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Old 09-07-2017, 12:29 PM   #1
Whaleman
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Default Differential experts Please

I have been chasing a clunk in my 96 base auto rear end. Happens on the 2-3 upshift. I changed the differential fluid again with the proper additive. No chattering at all. Finally gave up and took to a good mechanic but not a corvette expert. He quickly found that it has excessive ring and pinion backlash. It was not the worst he had ever seen. With the car on a lift where the rear wheels can not move and the car in neutral he could turn the driveshaft back and forth maybe 3-4-5 degrees. Each time there was a small clunk in the differential. He said that the 1-2 shift does not have a torque lag but the 2-3 shift requires 1 clutch in the transmission to disengage and another clutch to engage and the momentary torque lag is what allows this backlash to clunk. Is this something that will get worse? Does it need fixed right away? What does this repair entail? Thank You All, Dan
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Old 09-07-2017, 01:59 PM   #2
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The only way to check ring & pinion backlash properly is on a work bench. If no gear noise on acceleration or deceleration the gears are OK. Any banging while cornering = posi plate noise.

The play you have is "stack up tolerance", it is the combined clearance of all components from the companion flange to the wheels & is 100% normal.

Suggest inspecting the dog bone bushings any cracked, swollen rubber, evidence of rust = replace them.
Also check the torque arm bolts they need to be TIGHT.

Good luck.

BTW I'm not buying the "torque lag" theory.
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Old 09-07-2017, 06:48 PM   #3
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Churchkey, Thank you for your response. I went out and checked the two thinks you mentioned. They both seem fine. Car only has 30K garage kept miles. Placed a call today to a mechanic that was the top corvette mechanic at the largest local chevy dealership for 20 years and is retired but still works out of his home garage. Dan
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Old 09-11-2017, 07:31 PM   #4
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Hard to believe you could have excessive wear on anything with only 30k miles on it
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Old 09-11-2017, 07:34 PM   #5
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Quote:
Originally Posted by Whaleman View Post
I have been chasing a clunk in my 96 base auto rear end. Happens on the 2-3 upshift. I changed the differential fluid again with the proper additive. No chattering at all. Finally gave up and took to a good mechanic but not a corvette expert. He quickly found that it has excessive ring and pinion backlash. It was not the worst he had ever seen. With the car on a lift where the rear wheels can not move and the car in neutral he could turn the driveshaft back and forth maybe 3-4-5 degrees. Each time there was a small clunk in the differential. He said that the 1-2 shift does not have a torque lag but the 2-3 shift requires 1 clutch in the transmission to disengage and another clutch to engage and the momentary torque lag is what allows this backlash to clunk. Is this something that will get worse? Does it need fixed right away? What does this repair entail? Thank You All, Dan

i had same problem on 89. Turned out to be torsion bar bushing worn
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Old 09-11-2017, 07:56 PM   #6
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i had same problem on 89. Turned out to be torsion bar bushing worn
Probably a sway bar bushing. Torsion bars were used in the Mopars of yesteryear.

Last edited by 383vett; 09-11-2017 at 08:11 PM.
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Old 09-11-2017, 10:22 PM   #7
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Probably a sway bar bushing. Torsion bars were used in the Mopars of yesteryear.

you are correct, I should have said sway bar bushings
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Old 09-11-2017, 10:26 PM   #8
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I have heard a lot of loose differentials make noise. But never on one upshift and not on the garage shift.

IMVHO, I think he's wrong.
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Old 09-12-2017, 10:37 PM   #9
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Any repairs prior to clunk. not sure what 3-4-5 means. Normally if there's all of a sudden excessive backlash; there will be whining present. It is possible for drive shaft yoke to bind on output shaft splines. Synthetic grease applied to yoke splines trends to correct it.
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Old 09-13-2017, 08:46 AM   #10
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Quote:
Originally Posted by mlm0 View Post
i had same problem on 89. Turned out to be torsion bar bushing worn
Were you able to see or feel the worn bushing without taking it apart? All my bushings seem fine just looking at them. Thanks,Dan
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Old 09-13-2017, 05:36 PM   #11
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I couldn't tell with with car weight on them, so I just replaced them and fixed the problem.

