Sagging

 "Sustainability is living on nature's income rather than its capital." - Murray Gell-Mann

 I recently purchased a complex hydraulic system.  This system is computer-controlled and has the ability to make adjustments to four individual hydraulic actuators ten times per second through a high speed data network.  This ridiculously complex hydraulic system comes with a car attached to it.   

I'm referring to the Mercedes "ABC" Active Body Control system.  This ABC system uses hydraulic controls to pump or dump oil in the struts on each wheel, to control body sway during cornering, to minimize bumps affecting ride comfort, and to raise or lower ride height.  

I case you are unfamiliar with hydraulics, the overall concept is pretty straightforward.  An oil reservoir provides a surge volume, a pump supplies oil pressure, an accumulator stores oil under pressure for when the system needs a larger volume of oil than the pump can supply.  Lastly, there are control valves to direct oil as needed, and pistons that move actuators.

Below:  A simple hydraulic system.  The purple pump at the top left takes suction from the reservoir through the bottom left tube.  It's a high pressure pump, so a relief valve dumps excess oil back to the tank when the oil is not needed to operate the actuator at the right.  When the valve is operated, oil flows into one or the other side of the yellow actuator, causing the shaft to move in or out of the cylinder.  At the same time, oil is bled off the other side of the actuator and returned to the tank.  Pretty simple.

The S600 that I bought in November is showing signs of an issue, now that I've not driven it in a while - it's sagging a bit on the drivers side.  At first, I thought it was tires going flat, but it's not that.

 Looking back at the simple hydraulic system, imagine if 4500 lbs of weight were pressing in on that linear actuator at the right.  If the control valve didn't seal perfectly, that weight would eventually push oil out of the actuator, past the control valve seals, and the actuator arm would be compressed into the cylinder.  That's what's going on with the black car.  The weight of the car is squeezing oil out of the struts back into the reservoir, and the car is settling.  This is not a big deal.  Once the car is started again, the pump will run and hydraulic pressure will allow the system to level the car out.  But I would rather it wasn't doing this in the first place - it's supposed to be a nice retirement car, not a jalopy.

I did a bit of research, and it looks like I could either spend several thousand dollars to have Mercedes fix the issue, or buy a couple of rebuild kits off Amazon and have some fun doing the work myself.  I'm leaning toward the latter.  It's just a fancy brake caliper rebuild.  Hopefully it works out better than the motorcycle one did.  I think it will.

Below is a isoschematic diagram of the component locations in the car.

Below is a diagram showing the flow paths and how each thing works.  Both valves labeled "Y2" are lockout valves.  They are supposed to prevent the weight of the car from pushing hydraulic fluid out of the actuators when the car is not in use.  It appears they are leaking by. allowing the car to drop down.

I plan to rebuild the front and rear actuator valve bodies - dashed block Y36/1 and Y36/2.  

While apart, I'll also check the accumulator bladders 4, and 14.  Then check the pulsation damper bladder 52a, and replace the hydraulic filter 2a.  The previous owner said that the actuators (40,41) have all recently been replaced - probably due to lack of understanding why the car was sagging when not in use.

So with the explanation of what's happening and what I'm going to do about it out of the way, here's what's needed:  A bunch of seals and O-rings to lock the hydraulic fluid where it belongs - in the actuators.

That's a $75 rebuild kit for the front valve set and another $75 for the rear control valve set.  For $150 and a day or two of my time, I can rebuild all the control and lockout valves.  Throw in a filter and some fresh hydraulic oil, and we are up to $200-225.  I'm actually looking forward to this.  Below are a couple of videos showing the process.  The first video shows how to get the valve assembly off the car, and the second shows the valve rebuild process. 

I'm not sure where the free time is going to come from, but I'll find it!