Positraction Question

[Steven A.]

While doing a little pre-car show cleaning on my 1966 smallblock coupe, I had one rear wheel jacked up off the ground and the other on the ground. I noticed I was able to rotate the raised wheel fairly easily. I read in a previous post that there should be some resistance to the raise wheel spinning if the positraction unit is working properly. I'm pretty sure the car has posi (i.e. it has the little red plastic tag on the fill plug), so I have a couple of questions.

1. Even with the motor not running, and hence no input torque, will the posi unit create friction between the two rear axles?

2. If there is supposed to be some resistance and mine doesn't have any, could this be a result of not having the GM posi additive in the rear end? I had my mechanic change the differential fluid over the winter, but now I'm not sure he put this in. Can I add this "after the fact"?

3. Does driving in small figure eights help this situation?

3. If the posi unit is just worn out, is it a big deal to replace it? Is the Eaton unit I see advertised a good replacement? Is this a "bolt on" operation?

4. Is there a downside to having a worn out positraction unit other than being able to "burn rubber" easier than I would otherwise (I never do this, of course). The car is never driven in inclement weather.

Thanks.

[John H.]

Re: Positraction Question

Steve -

In the situation you describe, if the car has Posi, the wheel that's off the ground shouldn't rotate freely unless the plates are really shot; if it really is a Posi, there's lots of debris floating around in there that doesn't do the bearings any good. Eaton Posi units can be rebuilt (new plates, spring pack, etc.), but nothing associated with a diff teardown is a "bolt-in" job, and needs to be done by a pro with the right tools and experience, and this work doesn't come cheap. The GM Posi additive makes the plates a bit more "slippery" so it doesn't chatter going around corners, but that obviously isn't your problem. The ratio and date code is stamped on the bottom of the diff carrier just forward of the spring mount area, and will tell what was originally inside it, although the guts could have been changed out sometime in the past while using the original carrier.

[Steven A.]

Re: Positraction Question

Thanks, John. I'll take a look.

[William V.]

Re: Positraction Question

1. Even with the motor not running, and hence no input torque, will the posi unit create friction between the two rear axles?

Yes. I just checked mine. Both tires off the ground, placed the car out of gear and had the wife hold one tire while I turned the other. She immedately yelled I can't hold it! as both tire turn in the same direction.

2. If there is supposed to be some resistance and mine doesn't have any, could this be a result of not having the GM posi additive in the rear end? I had my mechanic change the differential fluid over the winter, but now I'm not sure he put this in. Can I add this "after the fact"?

No The rear should react the same with GM + GM additive or a good quality 80/90 fluid for POSI rears.

3. Does driving in small figure eights help this situation?

No If they were binding this would exercise the posi clutches in a effort to get them oiled so they would move more freely.

3. If the posi unit is just worn out, is it a big deal to replace it? Is the Eaton unit I see advertised a good replacement? Is this a "bolt on" operation?

The rear needs to be removed totaly disassembled and all warn parts changes bearings clutches ect. Then set up for proper pinion load and back lash. Suggest bring it to a qualified shop.

4. Is there a downside to having a worn out positraction unit other than being able to "burn rubber" easier than I would otherwise (I never do this, of course). The car is never driven in inclement weather.

If the posi is badly warn, there is a bumch of stuff mixing it up in the rear fluid floating around. The fluid change helped but if you have very badly warn clutches I suspect other parts may also be warn.

If you don't have posi: Then with both rear tires off the ground and the engine off and out of gear, one wheel will turn easly while the other turns in the opposite direction.

[Peter Ansted]

Re: Positraction Question

As uusual, John is right on with his advice. I recently pulled the diff from my 63 due my improper repair of the front seal. This is a labor intense job that will require getting the car safely in the air and a floor jack to support and lower the diff. If you take your time, you can get that unit out in a couple of hours. Check the archives for the details on the how to. Once the diff is out, find a reputable place for repair - the parts are all still available for a 66. Seals, bearings and clutch pack are can be replaced in the 350.00 range and up. If you take it to a shop and have them remove and repair, you can expect to double that cost. Also, the half shafts need to be removed as well, now is a good time to replace those u joints - another job left for people that do it for a living.

[Clem Z.]

Re: Positraction Question

with both wheels off the ground if you have a posi when you turn one wheel by hand the other will turn in the same direction and if the rear gear is open the other wheel will turn the opposite direction

[Steven A.]

Re: Positraction Question

Thanks, to everyone, especially Bill Voss's wife!! What a trooper!!