HIS

Hello everyone.

My SL500 2003 has same common problem with the hydraulic pump as many other MB cars. A humming sound. Very annoying!
I would like to have more detailed information what is causing this sound. For me its sound like worn out bearings.
There are many discussions on different MB forums about this pump noise and explanations what may be the reason but none of them really fits my problem.
My pump has always the noise and it’s not RPM dependent. Noise is higher when engine is cold. I have exchanged oil/filter and all four accumulators, cleaned both valve blocks thoroughly. I check the oil pressure, its 200bar/ 20Mpa and drop to 180bar/18Mpa few second when elevate the car. Otherwise the ABC works fine.
My guess is that the periphery equipment is not the reason for this noise in my car, it must be the pump that is broken.
My question is if someone has experience of disassembling the hydraulic pump and can tell what causes the noise?

Thanks in advance!

Rudeney

If the noise does not vary with engine speed, then I doubt it's the pump.

HIS

Well I expressed myself in wrong way. Off course the noise follows the engine speed but the sound does not diminish with anyone special rpm. it's always heard.

HIS

Well I expressed myself in wrong way. Of course the noise follows the engine speed but the sound does not diminish with anyone special rpm. it's always heard.

mercy-me

Might not be the pump at all. Try a mechanics stethoscope to isolate the noisy component.

Bob

Rudeney

Have you replaced the pulsation dampener?

HIS

Yes, all of them. (Four pieces)

Rudeney

I assume by "all four" you mean that you replaced the pulsation dampener, front and rear accumulators, and the return flow accumulator?

Look here: http://mercedes-abc-drive-carefully.blogspot.com/

The critical one would be pulsation dampener. If all of those are good, then it might be the pump itself. I second the advice from Mercy-Me to get a mechanic's stethoscope and start listening.

HIS

Yes, i changed all of them. Pulsation damper for the pump and pressure reservoir for front and rear axel. My car has now 100000km / 62137 miles. I have another question. How to check the restrictor valve to be sure it works correctly? There must be a reason why the the pulsation damper dosent reduce the oscillation from pump. What more parts can be involved into this problem?

Rudeney

It still could be a defective pump and not the dampener.

bobterry99

Activate and deactivate the valve while monitoring the system hydraulic pressure supply. A working valve will cause a significant fluctuation. There should be a test within Star Diagnosis for this.

HIS

Thanks for your suggestions but right now the pump is removed from car. I will dismantle it later to see if I find something wrong inside. About the restrictor valve, I found information that valve work with 0-5 volt DC. 0V=closed 5V=fully open. I checked my valve with 5V and it work fine. I need to check the pressure sensor as well. Maybe it gives wrong value. Has any right output value for this sensor??

HIS

Does anyone know what the pressure should be inside pulsation damper?? There is a stamp on the outside 200bar. Is this the maximum pressure it can withstand or is it the back pressure on the inside??
I measured the pressure in my new damper and it's only 80 bar.

Rudeney

I believe ABC operates at over 2500PSI. My guess is that the 200PSI is the pressure on the backside of its internal diaphragm which does the dampening.

bobterry99

100 bar.