Diff issue, vibration, several questions

[Richard Oben]

Finally have had 9070 (2400 automatic) on the road for about a year. Enjoying it more and more as I work some of the bugs out.

The worst bug is a vibration. On applied throttle is horrendous, at regular cruise it is not bad but add any throttle and it shakes pretty hard. We checked everything we could think of and finally pulled the whole center section (diff) out of the parts car, put it in and it was a LOT better. I still have some but it is drivable now, before it was just plain scary.

All the mounts, all the U joints, everything is new. I mean everything but the center section, we looked it over and put seals in it.

My questions are as follows:

What is causing the vibration? Why would a used diff fix it partially?

Is some applied throttle vibration common? The early cars have the forward mounted diff and I understand some vibration is common. I have had 4 other Z cars but do not remember the vibration being in any of the others.

Second is there someone that I can send the diff (whole third member) too that can redo it completely. The reason I ask is it feels the same as the replacement diff but the vibration is much less in the replacement. It is easy enough to change (have a lift) that I would like to get the original back in but fully corrected.

Are the parts available to rebuild the diff in the first place?

Any insights would be appreciated. As I have said before this forum is invaluable when it comes to fixing an early Z. Thanks Richard.

[djwarner]

There was a Service Bulletin TS71-18 issued 3/25/71 covering vibrations in automatic transmission vehicles. It was retrofitted on a "customer complaint basis".

Besides checking basics like wheel balance and alignment, it specified the installation of a rubber mass behind the differential. I have no idea where you find one today beside a salvage yard with a 71 automatic made after that date.

[Richard Oben]

I have the rubber mass behind the differential, sorry forgot to mention that, it hangs on the end of the diff at the mustache bar. Thanks Richard.

[Diseazd]

When I swapped out my 72 from auto to manual, there was a spacer between the tranny mount and tranny. Make sure your mount has this spacer.....it makes all the difference in alignment of the drive shaft.

[jmortensen]

On a 70 you can shim between the trans mount and the body to change the driveshaft angle. I would look into that and verify that the driveshaft angles are parallel. There are threads on hybridz.org on how to use laser pointers to measure the driveshaft angles more precisely, but a simple angle gauge also works. I have this one from Harbor Freight, but there are others: Digital Angle Finder / Angle Gauge

BTW, that image that Diseazd posted is wrong. You don't want the driveshaft perfectly in line as this prevents the bearings in the U-joints from moving. If they don't spin then you end up running on the same part of the bearing over and over and it fails. There should be an angle and it should be parallel.

There was another issue with the early cars, and that was the diff placement. Early cars had the diff forward and the halfshafts angled back to the wheels. After mid-71 they moved the diff back so that the halfshafts went straight out to the wheels. If you don't have the diff moved back, that's another possible solution.

Edited February 19, 2014 by jmortensen

[Zed Head]

On applied throttle is horrendous, at regular cruise it is not bad but add any throttle and it shakes pretty hard. We checked everything we could think of

All the mounts, all the U joints, everything is new. I mean everything but the center section, we looked it over and put seals in it.

What is causing the vibration? Why would a used diff fix it partially?

Is some applied throttle vibration common?

Vibration isn't common in a car with everything in good shape and the engine running right. It would help your case if you listed specifically the parts you checked and or replaced. Even better, how they were checked. "Everything" to someone who doesn't know where everything is would mean some areas might have been missed.

One possible reason that things got better when you changed the diff is that things ended up in a different place when put back together. Worn bushings and worn half-shaft u-joints come to mind.

Also, is the engine running well? A poor running engine can cruise at part-throttle just fine, but shake badly when the gs is given.

I don't think that I've read of a case where a bad diff was causing vibration alone. And these Nissan diffs seem pretty tough anyway. It's probably not the diff.

[Richard Oben]

All the bushings are new, the half-shafts have been rebuilt as well as the driveshaft, both done professionally and balanced. The old diff feels a little looser than the new one. Both are over 100K.

The car pulls hard and seems to run fine at all speeds. Only vibrates at that speed and on throttle, it is not near as bad now as it was but I would like to find out what it is. I do agree we may have put stuff back different than the first time. Should I check the balance on the driveshaft and the half shafts again?

Here is an iPhone pic of the car. Everything is as new as I could make it. All original sheet metal except the hood.

[madkaw]

Maybe I missed it, but did you replave your tranny mount, or atleast inspect it.

Like said, driveline vibrations are not uncommon on these cars. I have fought one for the longest time now, but my car is a hybrid, so lots of mixing parts.

I just ordered the mentioned measuring tool to recheck my stuff.

[Mark Maras]

Is the vibration relatable to wheel speed or drive shaft speed? Drive shaft speed is 3.5-4 times faster than wheel speed. In my experience, drive line vibration will sound and feel like an 800 lb. bumblebee attached to the car. Wheel vibration is much slower. You've felt it in an out of balance tire about 70mph. Half-shafts are wheel speed. Most high speed vibes will be found at the front of the drive-shaft or rear of the tranny. Mark in Portland

[Zed Head]

MM makes a great point. Standard noise/vibration troubleshooting is to separate engine from transmission from diff from wheels, etc. Shift through the gears (even with an automatic), speed up, slow down, apply brakes while the vibration is happening... If you can isolate to a more specific set of conditions things will make more sense.

Also, before and after is always a good clue. Don't assume that "new" is better. Don't assume that professionals know what they're doing, the word just means they make money doing it. Don't assume that an experienced pro with new cars knows anything about these old cars. Just saying, we all tend to assume that something will be better after it's "fixed", but it's not guaranteed.

Did you ever drive the car before all of the new stuff was installed?

[Mark Maras]

Years ago I fought a high speed vibration in my 71. After replacing drive shaft & half-shaft u-joints as well as all the mounts, I finally took my drive shaft in to check the balance. Sure enough it was out of balance & ever so slightly bent. A retube & rebalance fixed it all. They couldn't balance the shaft with the cheap u-joints I installed. They had to installed Spicer u-joints to be able to balance it. No more vibration after that.

Mark in Portland

[Richard Oben]

I will pull consider pulling the shafts and drive shaft and see what they balance out like. When I say pro, I mean guys who do nothing but drive shaft stuff all day long. I have sent them a ton of business over the years and they are really good, but everyone makes mistakes. I may pull the shaft out of the donor car and put it in, are the auto and 4 speed the same drive shaft?