Rear Diff Issues

[Gary L]

This is on my 76 280Z 2+2 with about 85K miles. The rear diff is noisy, whining on acceleration and coasting, but there isn't any clunking.  So I'm trying to decide whether I'll pull it and have it rebuilt or buy an already rebuilt one. According to the service manual it is a 3.54 ratio diff, but I don't know if it is open or a LSD. When I raise the rear end and spin one of the tires, the other doesn't move, I thought that if it was an open diff the opposite tire would rotate in the opposite direction and if it was a LSD they would rotate in the same direction. When I spin a rear tire, why doesn't the other one move? Could it be a LSD diff and the clutch plates or something in my diff carrier are shot?

Thanks

Gary

[Zed Head]

If the transmission is in neutral the drive shaft can spin instead.  A stock 1976 car wouldn't have an LSD.  The diffs are generally whiny and all it takes is some solid contact between metal parts, or compressed old rubber, to transmit the whine to the cabin.  People don't generally rebuild these diffs because they last a long time and you can find used units pretty easily.  Your best shot at getting rid of the whine is to check all of the mounting points for looseness and wear and replacing them.  Have you looked at them?

[Gary L]

Yes, the trany was in neutral. But if I put it in gear I won't be able to spin a wheel ? Or should I leave it in neutral and have someone hold the drive shaft from spinning? Obviously I don't know much about differentials But you say a stock 76 would have an open diff.

I have replaced most of the bushings including the mustash bar bushings and trany and diff mounts, except I haven't replaced the front and rear lower inner control arm bushings which I do plan on doing.

I'd rather not spend the money for a rebuild if not necessary. Like I mentioned it doesn't clunk or anything..I read quite a few posts where people have them rebuilt and they still whine. From old road tests I've read, they didn't come from the factory with noisy diffs. So I should probably check the last of the bushings I haven't replaced and try it again?

Gary

 

[Zed Head]

7 minutes ago, Gary L said:

I have replaced most of the bushings including the mustash bar bushings and trany and diff mounts, except I haven't replaced the front and rear lower inner control arm bushings which I do plan on doing.

Did you replace them with polyurethane or stock rubber?  The PU bushings can be tight and cause noise.

[djwarner]

The transmission should be in first or reverse if you want to check the differential action. This locks the drive shaft and the opposite wheel turns in the opposite direction at the same speed. Obviously both rear tires are off the ground when you do this. Duplicate the test with the other rear tire.

Clunks are normally caused by bad U-joints, but this is not the problem you describe.

Check for free play by seeing how far you can turn one rear tire before the other one begins to move. You can also get an idea of dragging brakes, or warped drums which can also cause rear end noise.

All that said, I would question the differential gear oil level first. Second is the gear oil itself. I would use a synthetic oil with an EP rating. EP oils act like a liquid until a thin film is subject to high loading as between the gear tooth surfaces when torque is being transmitted. At that point the oil stops acting like a liquid and acts like a solid as long as pressure is applied.

EP oils will run quieter because the metal parts never actually come into direct contact. 

 

[Gary L]

I used stock rubber for the ones I replaced, not poly.

I did go back out to the garage and put it in first and when I turned one rear wheel the other did turn in the opposite direction - open diff (I mentioned I didn't know much about rear diffs!). The wheels seemed to spin smooth and quite. I didn't actually note the amount of play before the other wheel starts to turn. And I not sure how much would be considered excessive.

I've been using Amsoil GL5 75w-140 synthetic in it. Once again I have to ask a stupid question; whats EP oil? Is the amsoil EP rated?

 

Thanks

Gary

 

[djwarner]

Hi Gary, 

EP stands for extreme pressure denotes a quality build into the oil. It will specify EP on the label. Don't know about Amsoil in particular, but you can find an EP gear oil in any auto parts store.

Drain a warm differential from the bottom plug and put one quart in through the top port. Use the whole quart and don't bother to measure the level as you would in a transmission.

Edited January 1, 2016 by djwarner

[Zed Head]

I think that the oil companies use EP additives to meet the GL5 spec.  Which is what the differentials require.

Sounds like you're doing everything right.  Other things that will help or hurt the noise level are the padding and/or carpeting in the back deck area..  I slid some of that silver foil insulation bubble wrappy material under my carpet in the back and it made a noticeable difference.  If you have no carpet or thin carpet, throw some more material back there.

Your description is a little different than many though.  Mine will whine or howl, actually more of a warble, at certain constant freeway speeds but is quiet at acceleration and deceleration.  So maybe you do have a differential wear problem.  If it were mine though, I would buy a used one and try it first.  If it doesn't work, at least you'll be able to drive while you get one fixed.  I have several used differentials and they all sound about the same.  They're pretty durable.

 

[Gary L]

Thanks guys, lots of good info as I plan my winter projects. Unfortunately, being I'm buried in snow, I won't be able to test drive it until spring. I do plan on doing some more insulating under the carpet and in a few weeks I'll probably pull the diff and take the cover off to see if there is anything obvious with the tooth pattern, etc.  Gary

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[djwarner]

As I understand it, the UP has three seasons, July, August and Winter.

[Gary L]

Yup, you got it. But now that I'm newly retired, I'll be free to take off when and to where I want. Another reason to get the Z running really well.

[djwarner]

I'm in central Florida, at 1:30 PM it is 82 and will get down to 65 tonight.