Hum with vibration felt through brake pedal - Mercedes-Benz Forum

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post #1 of 19 (permalink) Old 03-28-2016, 09:36 AM Thread Starter
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Hum with vibration felt through brake pedal

Car has about 61K miles. I have recently noticed a hum and the light pedal vibration occurring after a good run. It doesn't seem to occur when the car is cold or on short runs. A site search suggests that this might be the tandem pump pulsation damper starting to give out. Tempting fate, there are no other signs such as caution lights or erratic ABC operation. I haven't done anything that might cause the hydraulic lines to move up against the body (one of the other suggested causes). Any thoughts on other causes?

I have been thinking about changing the ABC hydraulic accumulators as a preventative measure anyway. Aside from buying them at the MB dealership, where do you recommend getting good quality units from?

Thanks
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post #2 of 19 (permalink) Old 03-28-2016, 09:46 AM
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The brake and SBC has nothing to do with ABC.

Humming at idle is ABC issue, that require damper to be replaced.

Pulsation in SBC pedal is probably an indication that you need to replace the SBC unit soon.

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post #3 of 19 (permalink) Old 03-28-2016, 10:01 AM Thread Starter
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Thanks for the quick response Bayhas. I don't have any pulsations in the brake system. I just feel the hum/light vibration being transmitted through the brake pedal and also a little through the floor if I think about it more.

Any recommendations on a quality source for the dampers?

Also, I have the WIS, but I haven't had a chance to read up on changing the dampers yet. Any tips on replacing these whereby experience may suggest doing it differently to what the WIS states (with all of the usual "at your own risk" disclaimers of course)?
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post #4 of 19 (permalink) Old 03-28-2016, 10:07 AM
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Quote:
Originally Posted by RocketBoots View Post
Any recommendations on a quality source for the dampers?
Your local parts desk at the dealership.

Quote:
Originally Posted by RocketBoots View Post
Also, I have the WIS, but I haven't had a chance to read up on changing the dampers yet. Any tips on replacing these whereby experience may suggest doing it differently to what the WIS states (with all of the usual "at your own risk" disclaimers of course)?
WIS is directed toward shops, not end users, and has detailed procedure to replace it, but naturally, because its target audience is professionals, it will not cover common sense mechanic basics (although it comes close).

WIS is always the best and most accurate source for info, however, there are a few DIY articles about the subject on BenzWorld.

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post #5 of 19 (permalink) Old 03-28-2016, 12:36 PM
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Have you considered the motor mounts? Generally doesn't create a hum but certainly chassis vibration, and they are known to wear quickly.

I purchased my front accumulator and dampener recently from Pelican Parts and did the job a few weekends ago.

The dampener is very simple to replace. I just removed the fasteners to the rear half of the wheel well lining, peeled the lining back enough to expose the pressure sensor + pressure limiting valve + pulsation dampener assembly, and used a slim wrench (21 or 24mm, I forget) on the dampener and an adjustable wrench on the assembly housing to provide counter torque, and cracked it loose. Then turned the dampener by hand like a light bulb to remove and put the new one on, and tightened. Then made sure the assembly had adequate clearance all around, and then put the lining back and was done. Simple as that. About 30 minutes work. There is no need to unbolt the assembly, but that is an option if you need more working room.

You aren't out much time or money if you are wrong.
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post #6 of 19 (permalink) Old 03-28-2016, 04:52 PM
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RocketBoots has a 2008. The pulsation damper is mounted on the tandem pump body. Models prior to 2007 have the damper mounted remotely.

WIS for the later cars creates a lot of unnecessary work by instructing to remove the pump. I unbolted my pump from its mounts and that gave enough clearance (just) to change the damper without unbolting any fluid lines. Just make sure the area is spotlessly clean before starting the work.
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post #7 of 19 (permalink) Old 03-28-2016, 06:24 PM
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RocketBoots has a 2008. The pulsation damper is mounted on the tandem pump body.
Good catch bekcon! I didn't check his profile. I stand corrected.
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post #8 of 19 (permalink) Old 03-28-2016, 06:32 PM
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out of curiosity, is there a seal or gasket that you have to worry about the integrity? Is there much risk that there will be leak afterwards? I agree your approach is much better than the WIS. Why spend all that time tearing down and reassembling for nothing.

I had to replace the right headlight bulb and WIS said the bumper needed to be removed. WIS definitely likes to take the scenic and expensive route at times! RocketBoots was smart to ask.
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post #9 of 19 (permalink) Old 03-29-2016, 05:41 AM
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The pulsation damper is supplied with the necessary o-ring already in place. It is simply a matter of unscrewing the old damper and installing the new one.
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post #10 of 19 (permalink) Old 03-29-2016, 09:12 AM Thread Starter
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Quote:
Originally Posted by bekcon View Post
The pulsation damper is supplied with the necessary o-ring already in place. It is simply a matter of unscrewing the old damper and installing the new one.
Thanks to all. A few questions as I haven't had a chance to get to my WIS yet.
- Is there any retained pressure to be concerned about?
- Does the car need to be supported to prevent the suspension from sagging due to an open line?
- Is there much oil spillage?
- Do you need to pre-charge the new damper with oil?
- Does the system need to be bled afterwards?
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