.

 

I feel like such an idiot for not ever checking the oil in the rear.

Edited July 26, 2023 by Machine Gun (see edit history)

[EmTee]

Not knowing what it looks like inside the housing, if it were in my garage I'd do the following: (1) open the differential and drain the oil; (2) check the bottom of the housing for debris and if there is anything there I would pull the axles and clean the crap from the bottom of the housing AND the axle tubes.  Use a rag zip-tied to a broom handle to swab the axle tubes.  Wet the rag with brake cleaner and ensure the inside of the housing is clean and shiny; (3) reinstall the axles and use some motor oil in a bottle to squirt the differential center section to wash it down and let it drain; (4) use clean rags to wipe-out the oil from the bottom of the housing and inspect for debris; repeat until the wash oil is clean; (5) button everything up and fill with fresh 80W90 oil and GM limited-slip additive (accept no substitutes).  Then drive for 20 ~ 30 miles to include some parking lot 'figure 8' turns.  Hopefully it will calm down... 

[Machine Gun]

The mechanic reportedly did all that when he did the axle bearings. There's a nice new dry gasket on the housing cover. The only thing I don't know is what type of additive he used. I'll ask, and insist that he use GM additive when he gets back in there. BTW, he did seem to agree with you that these things often clear themselves up after driving around a bit, but I put more than 100 miles on the car since the work was done and the problem is as pronounced as ever. There was no hint of chatter before the rear oil was changed even though it was way low, so you may be on to something about using only GM additive. I'll know more next week after he gets back in there. Thanx for the suggestions.

[NTX5467]

Earlier-style PTracs were locked until they unlock, using spring pressure to keep the clutches in contact with each other.  Regardless of what it might indicate on the lube bottle, PTrac additive IS needed for those differentials.  Finding a GM dealer with such in stock might be a trick, but it should be available via sources like Summit, Amazon, or similar.

 

On our 1969 Chevy pickup which we bought new, with a factory PTrac, it always had a "catch" when turning left.  Lube changes and new additive did nothing to stop it, but it wouild usually do it once and then that was it, in each lh turn, but by then, too, the turn was over and things straightened out.  Just something we lived with.  Even tried the lh turns followed by the rh turns and all of that . . . no change.

 

Perhaps new rear control arm bushings will lessen the feel of when it "catches"?  "Catch and release", LOL.

 

When I ordered my new '77 Camaro LT, I ordered it with PTrac.  It has never displayed any "catches" on lh turns, ever, in over 750K miles.

 

Enjoy "the feel" of increased performance and snow traction,

NTX5467

[EmTee]

35 minutes ago, Machine Gun said:

The mechanic reportedly did all that when he did the axle bearings.

That being the case, I'd just drain, refill (with the GM additive) and drive... 

[Machine Gun]

21 hours ago, EmTee said:

That being the case, I'd just drain, refill (with the GM additive) and drive... 

I'm curious...let's say that the verdict is that the clutches are shot and that a rebuild is needed. Not knowing precisely how Positraction diffs work, what's the harm in just living with the problem as you suggest? It does just fine moving down the road, around curves, and in most normal turns. It's hardly noticeable until I make a sharp turn at very slow speeds, like at "creep-speed" when going into my garage. Assuming the problem doesn't eventually clear itself I can easily live with it. My question is whether it will cause the whole works to eventually fail unless corrected.

[EmTee]

34 minutes ago, Machine Gun said:

It's hardly noticeable until I make a sharp turn at very slow speeds, like at "creep-speed" when going into my garage.

It sounds as if it's already improving.  Worst-case scenario is the chatter gets worse and happens more frequently.  It's basically a spring-loaded clutch pack, so I don't think anything is likely to break.  If the clutch pack completely wears out, I guess maybe it could get to a point where power would no longer get to the axles.

[NTX5467]

As noted, there is constant spring pressure on the generally flat clutch plates.  Which means there is more propensity for the lube to be squeezed from between them.  Hence, the sticking or reluctance to release initially when needed to release/slip.

 

Even with the low installation rates of PTracs in the later 1960s, I found several references to the "stick, pop.release" situation in car magazines I had from the middle 1960s.  In other words, it was more common than not, back then, especially on performance vehicles as Corvettes.

 

On our then-new '69 Chevy pickup, after we figured out why it was popping (usually just once, but noticeable more times than not), we had the dealer change the rear axle lube and add the additive.  Chevy used to have a lube for PTracs specifically, plus the additive, bvack then.  Or the normal lube to which the additive was added.  In some cases, it was suggested that TWO bottles of additive be added in "extreme cases".  Which some indicated might not solve the issue.

