Throbbing/Rythmic Vibrations at 40-65 MPH - PeachParts Mercedes-Benz Forum

legasea · #1 03-12-2008, 12:13 PM

I have this low level throbbing/rhythmic vibration when I am going about 40 - 65MPH. It is more like a humming than vibrating that comes and goes in rhythm at said MPH. I checked the flex discs and motor mounts yesterday which appear to have already been replaced at some point. What would the next thing to check? I have read about other culprits (centerline bearing/support, motor shocks, tranny mounts, bad axles and wheel bearings).

I am not certain where the tranny mounts are located and how to tell if the motor shocks are bad. My guess would be a bad centerline bearing/support or the wheel bearings. But I guess both of these just need to be replaced to find out if they are bad. Any thoughts or advice for troubleshooting on my own before bringing it in for service?

rrgrassi · #2 03-12-2008, 12:16 PM

Check the center support bearing and bushing. I had a truck that did this, but it was never diagnosed or found. Never had a failure either.

RichardLSU · #3 03-12-2008, 12:17 PM

My old pickup did the exact same thing. Changed the u-bolts on the driveshaft and it cleared it right up. BTW, that vibration is your driveshaft shaking around. So fix it fast!

RichardLSU · #4 03-12-2008, 12:20 PM

If your steering wheel isn't shaking then its not the bearings or unbalanced wheels. If it is, get your wheels balanced and go from there. My steering wheel was shaking and I got them balanced and it didn't really fix it. Once I got new tires I found that my belts had separated causing them to be uneven. So I guess it depends on where the vibration is coming from.

rrgrassi · #5 03-12-2008, 12:26 PM

Bad UCA bushings can cause steering wheel shake as well.

legasea · #6 03-12-2008, 01:06 PM

Quote:

Originally Posted by RichardLSU

My old pickup did the exact same thing. Changed the u-bolts on the driveshaft and it cleared it right up. BTW, that vibration is your driveshaft shaking around. So fix it fast!

Can the ubolts be replaced w/o removing the exhaust and putting the car on a lift? If so, I can most likely do this myself, if not, I will regretably have to bring her in for service. I believe the center bearing and mount need to have the exhaust removed, so maybe I will just do both of these items and call it a day.

Thanks for the advice.

Alberta Luthier · #7 03-12-2008, 01:16 PM

Quote:

Originally Posted by legasea

Can the ubolts be replaced w/o removing the exhaust and putting the car on a lift? If so, I can most likely do this myself, if not, I will regretably have to bring her in for service. I believe the center bearing and mount need to have the exhaust removed, so maybe I will just do both of these items and call it a day.

Thanks for the advice.

I don't think you have U bolts. Though I don't see what your car is. What (I think) you have is maybe bad center support bearing on the drive shaft. Yes you can do it. Do you have ramps? You will want preferably big ramps.

KarTek · #8 03-12-2008, 01:29 PM

You sure he wasn't talking about U-Joints? Depending on the car, there are 2 flex disks, a center support bearing and a traditional U-Joint right after the support bearing.

legasea · #9 03-12-2008, 01:35 PM

Quote:

Originally Posted by KarTek

You sure he wasn't talking about U-Joints? Depending on the car, there are 2 flex disks, a center support bearing and a traditional U-Joint right after the support bearing.

I was responding from an earlier poster who recommended replacing the ujoints. I have a 83' 300D turbo. So do I have two u-joints? One on each side of the center bearing? If so, is it recommended to drop the exhause and replace both ujoints, center bearing and center mount? Or can these items be replaced w/o dropping/disconnecting the exhaust? Any special tools required?

Thanks again.

RichardLSU · #10 03-12-2008, 01:38 PM

Haha, yeah, I meant U-Joints.

I dont know how to go about replacing them on a w123. My pickup is an old F100 so it was just a matter of crawling underneath and loosening a few bolts.

