Lost Positraction
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Instructor
Joined: Apr 2004
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From: Epsom Victoria
Lost Positraction
Last night I was working on the brakes of a friends C3 and noticed that his differential no longer has positraction. I know the car used to have posi as I have worked on it in the past. Can any one tell me how serious this is will the differential have to be pulled and repaired or can it be driven as a regular non posi with out any bad side effects
Race Director
Joined: May 2000
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You can continue to drive it without ill effect other than the loss of performance in hard driving.
You should also check side yoke end play as worn clutches cause it to increase, and you should measure actual breakaway torque to determine how degraded the Positraction unit is.
Duke
You should also check side yoke end play as worn clutches cause it to increase, and you should measure actual breakaway torque to determine how degraded the Positraction unit is.
Duke
Instructor
Joined: Feb 1999
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From: Waukegan,Il,USA
Lost Posi
It might depend on what caused the posi to go out in the first place whether to drive it or not. I'm not trying to frighten you but if there is some debris floating around in there it could possibly do further damage.
Bob L.
Bob L.
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Instructor
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From: Epsom Victoria
Thanks guys for the advice I will pop the rear cover and make sure it is clear and will also check the bearing float. My buddy is a retired vet and is short of cash so if we can keep him crusing that will be fine. He was a wild man once but these days he isnt doing anything to insane.
Tech Contributor
Joined: Aug 1999
Posts: 15,156
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From: Connecticut, USA
A lot depends on if someone was already in there. Assuming it is an original C3 differential then you may have either solid or "snowflake" clutches. I think the sloids were last used in '70? I know my 72 had the snowflakes. The snowflakes are weaker and break,I opened up a 75 and it was running fine only to find 2 clutches broken in 3 places.
As Duke said, as they wear your side yokes with have more play.
You can remove the cover on the car but it will be a bugger. If I'm going to check a posi I would check the torque required to break it loose,the procedure is in the GM overhaul manual. If in doubt then I drop the rear end out of the car. Be prepared though as a LOT of times you into repair one thing and end up rebuilding it. If the yokes have more then .050" and posi is shot expect to rebuild it all.
Good luck,
Gary
As Duke said, as they wear your side yokes with have more play.
You can remove the cover on the car but it will be a bugger. If I'm going to check a posi I would check the torque required to break it loose,the procedure is in the GM overhaul manual. If in doubt then I drop the rear end out of the car. Be prepared though as a LOT of times you into repair one thing and end up rebuilding it. If the yokes have more then .050" and posi is shot expect to rebuild it all.
Good luck,
Gary
Thread Starter
Instructor
Joined: Apr 2004
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From: Epsom Victoria
Gary thanks for the heads up I have checked the posi per the manual and it is vertually non existant. I have looked at what it takes to remove the cover and your right it is a bugger. I think I may have come up with a scam of sorts. My 66s posi is below spec but still in the ball park so I will order a rebuilt pumkin for mine, put my old pumkin in his and return his pumkin as the core. Many thanks to you all for your assistance.
Rick
Rick
Tech Contributor
Joined: Aug 1999
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From: Connecticut, USA
Rick,
Glad to help.If you have a posi out why not give it a shot at rebuilding it yourself? If you have some basic tools and mechanical experience they are not too bad to do. You'll need an old side yoke to set it up on. I have a procedure that might help you if you decide on giving it a try. You can PM me or go to www.corvettefaq.com and do a search for "how to rebuild a differential" it should be there.
Good luck,
Gary
Glad to help.If you have a posi out why not give it a shot at rebuilding it yourself? If you have some basic tools and mechanical experience they are not too bad to do. You'll need an old side yoke to set it up on. I have a procedure that might help you if you decide on giving it a try. You can PM me or go to www.corvettefaq.com and do a search for "how to rebuild a differential" it should be there.
Good luck,
Gary
Thread Starter
Instructor
Joined: Apr 2004
Posts: 115
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From: Epsom Victoria
Hi Gary thanks for the pointer to the instructions, I have access to a good shop and all the equipment shown in the rebuild instructions. All I have to do now is round up a couple of rebuild kits and get on with it. Many thanks for your help. 
Rick
Rick
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