 

So, what you have is an indication that the clutches are still in decent condition.  Add another bottle of additive if you desire and see if it helps.  Other than just a "live with it", with the orientation that things are not good, but put up with it . . . or use the pops to verify that things are as they should be (which can be confirmed with some quick take offs with the wheels on dissimilar traction surfaces ( one on dirt, one on pavement) to confirm everything is still working.

 

In these "tests", it is completely normal for lh rr wheel to lay more rubber than the rh rr wheel does.  Even on dry pavement, more "evidence" of lh wheel spin.  Which is just the opposite as a non-PTrac rear axle.  So, easier to spin the lh rr wheel in a corner on a PTrac vehicle.  Why??  In a normal non-locked rear axle, it is the torque reaction of the quickly-turning driveshaft (and the normal tendency of its direction of rotation to "take the housing with it" as it quickly spins when the vehicle is at zero-to-low speeds.  With a PTrac, the locked clutches counteract that spin action and put more rotational resisstance to better plant the rh rr wheel, which then can tend to lift the lh rr wheel as a result.

 

The OTHER thing is that the spring-loaded PTracs do not completely lock-up, but always allow for a certain amount of slip, only being about 75% locked.  Whereas the limited-slip differentials in 3/4 ton pickup trucks to fully lock as the speed differential between the rear wheels moves the clutches closer together mechanically via ramps, so that they lock-up solidly.  Hence the term "locker" used for them.  

 

In about the later 1980s, Chevy/GMC went to a "governor based" limited-slip/locker differential.  Using a pair of teardrop-shapped fly weights, swung outward by a small wheel which turned when the rear wheel speed variation increased.  Leaving it unlocked in normal driving, until one wheel slipped a certain amount.  "Unlocked until it locks", which also changed the terminology for it to "Locking Differential" in the sales literature.

 

NOW, in the sales training tools Chevrolet used to indicate how well it worked, they would drive a Chevy 1/2 ton pickup and its competitors up a steep hill with the rh rr tire's rear surface wet with Murphy's Oil Soap.  After about 40" or so, the competitor would cease to move upward as the Chevy continued onward to the top.  In later demo videos, they would slowly drive the competitor across a deep V-shaped drainage ditch, putting the body/frame into a torsional twist of sorts, but also resulting in the rh rr wheel hanging in open air, where the truck would stop and could not move forward.  In contrast, the Chevy would drive to the same middle spot, pause, then the rh rr wheel would slowly rotate.  After about a full turn, the truck would visibly shake and the Chevy would continue across the ditch, with the lh rr wheel moving it forward.  There should be some YouTube videos of these, I suspect.  Which supported the term "locking differential" for the same G80 option code.

 

Not EVERY car can leave TWO strips of rubber!

 

NTX5467

[EmTee]

21 minutes ago, NTX5467 said:

Which means there is more propensity for the lube to be squeezed from between them.  Hence, the sticking or reluctance to release initially when needed to release/slip.

Find a parking lot and do a couple of figure-8 turns each time you're out in the car to give the new lube a chance to work-in and clean the friction surfaces.

[Machine Gun]

@NTX5467 Thanx for the insight into how posi rears work, as well as the historical perspective on what owners experienced in the past. I may sleep a bit better tonight, secure in the knowledge that my diff is not likely to self-destruct in the next few days.

 

@EmTee I will take your suggestion to "exercise" the diff when I get the car back. I've done only normal driving since getting the car home last week.

 

Considering the insightful comments you guys provided I'll probably take the car home before the shop opens up the thing next week and I incur additional expense. Here's what the plan looks like for now: First, I'll ask which brand additive he used. Second, if he used the AC stuff I'll simply take the car through the exercise routine and see if things clear up. If not, I'll try adding a second bottle. If he didn't use AC additive, I'll either add a bottle of AC or drain, clean, and refill with AC additive.

 

I'll be sure to update you guys. I really appreciate the thought and time you've put into helping me with this.

[EmTee]

I'll be surprised if it doesn't quiet down after accumulating some miles with the additive...

[NTX5467]

You might need to remove a little bit of lube before adding the second bottle, as it will need "space" to be added.  I suspect that most brands are the same, but the ACDelco stuff, we know what it's supposed to be.

 

NTX5467

[Machine Gun]

I have a small update on the chatter. I picked up the car on Monday and have been driving it on the road as well as making sharp, slow-speed turns in parking lots. I will continue exercising it, but so far I detect no hint of improvement. I may have to take other steps to cure it.

