drivetrain hum / whine - PeachParts Mercedes-Benz Forum

#1 10-24-2005, 08:12 PM

Until yesterday, I had a vibration at 65 mph and above. After reading up on the subject, I suspected the flex discs and center bearing. Turns out the front flex disc had a crack between two of the bolt holes, and the center support was dried out, weak and cracked. I replaced both parts, put it back together, and went for a test drive. Now, no vibration at all even up to 90 mph. However, starting at about 60 mph there is a hum or whine. It gets louder when I hit the gas. Any ideas? I tried to mark and realign the driveshafts as best I could, but maybe I was off by one tooth on the spline...would this cause the noise?

Rich Mayer
82 500 SL

t walgamuth · #2 10-24-2005, 10:10 PM

be the diff. probably was there before but masked by the other sounds.

tom w

#3 10-25-2005, 06:26 AM

thanks for the response. the noise is loud enough that I doubt the earlier vibration could have masked it. regarding the diff, is the diff. flange supposed to line up with the trans flange in any way or are they independent? I am just wondering if I rotated one or the other by 1/3 while the driveshafts were out. would that make a difference? also, is it possible that the whine could be caused by overtightening the flange bolts on both ends or the bolts for the center bearing carrier? I tightened everything by the book, but they seemed not to be very tight, so i gave them an extra turn.

Does anyone know for sure what the sound would be like if the driveshafts were not properly aligned?

Rich Mayer
82 500 SL

andmoon · #4 10-25-2005, 06:52 AM

Could it be motor and tranny mounts letting the parts sit off center enough to cause the noise?
I had a hum at 60 in my Subaru that I couldn't track until it was put on a rack (4 jackstands actually but that's not what I can suggest

) and driven. Stethescope wasn't needed.

#5 10-25-2005, 09:44 AM

motor and trans mounts are new, and the car now drives very smoothly at high speed.

Rich Mayer
82 500 SL

Bruno_300TE · #6 10-25-2005, 10:10 AM

Quote:

Originally Posted by diqmayer

I tightened everything by the book, but they seemed not to be very tight, so i gave them an extra turn.

What about the large nut that at the center junction of driveshaft? If this one is too tight the driveshaft cannot accomodate in length while driving. This could lead to a hum. I think the book asks for 30-40 Nm which is not very much for a 46mm nut. (Personally, regarding torque specifications I'd stick to the book values anyway.)
Regards, Bruno

104.992 · #7 10-25-2005, 10:30 AM

So Rich,
The did the replacement of the motor and tran mounts solve the hum or whine at 60mph?

#8 10-25-2005, 04:23 PM

OK, Bruno, you got me interested there...of course, I did not have a torque wrench when I tightened that one. I tightened it pretty snugly with large adjustable pliers. Are you saying that it is supposed to move some as the driveshafts turn? Now that I think of it, this large nut was very loose when I dis-assembled the driveshafts. In fact, I removed it by hand. Why would the driveshafts need to move forward/aft as rpm's increase?

As to the engine and trans mounts, I replaced them all before this episode even started. All of my rubber was original when I bought the car from hot, dry Texas a few years ago. I have systematically replaced a lot of rubber. Before I did the mounts, the engine was in contact with the steering box, causing some noise problems there. Replacing the rubber has made an incredible difference in how the car runs and handles.

PS I just went out and removed the forward drive shaft and flex disc to see if the shafts were aligned. It seems that they were correct.

Rich Mayer
82 500 SL

Bruno_300TE · #9 10-25-2005, 05:51 PM

Quote:

Originally Posted by diqmayer

I tightened it pretty snugly with large adjustable pliers. Are you saying that it is supposed to move some as the driveshafts turn? Now that I think of it, this large nut was very loose when I dis-assembled the driveshafts. In fact, I removed it by hand. Why would the driveshafts need to move forward/aft as rpm's increase?

Rich
I understand that the driveshaft will need to adjust slightly in length in order to be stress free. I hypothesize that this is because of all the rubber involved (flex discs, transmission mounts, diff mounts...). In this context it may also be worth mentioning that the center bearing bracket should be positioned and tightened only after the driveshaft sits in place without any stress.
On my car I could cure a hum (after having removed the driveshaft before) by loosening the clamping nut somewhat. Regards, Bruno

t walgamuth · #10 10-25-2005, 07:30 PM

the driveshaft you are supposed to mark everything and put it back in the same places. dont know if this would cause it or not.

tom w

Roncallo · #11 10-25-2005, 08:42 PM

When I rebuilt my drive shaft I noticed that the large nut was actually a collet type arrangement that clamped the driveshaft at one specific length and not allowing extension of the spline coupling like on a rear wheel drive American car. The differential of the SL is fixed so extension is not necessary. Therfore I tightened my nut with all other shaft hardware installed and tight and the engine and transmission mounted in place. Slight axial extension of the drive shaft due to engine torquing will be taken up by the flex couplings.

I have no whine.

John Roncallo

hanno · #12 10-25-2005, 09:02 PM

if you leave that driveshaft nut loose the splines will wear resulting in purchase of your future new driveshaft. It allows the shaft to be shortened for replacement of flex discs and its removal (DB didn't see the need to remove the differential just to access the driveshaft). After new discs, center bearing, etc. you rotate shaft a few times to allow final length set, then tightening the large nut is the last thing you do. Nothing moves enough to need a domestic type sliding spline. I just did this on newly aquired 280SL, had a whine/noise so bad I thought the differential wouldn't make it the 1600 miles home. Fluid checked fine, but found the worst flex disc I ever saw (almost metal to metal on the yoke mounts). Since it was only a flex disc, opened window, didn't hear it as bad, and motored home. Replacement of both discs and tightening the center nut (found loose), plus rear subframe mounts eliminated almost all noise and play in the drivetrain.

Bruno_300TE · #13 10-25-2005, 09:17 PM

I do not recommend to have the nut loose but to use the proper torque of 30-40 Nm (book value) which is relatively little for a 46mm nut (think of the long lever one normally uses to tighten such nuts... I am sure "tight" is way more than 30-40Nm).
Regards, Bruno

#14 10-26-2005, 08:19 PM

The hum is gone. I got back under there and removed and reset the front driveshaft. My best guess is that in my first attempt I tightened down the 46mm nut before the driveshaft reached its equilibrium length(or "settled" if that's the term), somehow causing it to bind along the way. Anyway, after resetting it, I drove at high speeds today and the car was very smooth and fun to drive. Thanks for all your comments.

Rich Mayer
82 500 SL