Crunching/Clunking 79 rear end
Thread Starter
Racer
Joined: Jul 2004
Posts: 400
Likes: 1
From: California
Crunching/Clunking 79 rear end
well, this has been happening ever since i got the car about a year and a half ago, and im wondering if i have something wrong in the rear end or what. i looked at Ecklers.com and they sell a kit that has the gasket and original GM posi-fluid which i ordered and should be here shortly. i guess what my question really is: has anyone ever experienced this and if so, what has been your solution and can you recommend?
Drifting
Joined: Nov 2003
Posts: 1,634
Likes: 4
From: Northern NJ
St. Jude Donor '04 & '05
Welcome to the insanity! 
This is a very common issue with C3 posi units. Assuming it's not completely worn out (usually not), a simple removal and replacement of the diff fluid along with a couple bottles of GM posi additive will usually quite the noisy rear down.
After you change the fluid, go to a empty parking lot and do a bunch of tight figure 8's to work the new fluid into the posi unit.
And assuming the cover gasket is not leaking now, you don't have to replace it. Just get a suction pump and extract the fluid thru the overflow/check plug hole and replace the new fluid thru same. Add two bottles of posi additive and then top it off with whatever diff oil you choose until it just starts to run out of the hole.
Hope this helps. Good Luck!
This is a very common issue with C3 posi units. Assuming it's not completely worn out (usually not), a simple removal and replacement of the diff fluid along with a couple bottles of GM posi additive will usually quite the noisy rear down.
After you change the fluid, go to a empty parking lot and do a bunch of tight figure 8's to work the new fluid into the posi unit.
And assuming the cover gasket is not leaking now, you don't have to replace it. Just get a suction pump and extract the fluid thru the overflow/check plug hole and replace the new fluid thru same. Add two bottles of posi additive and then top it off with whatever diff oil you choose until it just starts to run out of the hole.
Hope this helps. Good Luck!
Last edited by ACECO; Jul 17, 2004 at 02:07 AM.
Melting Slicks
Joined: Jul 2002
Posts: 3,128
Likes: 3
From: Deltona (son of Deland and DAYTONA) FL
That worked for me for about 1.5 years. But as the whine from the worn ring and pinion grew louder, so did the growling around turns. I finally broke down and bought a newly rebuild unit from Van Steel and installed it myself.
No more problems. If you DO end up needing a new one, get it tapped for a drain plug. The originals don't have one and you have to suck the old fluid out with a oil transfer pump. I to change the oil in my old diff, I bought the pump from Walmart in the boat/fishing section.
Thread Starter
Racer
Joined: Jul 2004
Posts: 400
Likes: 1
From: California
Thanks guys! hopefully the kit for the Gm posi-fluid gets here by Wednesday of this week like its supposed to, oh and by the way, how hard is it to take the gasket and old cover off the rear differential housing?
Melting Slicks
Joined: Jul 2002
Posts: 3,128
Likes: 3
From: Deltona (son of Deland and DAYTONA) FL
I wouldn't mess with the rear cover unless you have to. The rear cover is also the center mount for the transverse suspension spring. You'll have to remove the rear spring and I think disconnect the strut rods to get it off. That kit was probably intended for those buying rebuilt diffs. I got one from Van Steel when I bought mine. Either way, you need the fluid, but unless replacing or opening the diff, not the gasket.
Most of us suck the old fluid out with a pump like I mentioned above. You can put the new fluid in with that same pump, or with a 6" section of clear tubing (clear is nice because you can see if the fluid is flowing) attached to the nozzle on the bottle. Add a little gear oil, then pour the limited slip additive in the first bottle of gear oil. Finish adding that first bottle of oil, then the second until the fluid starts overflowing the fill hole.
Most of us suck the old fluid out with a pump like I mentioned above. You can put the new fluid in with that same pump, or with a 6" section of clear tubing (clear is nice because you can see if the fluid is flowing) attached to the nozzle on the bottle. Add a little gear oil, then pour the limited slip additive in the first bottle of gear oil. Finish adding that first bottle of oil, then the second until the fluid starts overflowing the fill hole.
Last edited by isosceles; Jul 17, 2004 at 10:58 PM.
Advanced
Joined: Jul 2003
Posts: 52
Likes: 0
From: ware ma
I had this problem with my 78. I tried the GM additive and it still didn't go away. I went to a guy who has done a bunch corvette differentials and he told me to try more additive because he was positive it would go away. He was right! I flushed it out again and added some more additive and the problem went away.
Race Director
Joined: Oct 2000
Posts: 17,102
Likes: 7
From: Fair Oaks, OK
The suction pump idea works. I used it the other day. I bought a $7.99 pump at a local farm supply store and it worked perfectly. Out with the old, in with the new. I used 2 bottles of GM Axle Lube and 2 bottles of GM limited slip additive. I went to a parking lot and did a bunch of figure 8's. Now I have no noise, and my posi unit works flawlessly. After I get my motor mount fixed I should be able to lay 2 strips of rubber. 
Thread Starter
Racer
Joined: Jul 2004
Posts: 400
Likes: 1
From: California
so how much bottles of additive should i use? 1 or two, because Ecklers.com gave me one...or do u know where i could buy some for cheap at a local shop? maybe a corvette shop??? i live in Sacramento BTW so if ur around there any tips would be helpful for shopping around. 
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Race Director
Joined: Oct 2000
Posts: 17,102
Likes: 7
From: Fair Oaks, OK
Canada cars get SAE 80W GL-5 gear lube from my Haynes repair manual. I use GM 80W 90 (2 bottles) and GM Positraction additive (2 bottles). 2 and 2 are common recommendations here on the forum.
Intermediate
Joined: Jul 2004
Posts: 38
Likes: 0
had the same thing on my 69, thought the thing was ruined and going to fall apart. i removed all the fluid with a suction gun, (10 bucks autozone) filled it up with automatic transmission fluid (lots of detergent) went to a parking lot and did lock to lock turns, after 3 turns the noise was gone, drained and refilled with lube with 2 bottles of gm posi-additive, not a peep since.
Pro
Joined: Jan 2004
Posts: 522
Likes: 0
From: Grayslake IL
I too had this problem with mine in my 74. Turns out that the side yolks were worn and the ring and pinion gear were screwed due to a worn pinion bearing. Get it rebuilt, GM posi units are not as bullet proof as everyone claims.
Racer
Joined: Apr 2001
Posts: 395
Likes: 0
From: Shoreline WA
Originally Posted by EVIL_GRIM
...or do u know where i could buy some for cheap at a local shop? maybe a corvette shop???
Mike
Tech Contributor
Joined: Aug 1999
Posts: 15,188
Likes: 4,010
From: Connecticut, USA
Available for about $6-$7 a bottle from any GM dealership listed as "limited slip additive", use 80-90 wt gear oil.It's not practical to remove the cover unless the differential is out of the car. If the 2 bottles and oil don;t solve it then you have to drop the differential. The posi most likely has the snowflake clutches and they break over time.
good luck,
Gary
good luck,
Gary
Thread Starter
Racer
Joined: Jul 2004
Posts: 400
Likes: 1
From: California
what sucks about having that crunching for sooo long is now i expect it everytime i turn so i prepare for it and start to feel embarassed about the upcoming sound of "Popcorn" in my rear end then realize its not gonna happen. what i have noticed is that i do feel a slight rubbing in the rear end at low speed like 3 mph, i think i need another bottle of GM additive. 1 problem down! a Bagillion more to go!! 
