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Old 06-04-2005, 10:41 AM
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Limited Slip and Posi noise

I've read that posi's and/or limited slip diff. are noisy. I'm going to describe some noises I get from my Firebird and Blazer that I think are diff. related. Tell my if you think this sounds correct/incorrect.

Firebird: Currie rear with clutch type limited slip

When making 90* turns at about 20 mph (no power applied) I hear a ticking/ratcheting sound. Does that sound like the diff? I thought it was front wheel bearings but when I pulled the rotors off the bearings looked new. I contacted the previous owner and he said they are new.

76 Blazer 4x4: Factory posi rear

When going around curves at about 20+ mph (like 270* Hwy on ramps) I hear a ticking/ratcheting sound (maybe a little grinding/rubbing/hissing sound also). It is considerably louder than the Firebirds noise but is a similar noise. Does that sound normal? I have no idea when or if that rear has ever been gone through.

Thanks in advance.

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Old 06-04-2005, 11:57 AM
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Limited Slip diffs require a anti chatter additive. This is why factory GM vehicles came with the tag on the diff cover "use limited slip differential fluid only" This is still avalible from GM and it eliminates chatter. However, royal purple, as well as many other synthetic lubes eliminate the need for the additive, When I went to a posi diff it popped all the time until I got some of the GM fluid, when I recently changed over to royal purple I didn`t add it and it hasn`t popped yet as it says on the bottle that no other additives are needed to eliminate chatter. Some parts stores carry a anti chatter lube I think under the name positraction, I tried it a long time ago and it didn`t work. If you get the GM additive it costs about $11 for about a 4 oz bottle, so it`s really cheaper in the end to go with Royal Purple or the like, synthetic lubes provide better protection than plain fluids, they also reduce rolling resistance.
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Old 06-04-2005, 03:48 PM
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DoubleVision is dead on with the additives. The GM posi additive is the best one out there. I've even heard of ford guys using it in their trac-loc (that is the ford brand name right) diffs. A good full synthetic is the only other (better?) way to go.

As for the K5 they never came with a posi (limited slip). GM (for the last few decades) uses Eaton Gov-Lock diffs in all their trucks. Do some searching on here and you will see that they aren't the best diffs out there (not even close!). IT SOUNDS LIKE YOURS IS ABOUT TO BLOW UP AND TAKE OUT THE REST OF YOUR CENTER SECTION WITH IT!!!! I would pop the cover and see what is going on in there before driving it any more. A gov-lock should NEVER make any noise on the street unless your tires are spinning (and even then the only time you can hear it is in the rain because you just hear the tires otherwise). You might have caught it soon enough to only need to buy a new carrier (open or aftermarket limited slip) and not an entire new axle.
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Old 06-04-2005, 04:35 PM
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My Firebird has a Currie 9" so I used the Ford friction modifier. Currie says not to use synthetic in their limited slip rearends.

Triaged,

I'll pull the cover off in a while and check it out. I'm getting ready to cut off the exhaust right now and put headers on.

What should I look for when I get the cover off?
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Old 06-04-2005, 05:45 PM
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It should be fairly obvious if there is a problem.
Look for shavings/chunks in the gear oil.
Look for cracks in the carrier (the carrier sometimes cracks in half)
Look for chipped teeth on the spider gears and side gears.
Look for broken govoner parts.

Someone posted a pic of a whole one not too long ago.
This is a pic of a failed one:


I'm going to have to ask my budy to email me a pic of his (from an 89 K5) because it looked like you put a gernade in the diff and bolted the cover back on His even took out the housing and R&P so he had to scrap the whole thing.

The problem is that the 8.5" 10-bolt is just too weak for a full size truck and the gov-lock is also a weak diff. Some of the 88-up trucks got the 9.5" 14-bolt which is a much better axle (if you can find one).
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Old 06-04-2005, 06:29 PM
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I have a 12 bolt rear diff.
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Old 06-04-2005, 07:42 PM
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oops...I guess I read the year wrong in your post.

It is still a gov-lock as GM started using them _exclusively_ in 1973 in trucks. It shouldn't be making any noise. The 12-bolt however isn't quite as weak as the 10-bolt so I wouldn't worry about finding a stronger axle...just swap out the gov-lock for something else. Summit has some very inexpensive limited slip diffs you might look into. If you don't mind forking over some more cash you could also upgrade to 33-spline

Edit
I take some of that back. I don't think they make any 33-spline truck carrers I think they only make car ones. Not sure why that is...
The cheepist full case diff summit has is $149.39 but that is an open diff. The cheepist full case limited slip is $479.95.
End Edit

This is some good info about gov-locks
http://www.ringpinion.com/content/te...lt.asp?pid=132

I on the other hand had a bunch of problems with my 12-bolt (in my 71 K5) but I am a bit harder on my trucks then most

Last edited by Triaged; 06-04-2005 at 08:06 PM.
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Old 06-04-2005, 09:20 PM
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I have a 12 bolt with an eaton posi unit. It make no noise at all and if I hadnt had the cover off you would never know it had a posi by the noise.
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