 

The chatter occurs only after the car has been driven for at least a half hour. Never does it when cold. The mechanic treated the 80/90W lube with limited slip additive, but it wasn't AC/GM brand. The general consensus is that it's important to use the GM brand, so I plan to treat the lube with a bottle of the "right stuff."

 

@NTX5467 suggested drawing out some of the lube to make room for the additive. That sounds like a reasonable and painless thing to do, but I have a couple of questions about that method:

• Would there be any issue with mixing different brands of additive? My gut feeling is no.
• The manual states capacity of the rear axle as two pints, or 32 oz. One 4 oz. bottle of additive would normally make up 12.5% of the lubricant. Adding a second bottle would bring the additive up to 25% of the lubricant. Would that much additive adversely affect the lubricating properties over time? 

My other option is to drain and clean the rear, and start over with the AC additive. Opinions welcomed.

 

Jim

 

[Loren@65GS.com]

I had a full-size Blazer back in the 80's that the differential was serviced on by the dealer. It took two of the GM additives to quiet it down. On another vehicle, a Ford one ton, the dealer didn't have the correct additive in stock. They used something from an auto parts store. That caused a noise issue. From my experience, I feel it's important to use the correct manufacturer product. 

 

  Loren@65GS.com 

[JohnD1956]

Drain and replace with GM lubricant in recommended proportion.  Had a 69 Cutlass once with a posi rear. Had very similar issues. Changing the rear axle oil with the GM posi lube fixed it. Its in the service manual for the 69s.

[60FlatTop]

2 hours ago, Machine Gun said:

My other option is to drain and clean the rear, and start over with the AC additive. Opinions welcomed.

 

"The level of success one can achieve is directly proportional to the number of times they are willing to do the job over." : Julio

 

That rearend has a cover plate doesn't it? An easy one.

[Machine Gun]

It seems that the best shortcut is no shortcut. I'll drain, clean, and refill with lube treated with the GM additive. - Jim

[EmTee]

On my '77 Century, I went so far as to buy both the GM axle lube and the additive.  I don't know whether the GM oil is mandatory or not, but given where you're at with it today I would bite the bullet and go to the dealer for both oil and additive.  That will give you the best chance for success.

[Machine Gun]

@EmTee I'd like to go with GM oil, but I've not been able to find any that's not synthetic intended for much newer cars. I'll search some more, but if I can't source any conventional GM gear oil I'll just go with a high-quality 80/90W and dump in the AC additive. The additive and the cover gasket are due to arrive here on Saturday. I'll do the work early next week and hope for the best.

[EmTee]

Found this: https://www.jegs.com/i/JEGS/555/28054/10002/-1

 

[Machine Gun]

Never did find GM gear oil. I assume they don't sell it any more. I ordered a quart of Valvoline. Can't wait to get his done so I can move on to other things like door weatherstripping and door side molding clips.

[NTX5467]

GM transitioned into the full-syn rear axle lube about 20 years ago, with the feature "Smells like grapes" on the bottle.  Any good gear oil should work.

 

NTX5467

[60FlatTop]

20 years ago, that must have been the rearend in that little S-10 thingie they made.

[NTX5467]

Nope, normal 1/2 to 1-ton stuff, too.  Plus cars.

[60FlatTop]

Ooops, I was stuck thinking Buick only and how few rearends there were back then.

 

I knew I was writing about the Rainier but kind of giving it the brush off.

[Machine Gun]

I replaced the differential lube on Sunday. I used Valvoline conventional 80W90 oil with AC limited-slip additive. I put about 100 miles on the car since the oil change and it appears that the chatter is gone. I would never have guessed that the diff would be particular about the brand and formulation of the additive. @EmTee wins the prize because he was the first one to come up with the solution. Mucho thanko! Now I'm free to look for something else to worry about.

Edited August 10, 2023 by Machine Gun (see edit history)

[EmTee]

9 minutes ago, Machine Gun said:

I put about 100 miles on the car since the oil change and it appears that the chatter is gone.

Good, that's precisely how it went when I did the same on my '77 Century 35 years ago.  I don't know what the 'secret ingredient' in the GM additive is, but whatever it is it works.  i wish I could remember who told me to use the GM additive, but that was a long time ago...

 

Now you can take that fall trip that you were contemplating! 

[NTX5467]

8 hours ago, EmTee said:

Now you can take that fall trip that you were contemplating! 

All the "chattering" will come from inside the car . . .