what are the symptoms of bad wheel bearings?
 

when i drive down the highway at normal highway speed, i get a rythmic groan. i cannot tell if it is speed sensitive but I suspect it is.

i think the only way to test wheel bearings is with a load and maybe a microphone close to the front wheels which is impossible for me.

so is the only way to see is to remove them and visually inspect the rollers?

they are fairly inexpensive parts anyway so I might just change them out. I just need help in the diagnosis.

groan....nothing.....groan...nothing...groan......ugh

You did not say which Wheel Bearings. But, I am guessing the front ones.

Safely jack up the front of the Car and rotate the wheels and listen for a grinding sound.

You also might as well Start the Engine and have someone brake hard and release and the rotate the Wheels. If the Wheels do not rotate easily something is preventing your Brake Caliper Pistons from retracting.

Remove the Wheel spread the Brake Pads enough so that they don't Rub; or you could pull the brake pads

If you have a Dial Indicator remove the Hub Cap and set it up to take a reading on the Hub end Play.
You can also pull it back and forth to see if there is any Play.
Rotate the Hub and see if you can feel any roughness or hear a grinding sound.

Of course visual inspection is the ultimate way to check but while simple is some work due to having to clean and re-grease just to see. Changing the Bearings is more work and you face the fact that even the well known name brands of Bearings have 3 or more grades of Bearings. The local Part Store is likely to sell you the made in China grade eve if it is a well known Company.

For Myself If I inspected the Bearings and they were good I would leave them alone and just change the Grease Seal.
Also since I was not successful at adjusting the Hub end play by Hand (I end up over heating the Grease and having to remove the that Grease and do it again) always used the Dial Indicator as the Service Manual tells you and have never had an issue since though I have had the Front Hubs off several times.

They can make all kinds of different noises, but considering how inexpensive it is to pull them and clean/inspect/repack(/replace if necessary), I would just do that. The worst part of the whole job is setting the free play, but you can buy the stuff you need at HF for $60 or so (runout indicator and magnetic mount).

I ran that all together, but unless they are actually worn (you can easily tell once you clean them up and start inspecting them) there's no point in replacing them. And if all they need is repacking, you need a pair of wheel seals and some grease, and you're good to go. ;)

Good luck.

Is the groan a single tone or does it increase/decrease proportionally to speed? If not, do a very thorough brake flush and bleed.
I have had this problem on my 300D and I've found that a brake bleed fixes it.
What is happening is the brake pads begin to chatter after the water heats up and expands and that chatter rubs the pad against the rotor until it hits a nice narrowband resonance.

If the tone increases and decreases proportionally to speed, then check your wheel bearings and if it's in the rear, check your CVs.

Phil Forrest

Does it sound like you have studded snow tires on?

NVH Noise Vibration Harshness issues are difficult to find. I doubt it is a front bearing due to the design ( tapered roller ) , when they fail there are more of a old roller skate gravely sound. Rear bearings ( double row angular contact ) make a more of less buzz that will change intensity when you load the car left to right.

What you are describing is something vibrating and going in and out of resonance. If you have a guitar string and subject it to vibration, it will resonate and make a sound when the frequency gets to the resonance point.

Cars make many frequencies at the same time. When the high point of frequencies consider, noise is amplified, when the are opposite , cancel each other out.

The usual suspects are motor / trans mounts that are crushing out, exhaust that has a failed hanger, something out of balance ( fan, drive shaft ) heat shield to tie bar.

My SL has a 36" ish long tie bar running from the front cross member to the body, in the center there is a 1" dia / 1" long rubber mount to hold the bar from vibrating. The mounts are broken and ,under certain conditions, it will resonate for a second or two. I'll get around to fixing it at some point but is isn't critical and I'll end up using industrial parts rather than MB.

Also, the microphone system you are suggesting does exist. It uses piezo electric vibration sensors placed near possible vibration sources. Some of these are wireless, some are hand held for use in diagnosing bearings in an industrial environment.

Remove the suspect front wheel, and retract the pads from the caliper. At this point, it should spin freely, and quietly. If it makes any noise, you need new bearings. To check the rear bearings in this fashion, you should remove the axles from the hub, quite a bit more involved. Replacement is also quite a bit more involved, so hope it's the front bearings making the noise.

Fronts, jack the car up, place hands at 2 o'clock and six o'clock, can you rock the wheel?

Repeat with hands at 3 o'colck and nine o'clock.

If there is play, you need new bearings.

You have two per wheel, an inner and an outer.

As mentioned, use a dial indicator to set the pre load on the outer bearings.

See post #2 here:
http://www.peachparts.com/shopforum/diesel-discussion/322075-front-wheel-bearing-end-play.html

I have had tires that were badly worn exhibit this kind of noise. Inspection of the tire/s revealed the extent of the wear.

New tires and a front end inspection/alignment fixed the issue.

Is your front end damper shock in good shape? I doubt if it would cause rhythmic groaning but it might dampen any vibrations.