Matt trans am

So I have an 85 trans am. Just bought a 95 firebird. Has a 6 speed lt1 and 4.11 rear. Awesome running car but got a smoking deal as parts car. So the original 3.42 gears were replaced for 4.11 gears in the rear. They were the correct gears for that carrier. So problem is when off throttle there is a loud humming coming from the rear end. When on throttle and accelerating it sounds perfect. Anyone have any ideas as to what the issue might be. Also I did confirm posi additive was put in. Thanks for the help

Matt trans am

And also all bearings were changed

mdtoren

Have you checked the fluid level in your diff? sounds like its either low on fluid or too much play in the gears - likely not pre-loaded properly on install, pretty common cause.

Matt trans am

Guy I bought it from admitted it probably wasn't adjusted properly. Any chance this would've caused any damage to the gears?

mdtoren

Also, if coasting and you ease the wheel to the right and left just swerving down the road, is there any variance in the noise? If it changes at all - either what it sounds like, or if it goes away, you have wheel bearings going bad. Alot of people confuse bearing noise for diff noise

Matt trans am

No it doesn't matter it's just as soon as I let off the gas and coast it's immediate.

mdtoren

Yes, if not properly adjusted there will be too much play in the gears, you dont hear it on acceleration because your literally forcing the gears together with torque and its constantly applying the force to the diff gears. Once you let off that noise you hear is the teeth of one gear hitting / banging into the teeth of another because theres no force behind it. If I were you, I'd have someone check it, and if there hasnt been any damage yet, have it properly preloaded and change the fluid out.

In the mean time, if you have to drive it, I wouldnt beat on it badly. You run a bigger chance of breaking a gear tooth.

big gear head

If the pinion depth, bearing preload and backlash are not set correctly to get the correct contact pattern the gears will make noise and possibly fail completely eventually. It is VERY important to set them up correctly. At this point there is probably nothing that can be done to correct this except to replace the gears. Once a set of gears has been run for a while they wear and can not be set up again.

sofakingdom

iTrader: (1)

Wouldn't be surprised if the crush sleeve isn't crushed all the way, and when in coast, the pinion gear is getting pulled into the pumpkin far enough (only takes a small movement) to where it hits the carrier. Might even be just the pinion nut loose but that's not as likely on a more or less "fresh" rebuild; not as likely as improper assy, for sure.

Pop the cover off and see if there's marks on the carrier from that. If so, the whole thing needs to be torn down and cleaned up (metal chips removed from EVERYWHERE... axle tubes, interior of carrier, etc.) and put back together CORRECTLY. All new bearings of course since they're going to be full of chips as well. Gears are probably already destroyed on the coast side from the severe misalignment.

BGH, I just passed through your town acoupla weeks ago... had to go to the hospital in Greenville...

big gear head

How did you wind up here? You must have got lost going somewhere else. Hope you didn't have something serious. We don't even go to the Greenville hospital unless it's something very minor.

sofakingdom

iTrader: (1)

No actually, I intended to go there. They purchased some equipment upgrades from us. Didn't seem too bad for a small-town hospital, from what I could see; I've seen ALOT worse, even in some major cities.

Matt trans am

So would a good mechanic shop be able to fix it? Is there any chance it won't be destroyed. The rear hasn't even run a couple 1000 kms

sofakingdom

iTrader: (1)

Depends; needs to be somebody good with setting up gears. Not to be found in just "any" shop. But, given that, of course; unless the housing is damaged somehow.

Of course, there's always a chance. There's a chance the aliens will land in my front yard this afternoon too; just not a very big one. (although I wish they would, I'm ready to go back home) You're in a similar situation: yeah there's a chance, but if I were the betting kind, I wouldn't be betting on it, even if it's what I "want" it to be. But it's not a big deal, gears are only about $200, whereas you're going to be into it for A MINIMUM of around $400 in labor and "soft" parts. (bearings etc.) I'd recommend expecting to replace them, especially since if you don't but they're too damaged to work well, you'll be RIGHT BACK AT THE SHOP in short order anyway, getting ready to bust out ANOTHER $400, plus the gears. Better to do it RIGHT, ONCE, than to half-a$$ it and have it fail AGAIN.

The carrier is probably an Auburn and is probably mostly used up as well. That will impact the correct decision.

Posting on the Internet, speculating about stuff, is pretty worthless. Pop the cover off and see what's in there. Not much point in a bunch of "maybe it's this maybe it's that" when it's SO EEEEEEEEZY to actually find out what's wrong. All ya gotta do is look.

big gear head

It's east to check the pinion nut and it's also easy to tell if the crush spacer was crushed correctly. Get under the car and hold on to the yoke. See if it will move in and out or up and down. Any movement other than rotation is not good. Even the very slightest movement in and out or up and down is bad. Usually this can be corrected by tightening the pinion nut to get the correct preload on the bearings.