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knocking sound from rear diff

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Old Jun 25, 2010 | 03:47 PM
  #1  
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Default knocking sound from rear diff

Hi everyone,
In my 77 vette, after about 20 minutes drive, usualy while turning or braking, I get a knocking sound from the rear left inner wheel side.
Two friends of mine, who own a C3 vette as well told me they have the same noise and they think that its from the wheel lockers (I think that's the right term) to the diff.
1. does anyone familiar with that?
2. if they are correct, are there any parts I need to fix it?
Hope someone can assist me

A.L.
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Old Jun 25, 2010 | 03:54 PM
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Probably just posi chatter , change your fluid and add GM additive
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Old Jun 25, 2010 | 04:19 PM
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Originally Posted by ...Roger...
Probably just posi chatter , change your fluid and add GM additive
Thanks Roger.
Forgive my English since it's not my native tongue.
I've filled the rear axle with Motorcraft SAE 80W-90 API SERVICE GL-5 (same spesifications as in the owners manual) and it didn't help.
If U think I did wrong, Can you elaborate what exact products should I use?

A.L.
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Old Jun 25, 2010 | 06:12 PM
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You should have put this additive in also. I put 2 bottles in and then fill.
http://www.ecklers.com/positraction-...-additive.html
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Old Jun 25, 2010 | 09:06 PM
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You need the additive in the above post by Roger along with the rear end oil you already put in.

Then go to a parking lot and do a bunch of figure 8's, tight left and right hand turns and the noise should go away


Use two bottles, you know us North Americans, if one works two should work twice as good

Seriously they say 1 bottle or 4 ounces but most put in 2 bottles or 8 ounces
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Old Jun 26, 2010 | 10:39 AM
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Thank you.

can I use AC Delco as well?
http://tld-corvette.com/prodinfo.asp?number=668532
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Old Jun 26, 2010 | 10:43 AM
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Originally Posted by la1234
Thank you.

can I use AC Delco as well?
http://tld-corvette.com/prodinfo.asp?number=668532
Wow $6 what the heck is it ? If you get this , smell it,if it stinks it probably won't work like the GM stuff.
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Old Jun 26, 2010 | 10:53 AM
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Thank you very much for your help.
I also found what you showed for 10$:
http://corvettepartsman.com/1963-96l...ential4oz.aspx
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Old Jun 26, 2010 | 05:44 PM
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You just might check all of your U-Joints while you are under there...if you have not done so...the Grease Zerks are easy to over look when servicing...and can get good and rusty inside the needle bearings...they make a noise when dry... Good Luck!

Last edited by TEXASDESERTVETTE; Jun 26, 2010 at 05:48 PM.
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Old Jun 26, 2010 | 05:47 PM
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Originally Posted by la1234
Thank you very much for your help.
I also found what you showed for 10$:
http://corvettepartsman.com/1963-96l...ential4oz.aspx
That stuff is more expensive per ounce the Hai Karate Cologne
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Old Jun 26, 2010 | 06:58 PM
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Originally Posted by MotorHead
That stuff is more expensive per ounce the Hai Karate Cologne
Yes it is more expensive than Hai Karate Cologne but it works better that Hai Karate!
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Old Jun 27, 2010 | 09:53 AM
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Thank you all for your replies.
Since they can not airborne it, I have a Motorcraft dealer that can sell me a "Motorcraft rear axle additive friction modifier XL-3 (4 oz. bottle)"
Can anyone tell me if I can use that?
As shown here:
http://i2.frsimg.com/FR-Motorcraft-X...il_p_2288.html
Specifications according to Motorcraft: "EST-M2C118-A".
Thanks.

A.L.
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Old Jun 27, 2010 | 10:00 AM
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Wonder what a friction modifier is ? I was told the GM stuff is a anti-foaming agent. Maybe some of our oil product engineers will chime in and tell us whats really in the additives.
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Old Jun 27, 2010 | 05:32 PM
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Originally Posted by la1234
Thank you all for your replies.
Since they can not airborne it, I have a Motorcraft dealer that can sell me a "Motorcraft rear axle additive friction modifier XL-3 (4 oz. bottle)"
Can anyone tell me if I can use that?
As shown here:
http://i2.frsimg.com/FR-Motorcraft-X...il_p_2288.html
Specifications according to Motorcraft: "EST-M2C118-A".
Thanks.

A.L.
Any limited slip or positrac friction modifier additive should be fine as long as it is compatible with your gear oil (GL-5 spec). The Motorcraft version you found should be fine as it says it is a universal product for any vehicle. The only concern is that it says it is for non-synthetic gear oils. If you have synthetic gear oil, you might need to find something that says it is compatible.

This is what I used last time I serviced my '73: http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Trans-X&174;-Posi-Trac&174;-Limited-Slip-Gear-Oil-Additive-K-&-W-Chemical_7070977-P_N3242_T|GRP2035____

It worked fine for me.

DC
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