Help with rear end noise diagnosis

[steve91tt]

I am trying to diagnose a rear end noise in a 1971 240Z that I am putting back on the road / track after sitting since 1989. Here are the facts...

1. There is a significant noise coming from the back end. It seems to change pitch slightly with speed. It only occurs when accelerating and decelerating but nothing while coasting.

2. The noise does not occur with the clutch in when the engine is rev'ed.

3. I can't feel or hear anything wrong when I jack up the back end and spin either tire.

4. Rear brakes are freshly rebuilt.

5. I pulled the differential, opened the cover, replaced the oil. No excessive metal shavings, nothing looked out of place.

6. The driiver's side rear wheel bearing was bad. I replaced it.

7. Stock R180 with new Wolf Creek CV joints. CV joints feel very smooth and have little play.

The noise it fairly loud so I want to make sure I fix what ever is wrong before something let's go and leaves me stranded on the side of the road. Any ideas of where to start or what to test?

[240ZGL]

Pinion Bearing

[Arne]

You mention a slight change with speed. Is the change road-speed linked, or RPM linked? The reason I ask is that I chased a noise that I was absolutely certain was in the rear end, but it turned out to be in the transmission instead. Noise seems to travel oddly in these cars.

[steve91tt]

The noise occurs in all gears and seems to be more related to speed than RPM. I would say the frequency of the noise changes with speed but not as much as I have heard in the past with parts rubbing on other parts for example. I guess that still does not tell me if I am looking at a transmission or diff issue. Any ideas on how to diagnose which it is? I would rather gather as much data as possible before throwing money at it.

[jmortensen]

I would also suspect pinion bearings, although the main shaft for the trans spins the same rpm as the diff, so it's possible you have a problem there as well. You might be able to pull the shift boot and use a mechanics stethoscope on the back of the trans housing to see if it is the trans, and if not, then you can be pretty sure it's the diff.

It's usually cheaper to find a new diff than repair one. You might see if you can pick up an R180 for $50, throw it in and see if the noise is still there. If you're wanting something more track oriented, you might look for the K-R180 from the newer Nissans that has 4.11 gears. John Hines provided pics of how to switch it to bolt in axles in this thread:

http://forums.hybridz.org/index.php/topic/49194-differential-cv-lsd-hp-torque-r160-r180-r200-r230-diff-mount/

[steve91tt]

4.11 gears may be an issue with my current setup. I have a 4 speed and smaller rolling radius track tires. When I use this setup on my other car I'm close to running out of RPM's on the long straights. I suspect that going with 4.11's will make this a bigger issue. I might be able to get away with it with the stock motor but eventually I will move my 3.0 liter motor to the track car. In other words, I'd rather stick with the 3.54 gear ratio for now.

Anyone got an R180 they want to sell?

[steve91tt]

I put a few more miles on the car tonight. Arne, I'm thinking you may be right. It could be the transmission as I was unable to get it to make noise in 1st gear. 2nd, 3rd and 4th but not 1st. Does that make sense?

[az240z]

I have both trannys and rear ends available.

[Arne]

Not as much sense as I would have hoped for, no. Fourth gear is direct drive, and there's no load on the countershaft. First through third (and reverse) all load the countershaft. So I wouldn't have been at all surprised had you said no noise in fourth, but noisy in all other gears. But quiet in first only? That is a bit tougher to figure. I'm going to need to stew on that a bit.

[jmortensen]

Could be that you just aren't going fast enough for the diff to start whining.

[steve91tt]

I decided to start with the transmission. I dropped the stock 4 speed out of my 1973 and swapped it for the one in the 1971 that I have been working on. The swap went without issue. I'm getting good at this, I completed everything before noon! Bad news, nothing changed. Next up...diff swap.

[steve91tt]

Swapped diff's with my street car. Noise is gone now. I guess I'm looking for a new diff. I'm thinking an LSD on the track car would be the way to go. Anyone got one for sale?