Today I was getting ready to put my 81 away for the winter. There had been a inch of snow the night before that had left a thin coating of ice on the driveway except for a clear and dry spot where my daily driver is usually parked. I backed the car out of the driveway and ended up with the passenger side tire on the dry patch and the driver's side tire on the ice. A few minutes later I went to put the car back in the garage. As I gently pressed the accelerator the driver's side wheel began to spin and the car wouldn't move. I gave it a little more gas and nothing changed (except the wheel spun faster). I backed the car up 2 feet (the driveway is on a slight decline away from the garage) and tried again and the same thing happened. I ended up having to roll back off the driveway to pick up a clear spot on the road for both tires and take a little bit of a run at it before I could get it in the garage. I thought the Posi differential was supposed to prevent that from happening. Over the summer I peeled out a few times and recalled a straight launch and two patches of rubber on the road so I am confident it was working pretty recently.

Is this just normal function for the Posi system at low speeds?

 

Most limited slip differentials can only transfer torque up to a certain traction differential. For example, it might only be capable of locking up and transferring torque when the passenger side wheel has no more than 4X the traction of the spinning drivers side wheel. There has to be enough resistance to wheel spin so the spider gears can force the side gears apart into the clutch packs which locks the side gears and differential case together.

 

If I remember correctly there was a GM jacking warning back in the day about not having both wheels off the ground and starting the motor incase it was in gear. It just may be that your posi clutches are excessively warn

 

I think most people believe that a posi traction rear end has no slip.
It has to so when you make a right or left turn the posi will slip on the inside axle so you don’t tear apart the gears in the rear end.
If one tire is on ice and the other is on dry pavement, the wheel on ice will spin.

 

Positraction wouldn't be worth much if it couldn't even get the car started on a slight incline...

 

How many miles on the car? Has the differential ever been touched? Sounds like some worn clutch packs in the posi.

 

It's toast!. A posi unit is made to allow a slight amount of slip from side to side, but you have nothing. My posi works fine in snow and ice. It's actually dangerous to drive on packed snow and ice. I've been caught out a few times and wished I had at least cable chains!

 

And then there is the Detroit No-Spin... Of course both wheels may spin 😁.

 

Posi slips so you dont tear apart your inside tire..

Read this. scroll down to the posi section...your diff has an issue

https://www.streetmusclemag.com/tech...differentials/


Heres the wiki on it

https://en.m.wikipedia.org/wiki/Limited-slip_differential

 

Apply a little parking brake. It sometimes help and this comes from a GM manual.

 

A posi will not transfer torque with 1 wheel on ice and the other on clear asphalt. It will with both wheels on ice.

Jack up the rear end and leave it in park or gear depending on transmission and try to turn one wheel. Either you shouldn't be able to get a wheel to turn or there should be a pretty decent resistance to rotating it.

Or, wait till the roads are dry and lay 2 patches again. If they are pretty much equal then it's OK.

 

Same goes for four wheel drive.
It isn’t really.

 

Some varied opinions on the matter it seems.

The car has about 80,000 miles on it so it's not super old, but I don't know how hard it was driven before I put the last 6,000 on.
The idea about the parking brake is an interesting one and a simple enough try. It'll be a while up here before I can get it on dry enough pavement to lay some rubber and check out if I'm leaving a "1" or an "11". I have a small pinion seal leak so I have to get into the differential over the winter anyway.
I'll have a look at the clutch pack and the rest of the posi unit when I'm in there and see what it looks like. I suppose while I have it apart the smart money would be to just change them out either way.

I'll respond in a couple months and let you know what I find.

 

Watch Tom's rebuild video. If it's easy to turn it will slip more than it should

 

That is why real 4X4 have a rear posi unit. Then you have 3 wheel drive. Like my IRS 2007 Ford Expedition. Off roaders even install front and rear posi units with air lockers so if you get one front and one rear off the ground they still have driving traction.

Just to simulate one wheel on ice today. I put my big floor jack from behind under my trailing arm and lifted one rear tire off the ground. Then I got in and started the motor and at idle put it both first and reverse and let the clutch slowly engage. My vette can drive with one wheel off the ground. Another time many years ago. I had the passenger side half shaft off and I needed to move the car. I just hopped in and it and moved it.

 

Thats why there was a service bulletin put out by GM years ago. But no one listens

 

I was going to let this thread go because you guys are probably tired of me talking about diffs all the time, but I am sitting here on lunch break so I figured I would post up some pictures to help the OP know what is in that aluminum diff he has. BTW, I do not work on these because I don't like anything about them and quality is more important to me then money however there are plenty of places that will take them in. I would recommend you take it apart yourself but that's your call.

Here is one hanging like a sack of ..... something 64 lbs without the batwing. So maybe 80-85 lbs with the batwing compared to 100 lbs for a complete and a lot better iron diff.


Inside, aluminum bearing caps, smaller spiders, thinner clutches, same pinion design with a square shoulder that was used in 63-4 only those were larger but the pinions still snapped in half with them.



Clips hold in the cross shaft which gets chewed up by the axles which are also soft and wear down as you can see here.



Spiders, DANA on left Eaton on right. I will take the Eaton all day long.


16 tooth side gear is small



Clutches are thinner and narrow. I would call this whole diff junk but then people get all bent out of shape so I won't.



Clutch retainer get chewed up too.





Pinion


Pinion seal is different than the iron seals.



Case shims are under the pressed on bearing, you need setup slip for bearings to dial in the lash and pattern.


So here you have it. Like them or hate them this is what they look like. I will never use one if I have a 80-82 and nice "heavy" iron would go in.

 

Air lockers are not positraction differentials.

A built positraction differential with heavy preload acts much different than an aged factory positraction differential. Stronger preload springs allows more initial grab of the clutch packs or in other words it means there is more initial drag that will transfer up to a certain input torque to both wheels. But, if you tried to put serious power down through one wheel you would slip the clutches. Once again, the hard locking of the differential so it can hold both wheels together under full engine power happens due to the spider gears trying to climb the side gears which forces the side gears out hard against the clutches locking everything together.

I've been around a posi with good pre-load. It would move the vehicle with one wheel off the ground until the vehicle encountered resistance to moving trying to climb a small hill and then it began to slip. It would actually start up the hill and then slip back down when the posi started slipping.

 

Never tired of your posts Gary! Keep ‘em coming!

 

Gary,

When I made this post I had very much hoped that you would chime in and share your knowledge. I very much appreciate the photos and you weighing in.