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Were you able to see or feel the worn bushing without taking it apart? All my bushings seem fine just looking at them. Thanks,Dan
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Old 09-13-2017, 06:35 PM   #12
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i had same problem on 89. Turned out to be torsion bar bushing worn
Thank You. The end link bushings do not look new. The big frame mount bushings do look new and seem to have no movement at all being pried with a bar. I ordered a new set of both and I sure hope that is it. They did not seem to have enough movement to make the clunk I have but I will sure take you advice and change them. Dan
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Old 09-16-2017, 06:57 PM   #13
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Just my $0.02 - but when someone tells me that I have "slop" in the drivetrain - I tend to think U-Joints - not excessive gear lash in the ring and pinion. You have a Dana 36 - which while not as tough as the Dana 44 used in the stick cars - is a pretty decent piece that rarely has problems with tock power levels - even at 3 - 4 times the mileage that you have on your car.

One relatively easy way to check - Jack the rear of the car up, and get the back wheels off the ground. Start the car - and have your friend stand on the brakes. Then have the friend shift from reverse to drive while you watch how much the driveshaft moves (you want to look where the driveshaft enters the diff (marking the driveshaft with a piece of chalk will help).

Typically a properly set up diff has on the order of 0.008 - 0.012 of backlash when measured on the outer edge of the ring gear.

If you see the drivehaft turn more than a little bit - repeat while looking at each axle shaft. (The U-Joints on the axles may spin slower than the driveshaft U-Joints - but they take a lot more torque than the ones on the driveshaft do).

I agree with the other posters - that a worn bushing is the more likely cause for what you're seeing - but it would be good to rule out the U-Joints.
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Old 09-16-2017, 07:27 PM   #14
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Just my $0.02 - but when someone tells me that I have "slop" in the drivetrain - I tend to think U-Joints - not excessive gear lash in the ring and pinion. You have a Dana 36 - which while not as tough as the Dana 44 used in the stick cars - is a pretty decent piece that rarely has problems with tock power levels - even at 3 - 4 times the mileage that you have on your car.

One relatively easy way to check - Jack the rear of the car up, and get the back wheels off the ground. Start the car - and have your friend stand on the brakes. Then have the friend shift from reverse to drive while you watch how much the driveshaft moves (you want to look where the driveshaft enters the diff (marking the driveshaft with a piece of chalk will help).

Typically a properly set up diff has on the order of 0.008 - 0.012 of backlash when measured on the outer edge of the ring gear.

If you see the drivehaft turn more than a little bit - repeat while looking at each axle shaft. (The U-Joints on the axles may spin slower than the driveshaft U-Joints - but they take a lot more torque than the ones on the driveshaft do).

I agree with the other posters - that a worn bushing is the more likely cause for what you're seeing - but it would be good to rule out the U-Joints.
Thank you for responding. I have new sway bar bushings coming very soon. After I install those if I still have the problem I will do the exact test you suggest. Since you seem to be very knowledgeable I replaced all four u-joints and both wheel bearings 900 miles ago. The mechanic who helped me did not want to lube (grease) the splines on the new bearing and teflon washer. Can this be a problem? Dan
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Old 09-19-2017, 06:13 PM   #15
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I'm hardly an expert on diffs - but I do know that the rear diffs on the C4's are normally not problematic until dragstrip launches with slicks become part of the equation... Search on the term Dana 36 and you will see lots of posts - but if you read the threads - most time there are problems - they are being asked to handle FAR more loading than the factory designed them to deal with

As far as your question - Truthfully - when you install the new rear bearings - the bearing typically slides over the splines fairly easily. While some people feel grease is necessary - I think it's optional - I doubt that a little will hurt anything but I don't see it helping very much either (very little relative motion between the stub shaft and the splined hub under anything resembling normal use. Truthfully - I see the biggest advantage in greasing the splines as far less potential for corrosion - meaning easier disassembly next time (and don't rule out a next time)....

So - if the U Joints are 900 miles old - it's HIGHLY unlikely that they're worn. It's always possible that you got a defective U-Joint - but it's pretty unlikely.

So - if the end link bushings solve the problem - great - if not - look for another bad bushing ...
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Old 09-19-2017, 07:22 PM   #16
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Have you wiggled the pinion (drive shaft)? or one of the axle driveshaft? If there is, you may have a bad bearing. I would rip apart the diff any ways. Probably time to change the fluid anyways. I removed my rear diff in an afternoon. It wasn't all that bad of job. My rear diff was leaky.

I have 85 vette 2.73 dana 36. I've never rebuilt a dana 36 or dana 44, but a few dana 70/80 rear diffs. Most dana's use shims, so there is no adjusters to turn.

To remove the dana 36/44 you need to remove the c-beam, rear drive shaft, both drive propeller drive shafts. Mine came apart pretty easy.

I used air/electric impact tools and four jack stands. Pretty easy repair.

I can easily see a rear end fairly at 30000 miles if there is no fluid in the diff. Best of luck.
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