8 3/4" axle whines. Gears or bearings to blame?

[Kern Dog]

I have a 3.91 diff with a clutch type SG in it. It is quiet as can be until about 50 mph. It sounds louder at light throttle but is still there at cruise or WOT. I can be cruising along at 65 and lift off of the gas and it goes away, ease back in it and the noise comes back.
The diff was rebuilt and is as tight as new. The bearings were original as was the ring and pinion. Years ago when I put this 3rd member in it was silent. I had a 4.10 3rd member in before that had the exact same thing happen. I use high quality 85w90 oil with the LSD additive.
Any ideas? Thanks, Greg

 

[Meep-Meep]

What case do you have? 741, 742 or 489. So it makes no noise under full load of deceleration? But it makes noise under load during acceleration? What about floating the drivetrain with no load? As in not accel or decel. Any erratic sounds then? It doesn't sound like a pinion bearing issue but could be carrier bearings.

 

[Supershafts]

Wrong set up

 

[Garys1969RR]

The 2 3.91 differentials I have now, and the one I had in 1972 all made a steady whining noise under load. Let off the gas, or coast it, and it goes away. About like you are describing. I think it is caused by improper pinion contact pattern on the ring gear. Like it is contacting the toe or the heel of the gear tooth first. It is supposed to make contact in the middle of the tooth, then it prob wouldn't whine. A good gear set up mechanic like Supershafts could fix it I'm sure. The next question would be is it worth it? For me, I can live with the whining sound. Seems to be loudest about 40 to 60 MPH, then pretty much fades away or gets drowned out by other road noise. I put many miles on the one back in 1972 to 1974, and no other probs came up. Good luck.

 

[Kern Dog]

Thanks Supershafts!
Yeah Gary I have the same symptoms you describe. My 3rd member is a '489 case. On light accel or WOT they make noise at say 50 and up. Coast or decell they are as silent as can be. I've tried over the last few years to make this car a better street car and a noisy axle is a bit annoying.
I do have a Richmond 3.91 ring and pinion as well as 2 other 3.91 3rd members.

 

[Supershafts]

The set up is wrong, the pin is running off the face

 

[Garys1969RR]

Thanks Supershafts!
Yeah Gary I have the same symptoms you describe. My 3rd member is a '489 case. On light accel or WOT they make noise at say 50 and up. Coast or decell they are as silent as can be. I've tried over the last few years to make this car a better street car and a noisy axle is a bit annoying.
I do have a Richmond 3.91 ring and pinion as well as 2 other 3.91 3rd members.

You could always take one of your spares to a gear set up specialist. Have him check the contact pattern. They put some kind of marking grease on the gears, and turn it thru under a slight load, and a contact pattern is transferred to the ring gear. Then they know what to adjust after that. The factory service manual explains it, but you have to have training to really understand it. A good gear mechanic can set it up and should guarantee it not to whine.

- - - Updated - - -

I have set the backlash on a few of them to the factory minimum tolerance of I think its .006 to .008. This can elimate the "clunk" sound heard when you go from reverse to drive.But you have to set the carrier bearing pre-load at the same time. But this wont make the whine go away.

 

[sleepar]

If it's got the crush sleeve ( 489) by nature they can loosen up under a lot of abuse the other kind 741 742 have shims that have to be stacked to set that pinion in the ring gear.... if it just got noisy and there's no twine bound in behind the pinion flange I would pull it out and have it gone through I've seen them gun metal blue and missing teeth and not be real noisy... at night after a night of burning rubber till the tires blew my 8 3/4 would glow a pumpkin color and you could hear the 90w boiling

 

[Supershafts]

Crush collars CAN NOT loosen up... If a crush collar is loose then the bearings are JUNK, the only way for a crush collar to get loose is the bearings and race wear... You can not crush it further over time driving it

 

[sleepar]

Bearings toast then.... pull the shaft and see if the pinion has got any play.... don't drive it till you get it indexed or on the way there..... There is a chart and a marking clay like plastic gauge and a fairly wide range.. you need a spanner fork tool and another exact same 3rd member to judge a feel from and a junk pinion bearing reamed out so it slips on to gauge the shims with out pressing it on and off a dozen times on the 741 742 early ones and a crush sleeve and 2 men and a boy on a tork wrench for the 489 one... the only diff in the diffs is the sm shim large shim and crush sleeve and the open axles take a different carrier bearing than the clutch and cone type limited slips are the same and Dr. Diff will want to sell you reconditioned bearings as new the first time around..... and under extreme load you can distort the crush sleeve.... I don't know the power you're running but a dyna 60 will cure you're problems with the 8 3/4 if your smoking them

 

[Kern Dog]

I appreciate the suggestion but a Dana 60 is overkill for me. I'm a cornering guy and do not need the added girth or strength. My tires go up in smoke as is too. The axle rarely sees all of the power that I'm making.