Posi unit slippage.
Thread Starter
Instructor
Joined: Dec 2009
Posts: 109
Likes: 0
From: Nassau co. NY
Posi unit slippage.
I was just under our '69 and finally got to the axle code. I found out it is a 3.08 Posi (not H.D. according to my numbers book). I'm pretty stoked about having the posi, but not about the 3.08's.
Anyway, I put a breaker bar through one of the rims to lock it, and with very little force, I was able to spin the other tire. I am not familiar with this eaton type of posi and would like to know if this is normal. On the rears I am familiar with, usually it takes 30-50 ft.lbs of force to slip it.
Am I looking at a posi unit rebuild?
Thanks,
Greg.
Anyway, I put a breaker bar through one of the rims to lock it, and with very little force, I was able to spin the other tire. I am not familiar with this eaton type of posi and would like to know if this is normal. On the rears I am familiar with, usually it takes 30-50 ft.lbs of force to slip it.
Am I looking at a posi unit rebuild?
Thanks,
Greg.
Safety Car
Joined: Oct 2006
Posts: 3,930
Likes: 15
From: Calgary Alberta
Someone might have removed the springs in the posi, and tuned the clutches. If the clutches were not shimmed correctly, then they won't be doing there job, and you will have a ton of spider gear backlash. I'd drop it and take a look...
Le Mans Master
Joined: Jan 2006
Posts: 6,505
Likes: 139
From: Butler Pa
I was just under our '69 and finally got to the axle code. I found out it is a 3.08 Posi (not H.D. according to my numbers book). I'm pretty stoked about having the posi, but not about the 3.08's.
Anyway, I put a breaker bar through one of the rims to lock it, and with very little force, I was able to spin the other tire. I am not familiar with this eaton type of posi and would like to know if this is normal. On the rears I am familiar with, usually it takes 30-50 ft.lbs of force to slip it.
Am I looking at a posi unit rebuild?
Thanks,
Greg.
Anyway, I put a breaker bar through one of the rims to lock it, and with very little force, I was able to spin the other tire. I am not familiar with this eaton type of posi and would like to know if this is normal. On the rears I am familiar with, usually it takes 30-50 ft.lbs of force to slip it.
Am I looking at a posi unit rebuild?
Thanks,
Greg.
Torque applied to the pinion causes a force from the spider gears which compresses the clutch packs and locks them up.
Safety Car
Joined: Oct 2006
Posts: 3,930
Likes: 15
From: Calgary Alberta
The key to all this is that you don't have excessive spider gear backlash when there is no torque being applied to the pinion. They need to be shimmed properly.
Thread Starter
Instructor
Joined: Dec 2009
Posts: 109
Likes: 0
From: Nassau co. NY
Thanks,
I like the "foot to the floor" test for the posi unit. I plan to try that after the new motor is broken in...(hopefully not "broken")
You know what they say...fourth motor is a charm.
Greg.
I like the "foot to the floor" test for the posi unit. I plan to try that after the new motor is broken in...(hopefully not "broken")
You know what they say...fourth motor is a charm.
Greg.
Pro
Joined: Sep 2009
Posts: 629
Likes: 6
From: columbus ms
i have some experience with the " foot to the floor " testing method. i would be happy to demistrate the correct form if you would like. lol you don't want to throw alot of money at a problem you may not have. save that money for the ones you find.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Actually, there's a GM tool, a torque wrench of sorts, that attachs to the five lug studs on the rear hub, and with one wheel (tire) on the ground, the resistance is measured on the other hub. I believe the spec was 75 or 80 ft/lbs. Perhaps somebody has this info.
Also, if somebody had installed a synthetic gear lube and/or too much friction modifier, the clutches could be slipping, reducing the effectivenes of your posi.
FWIW
Also, if somebody had installed a synthetic gear lube and/or too much friction modifier, the clutches could be slipping, reducing the effectivenes of your posi.
FWIW
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Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
Actually, there's a GM tool, a torque wrench of sorts, that attachs to the five lug studs on the rear hub, and with one wheel (tire) on the ground, the resistance is measured on the other hub. I believe the spec was 75 or 80 ft/lbs. Perhaps somebody has this info.
Also, if somebody had installed a synthetic gear lube and/or too much friction modifier, the clutches could be slipping, reducing the effectivenes of your posi.
FWIW
Also, if somebody had installed a synthetic gear lube and/or too much friction modifier, the clutches could be slipping, reducing the effectivenes of your posi.
FWIW
Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
and try 2-3 times and with a bit uneven ground or turning a bit the same time one or the other directions
7th Gear
Joined: Mar 2010
Posts: 7
Likes: 0
From: Reno NV
There are different preload springs used in that diff, from 300# to 800#. The test is what the turning torque is (torque required to turn one axle while the other is stationary). I can't remember the specs, but i think it's about eighty pounds of torque to make the 300# springs slip. Somewhere around two hundred and fifty to make the 800# setup slip. Depends on how it's set up. Also there are different clutch materials available, from stock to carbon fiber. they all have different coefficients of friction. It's best to talk to a shop that does alot of differential work. 
Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
There are different preload springs used in that diff, from 300# to 800#. The test is what the turning torque is (torque required to turn one axle while the other is stationary). I can't remember the specs, but i think it's about eighty pounds of torque to make the 300# springs slip. Somewhere around two hundred and fifty to make the 800# setup slip. Depends on how it's set up. Also there are different clutch materials available, from stock to carbon fiber. they all have different coefficients of friction. It's best to talk to a shop that does alot of differential work.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Yep. If everything seems good. no noise or excessive play and both tires hook up well on launch why pull it out or mess with it?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Even with a low-powered, late 70's SB, your car will spin one tire, if the posi isn't working properly. If you can't leave any tire marks when you 'floor' it, the posi is working...or you don't need a posi anyway. 
But, you should also do some tight figure-eights at 'casual' speed in a Wal-Mart parking lot to see if you get any popping or slipping of the posi plates when doing tight turns. If you do get some unwanted sounds, add a tube of the GM posi-trac additive. And if it has been a while since the fluid was changed in the diffy, suck out the old stuff and put in a fresh bottle (or so) of rear-gear lube along with the posi additive.
But, you should also do some tight figure-eights at 'casual' speed in a Wal-Mart parking lot to see if you get any popping or slipping of the posi plates when doing tight turns. If you do get some unwanted sounds, add a tube of the GM posi-trac additive. And if it has been a while since the fluid was changed in the diffy, suck out the old stuff and put in a fresh bottle (or so) of rear-gear lube along with the posi additive.
Racer
Joined: Dec 2004
Posts: 410
Likes: 0
From: Wichita Kansas
Interesting, I have always thought the way to test a posi is, see if it spins one tire a lot with the power on, going around a corner. Mine will spin the inside tire a small amount. I know on later model Camaros when the inside tire starts spinning too much, it is time for a rebuild. Vette diffs hold up much better.
