72 differential noise?

pttsls5 · 10-31-07, 08:05 PM

72, 454, automatic trans., stock rear gear. I noticed a slight grinding/clunking noise coming from the rear. The noise only happens when I turn left at idle speed anything faster there is no noise. I replaced u-joints drive shaft and half shafts 4 years ago. I checked and added the special GM posi-lube additive but I still have the noise. Is it the differential ready to go south?( original differential never been rebuilt). Should I get it rebuilt or replace? Should I stick to the original 3:08 ratio or would I benefit performance wise with a 3:36 ratio? Thanks for your help!!

fine69 · 10-31-07, 08:54 PM

Quote:

Originally Posted by pttsls5

72, 454, automatic trans., stock rear gear. I noticed a slight grinding/clunking noise coming from the rear. The noise only happens when I turn left at idle speed anything faster there is no noise. I replaced u-joints drive shaft and half shafts 4 years ago. I checked and added the special GM posi-lube additive but I still have the noise. Is it the differential ready to go south?( original differential never been rebuilt). Should I get it rebuilt or replace? Should I stick to the original 3:08 ratio or would I benefit performance wise with a 3:36 ratio? Thanks for your help!!

It sounds like a commom problem. You'll have to drain the old fluid first, and then replace it with the GM posi lube. Did you do that?

Also be sure to drive in several figure-eights (in a open parking lot for example) to get the fluid mixed up really well.

Ralph

pttsls5 · 11-01-07, 08:32 AM

Yes, when I did the u joints about 4 years ago I did the fluid change and only now am I having this noise. I didn't put the GM special additive in until the noise appeared and it doesn't seem to help. Just wondering if noise is coming from turning left only that something inside is worn and is a sign that eventually it will make the same noise turning bothleft and right? If the rear gear breaks will the rear tires lock-up?or will it just scramble the inside?

GTR1999 · 11-01-07, 12:54 PM

Check the LH yoke for endplay and possible hitting of the housing. If there is a lot of endplay then the yoke will hit and grind the housing.

Sounds like posi clutches. If the additive didn't help then I would consider removing it and looking it over. If you're not sure then drain out the old oil and add Lucas 85-140 gear oil with 2 bottles of GM additive. The posi clutches in your stock 72 are what I refer to as snowflakes and are not too good. Same goes for the newer fiber coated clutches.

If you have to drop the diff out to inspect and rebuild be sure to install solid steel clutches.

The diff usually fail from the posi spiders and cross shafts. Sometimes the rign gear bolts back out but not too much with the early 70's cars, this happens more with the 76-79 units. The 72 posi case is a good one and it has the better 10-17 spiders as well.

Also check for the noise in the parking brake area.

I have a full diff procedure avail if you need it, PM me and I'll get you the link.