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1970 350/370 HP positrac differential shudder

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1970 350/370 HP positrac differential shudder

Old 09-17-2018, 04:50 PM
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DonnaW
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Default 1970 350/370 HP positrac differential shudder

What additives can be used to eliminate shuddering in positrac differential when turning corners after car driven for a period time & has heated up. Or other suggestions to eliminate.
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Old 09-17-2018, 07:42 PM
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GTR1999
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2 bottles of GM limited slip additive, 4 oz per bottle. 90 or 85-140 gear oil.
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Old 09-17-2018, 07:46 PM
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Thanks. Had tried GM, but still having problem. Will consider trying again. Thanks again.
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Old 09-18-2018, 10:03 AM
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Please clarify gear oil -- do you mean 90 - 140 or 85-140? And any specific amount? Thanks.
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Old 09-18-2018, 11:43 AM
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You can use either a 90 wt gear oil OR 85-140- both will work. I have used Lucas for many years, change it out between 8-10k miles as part of a PM schedule and never had a problem. You can of course use any quality gear oil but I would stick with the GM additive and use 2 bottles of it. If you still have posi chatter then oil is not going to make a difference. Do the slow figure 8's in an empty parking lot and work it into the clutches. When I build a posi I run them in on the machine I built and break in the new clutches, the parking lot figure 8 is doing the same thing.

The diff should be level if in the car, drain, add the additive and fill until the oil weeps out the fill hole or just below it. There will still be plenty of room in the diff for more oil but you won't be able to get it past the fill plug and you don't need to. If was on a stand 55 oz would be the most added otherwise it will get blown out the vent in use.

Last edited by GTR1999; 09-18-2018 at 11:46 AM.
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Old 09-18-2018, 12:50 PM
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Tks so much for additional information. The other 3 times have drained/added additive/grease only added one can of additive. Have done figure 8's, but don't seem to have a problem until "system" heats up considerably. Kinda of an intermittent scenario. Will see what happens after redoing & adding 2 cans additive. Thanks again for your help.
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Old 09-18-2018, 05:36 PM
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I know I replied to this post. Where the hell did my posts go?
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Old 09-18-2018, 05:39 PM
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Got your posts. Don't worry, I have trouble figuring out how the posts work & where they go, too!!! Will get more additive & grease & try to "redo" in the next few days & test it out. Thanks so much for all your help.
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Old 09-18-2018, 05:44 PM
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I like to keep up with threads that I reply to because it helps me lean and hopefully I can help some as well. When I can't search for my recent topics it make it hard to keep up with the topic.
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Old 09-18-2018, 05:49 PM
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Understand. I'm doing this for my husband who maybe some day will become "computer savy" -- I have a long way to go, but figured it was faster to do it for him. He does so appreciate all the help you've given him & when he gets it redone & tested, I/he'll let you know.
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Old 09-19-2018, 08:55 PM
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I had a similar issue, but it wasn’t happening all the time only intermittently when turning corners. Hopefully the additive will fix the problem on yours

for us it ended up being one of the circlips that attach to the driveshaft in the diff (?) had come off and were crunching around in the diff. Took it apart and had to rebuild it
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Old 09-19-2018, 09:12 PM
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Just did all this good luck. Mine at first ,once it got hot then all the time.
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Old 09-20-2018, 03:22 PM
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Thanks for additional info. A few years ago had someone else do some work on the differential & then didn't drive it much after that. When started driving again, had this problem start, so don't know if something wasn't put back together right or not. Seems like my husband said the individual put clips or pins on, so maybe they didn't get put back together right. Do appreciate all the information & will let you know when he gets it "redone."
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