Zachmac

Recently my wife mentioned her 112k mile 2009 R350 was "groaning". I checked it out and sure enough the power steering reservoir was low. A quick visual check confirmed the back of the reservoir where it slots onto the pump was leaking, a common issue. I topped it off and ordered the o-ring and clip. System seemed to be working fine otherwise, good steering and no noise. I mentioned it to my indy who does a lot of MB work and he cautioned me to be sure I was using the correct PS fluid as most European cars use a Petosin base rather than the average red stuff. He gave me a quarts of the correct Fuchs fluid. Parts came and I set them on the shelf with the intent of getting around to it as the leak was very slow. Then we made a trip that included lots of two lane black top on winding up and down roads, not a lot of passing opportunities and a lot of Sunday drivers poking along. So we get to a stretch where I can pass but I need to not dawdle. I step all the way down on it and of course the RPMs climb. When I go to pull back into my lane, NO POWER STEERING, had to muscle it hard as an un-boosted R-class is a bear! Few seconds at lower RPM and back it came. I waited for some open road and tried it twice more, kick down, high RPM, power steering goes away. Clearly either the belt is slipping or the pump is cavitating. I took it easy the rest of the trip and got home without incident. Once home I changed out the belt, flushed the system, replaced the leaking o-ring and topped off and vented the system with factory fluid. Test drive goes great with steering like new UNTIL I try the high RPM load and crap, it still does it! So at this point a few questions: 1) anyone else experience this (you may want to stomp it on a safe long stretch and see as just driving around normally you'd never know), 2) does it sound like maybe the pump is worn but not totally meaning as lower RPM it is fine but at high RPM there is enough leakage past the impellor to cause it to lose pressure, 3) should I just keep driving it being careful to avoid redline and wait for it to fail or go ahead and change it out now, 4) can anyone think of anything other than the pump it could be? BTW, belt tensioner seems fine.

Zachmac

Some further research reveals an SPS Solenoid on the steering rack that could be going bad. It basically gets a signal from the ECU based on speed and sends an electrical signal to the solenoid to control a spool valve that regulates the pressure to the rack versus fluid returning to the reservoir. It sends max voltage at stand still and decreases that voltage as speed increases allowing a spring to open the solenoid. Perhaps something is happening at high RPM (although independent of speed) to cause that voltage to go to zero thereby porting all the fluid right back to the reservoir?


SPS solenoid is about $300 bucks and only two bolts BUT requires removal of entire steering rack to replace!!!!!


Guess I will just not redline the car for now as most likely the pump is fine and hope the solenoid doesn't crap out completely.


BTW, in case you weren't following closely, that means on a failure of the electrical signal or the solenoid Mercedes cars with speed sensitive steering default to no power steering (or at least minimum boost). I guess they decided that was better than maximum assist kicking in at high speed?