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740 Driveline vibration from 35-60 mph

oifish

oifish

I'm feeling extra zazzzy
Joined
Jul 30, 2014
Location
Indiana
Car is the 740 in my sig. I recently replaced the u-joints in my driveshaft with Moog components. After reinstalling it, there was a bad vibration above ~20-30 mph. Being fed up I took the driveshaft to a specialty shop to have them make sure I did the u-joints correctly and balance it.

They replaced the central bearing and supposedly balanced the driveshaft. I installed it today and thought everything was OK. Nope, starting at 35 mph a vibration starts and gets really rough in the low 50's. Once above 60 it somewhat smooths out. The vibration persists with the car in neutral coasting at those same speeds.

I never separated the 2 halves while installing it, and I mads sure (at least I think so) that central bearing was aligned correctly on its mounting plate.

The only thing I didn't double check was that it attached to the diff in the same orientation it was in before originally removing it. My next step is to flip that attachment 180* and see if any thing changes. Besides that I'm at a loss on where to look next.
 
shoestring said:
Did you do anything else at the same time, because this sounds like a tire vibration.
Click to expand...

I did put on my summer tires, but they've been balanced and I've never had problems with them before. I'll take the rear wheels off with the car jacked up and run it up to 45.

VB242 said:
Did you make sure they put the 2 pieces of driveshaft back together in the correct orientation? Just because someone else did it, doesn't mean they did it right.
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The markings line up on the driveshaft. I guess I could flip it 180 and see if anything changes.
 
Without the wheels on the vibration is the same. I double checked the alignment of the driveshaft and all the markings are lined up. The rubber for the central bearing is new. This is weird...
 
oifish said:
Nope, starting at 35 mph a vibration starts and gets really rough in the low 50's. Once above 60 it somewhat smooths out.
Click to expand...

That's because 740s are only meant to be driven by grandpas, under 35 mph. Its like the rumble strips to tell you where the ditch starts.
 
oifish said:
Without the wheels on the vibration is the same. I double checked the alignment of the driveshaft and all the markings are lined up. The rubber for the central bearing is new. This is weird...
Click to expand...

I?ll admit, this part is gonna be unsafe, but did you watch the driveshaft as it was turning?
If it?s bouncing around, it needs rebalancing.
Another idea: unbolt the driveshaft from the transmission flange, and give the flange a wiggle. If the flange moves, you need to replace the tail shaft housing bushing.
 
How are the rear end bushings? Worn suspension bushings, and also lowering the car in the rear can cause pinion angle to change. If it isnt right you will get vibration. Adjustable torque arms are used to correct the pinion angle on a lowered car.

Bad driveshaft vibration can take out both the pinion and tailshaft bushings/bearings. Take the shaft off, and check for play in these. While you have it off, turn the yoke on the tranny and the rear end lightly and feel for the backlash. The yoke should not turn far before you feel the gears engage in either direction.

11BC2 said:
I’ll admit, this part is gonna be unsafe, but did you watch the driveshaft as it was turning?
If it’s bouncing around, it needs rebalancing.
Click to expand...
The safe way to do this is to put a magnetic base dial indicator under there measuring the runout of the shaft and flanges. Point a gopro at the dial indicator face. Mag base indicators can be bought here for under 35 bux:
https://www.shars.com/products/measuring/dial-indicators/1-dial-indicator-001-3
https://www.shars.com/products/measuring/magnetic-base-stands/magnetic-base-with-fine-adjustment
 
THere is a factory fix for this; I've done it. You Raise the center bearing closer to the body/tunnel, with spacers on the mount plate. The plate stays the same, the center bearing gets closer to the tunnel. This counterintuitively increases the angle of the two pieces. Maximum is 12 mm, I did 11 mm and it worked.
 
The bushings aren't in the greatest shape, but they are not completely blown out. I'm starting to think one half of u-joint was installed 180* in relation to the other. If that happened the 2 halves of the driveshaft would be 180 out of balance even in the marks line up, right? So I'm going to rotate the halves 180 then report back.

It just doesn't add up that the pinon or tailshaft bearing/bushings would go out when the car has been on jackstands. I will test the play in both with the shaft out.
 
oifish said:
Without the wheels on the vibration is the same. I double checked the alignment of the driveshaft and all the markings are lined up. The rubber for the central bearing is new. This is weird...
Click to expand...

Yes, new central bearing bushing... There's your problem. I just used a sharp scalpel and cut away some material from the bushing. The damn driveshaft was held too tightly in that rubber. If you look in that bushing from the back you can see how it sits against itself if its moved just a bit...

Car was droning/vibrating horribly, when i tried before on jackstands. Tried lifting the support bearing aswell, it just shifted the ranges a bit. Cut some rubber away, became silent and nice...
 
I have heard that new support bushings can be too stiff and transfer vibrations, they typically wear in and calmn things down.
 
Good to know about the new central bearing wearing in. I rotated the drive shaft 180*, but haven't tested it yet. If it's worse, I'll flip it back and just run it for a while to see if things need to calm down.
 
I had a rumble/deep bass tone in my wagon from 40mph to 55mph, I tried shimming up the center bearing, different tires, different driveshaft, different transmission and nothing worked. I ended up selling it so the mystery was never solved. I am guessing worn rear bushings may have been my problem.
 
OP, did you properly clock the drive shaft plate at the rear of your trans with the plate (and U joint) at the rear end flange? The green books show how this must be done. Basically they must be opposite......if the rear pin on the plate is horizontal, then the front must be vertical.

This clocking is separate to the center joint where you disassembled the halves..... I’ve seen some factory match marked, some not, and late versions are machined so they cannot be improperly assembled.
 
petiww said:
Yes, new central bearing bushing... There's your problem. I just used a sharp scalpel and cut away some material from the bushing. The damn driveshaft was held too tightly in that rubber. If you look in that bushing from the back you can see how it sits against itself if its moved just a bit...
Click to expand...

Hey, sorry to bother and interrupt the thread but could you explain a bit more about what/where you cut away at the bushing?
Are you talking about where the center bearing metal "dust shield" meets the rubber carrier bushing? Something else?

I ask as I noticed that the dust shield for the bearing is pressed onto the driveshaft and also essentially gets pressed into the rubber carrier. But maybe your taking about something else entirely.

Sorry again and thank you
 
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