KarTek · #11 03-12-2008, 01:46 PM

What car do you have? From the front to the rear, my 300 has:

Flex disk
Center carrier bearing
U-Joint
Flex disk

I was getting the same rhythmic thrumming you're describing and I went under and wiggled the shaft up and down but I can detect no looseness... Still have the problem.

Alberta Luthier · #12 03-12-2008, 01:47 PM

there is 1 universal joint but it runs almost straight (if this is a W123) I haven't heard of one failing or being replaced what does go is the center support bearing and it's rubber holder .... then if you leave it one of the two flex disks goes and that is bad sometimes VERY bad. I have a an old set of very large ramps built out of 2x12 fir and 7 or 8 feet long, worked well for me. (I built them for a 67 3/4 ton GMC truck so they are pretty sturdy) I don't know how it would go on those puny stamped steel types, I haven't had to use them.

mobetta · #13 03-12-2008, 02:01 PM

YOU DO NOT NEED to drop the exhaust for the carrier bearing.(w123) jack up the rear end, support w/ stands,(if its on ramps, you cant rotate the driveshaft) put car in N and crawl under w/ 15mm & 17mm wrenches. also a 12mm socket and at least 6" extension. remove the 3 bolts which attactch the flex disc to the rear diff. remove 2 bolts that are on bearing support. pry the flex disc off rear end, remove to the rear of the car.

to do the bearing, a big puller is required. I used a pvc tube to drive mine on, but it was weak. a 1 1/4"X10" pipe nipple will work.
I would order a new bearing, mount, and the 2 metal shields which are on either side.

legasea · #14 03-12-2008, 02:09 PM

Quote:

Originally Posted by mobetta

YOU DO NOT NEED to drop the exhaust for the carrier bearing.(w123) jack up the rear end, support w/ stands,(if its on ramps, you cant rotate the driveshaft) put car in N and crawl under w/ 15mm & 17mm wrenches. also a 12mm socket and at least 6" extension. remove the 3 bolts which attactch the flex disc to the rear diff. remove 2 bolts that are on bearing support. pry the flex disc off rear end, remove to the rear of the car.

to do the bearing, a big puller is required. I used a pvc tube to drive mine on, but it was weak. a 1 1/4"X10" pipe nipple will work.
I would order a new bearing, mount, and the 2 metal shields which are on either side.

GREAT INSTRUCTIONS. Good thing, because I was just searching online and could not find ujoints for sale on any sites. the center bearing and rubber mounts are plentiful ($50 for the pair). I have not seen the metal shields though. I will pick up some jack stands this weekend and order the parts, I am just hoping I am not replacing a part this has already been replaced recently. Hard to tell, very dirty under there. $50 is not a bad loss though if problem persists.
Thanks again for all the advice. I was also thinking about replacing flex discs as well even though they do not appear to be cracking.

Alberta Luthier · #15 03-12-2008, 02:13 PM

Quote:

Originally Posted by mobetta

YOU DO NOT NEED to drop the exhaust for the carrier bearing.(w123) jack up the rear end, support w/ stands,(if its on ramps, you cant rotate the driveshaft) put car in N and crawl under w/ 15mm & 17mm wrenches. also a 12mm socket and at least 6" extension. remove the 3 bolts which attactch the flex disc to the rear diff. remove 2 bolts that are on bearing support. pry the flex disc off rear end, remove to the rear of the car.

to do the bearing, a big puller is required. I used a pvc tube to drive mine on, but it was weak. a 1 1/4"X10" pipe nipple will work.
I would order a new bearing, mount, and the 2 metal shields which are on either side.

You will also want a large set of channel lock pliers to loosen that big nut so you can shorten the drive shaft when prying it off the rear end.
I'd suggest a good look see with a trouble light, before you buy parts in case your flex disks look iffy(little cracks in the rubber around the bolt heads). If they look bad you would want to change them while you are 1/2 way there.