PLEASE READ BEFORE REPLYING: I have more than enough evidence and reason to believe it is NOT the pump. Please do not just give me the typical "it's the pump" diagnosis. I am looking for a legitimate reason why the vehicle might be doing this and ways of testing it if possible. Yes I already tried to bring it to an independent, after calling every single one in my area and getting "it's too difficult/complicated" I'm pretty much stuck doing this myself unless anybody knows a independent that works on these in south coast MA. I am a fairly experienced DIY mechanic but I'm most certainly out of my league when it comes to this. Any legitimate information, workshop manuals, diagrams and references would be greatly appreciated I am very willing to learn.

After blowing up my ABC pump months ago I took on the task of replacing it, to make a long story short this is the 4th one because finding this exact part is extremely difficult and I was given wrong parts or defective units. The ABC pump in this car is from a reputable company that I trust, it held pressure outside the vehicle and still easily gets up to 2900PSI+ inside the vehicle during initial startup for about a minute, hot or cold at idle fluctuating perfectly with the B4/5 suction valve's request. Yes I did the priming process correctly. I do not have SDS from which I understand can pinpoint exactly what is causing the issue, I have a very nice snap-on scanner that can do all the functional tests and read all the data but cannot pinpoint the issue and can only do so much.

Here are the symptoms and relevant data:

-There are absolutely no leaks and no air bubbles in the reservoirs

-Upon startup hot or cold, codes cleared, the ABC pump works perfectly fine for about a minute getting up to 2900PSI+ when requested to at idle

-Vehicle during this time lifts very fast, works perfectly fine, levels itself quickly and will even go through the rodeo functional tests fine.

-After about a minute a groaning sound can be heard and then what sounds like a sort of pressure relief valve and then the pressure in the system rapidly drops and sits at 1400-1500PSI at idle.

-At this point car will "fail out" of any tests.

-Code "pressure supply malfunction" is kicked on, then after a while "pressure supply low"

-Car will drop very low and disable the lift feature

-If RPM is held slightly above idle 800-900 RPM the pressure will go up to 2400-2500PSI and stay there as long as the RPM is held

-Car will pass any tests and will raise itself back up and if restarted will enable the lift feature again

-When the RPM is dropped it will immediately start lowering itself back down and drop back down to 1400-1500PSI

-When driving the car rides amazingly, stays at 2400-2500PSI and reacts immediately

-After a really, really long time it will eventually kick on the red ABC light if left to sit and restarted a few times with no RPM hold and no codes cleared (1-3 days typically)

-Side note, occasionally it will also give an error code for front right and rear right struts moving when valves were closed? Don't know if this is important

I personally think it might be the valve block being stuck open? or valve body sending unit? Which might be why suddenly it can't build full pressure and the entire suspension is directly reliant on the current output of the pump? If any more information is needed to properly diagnose it, feel free to ask, I forgot the actual Mercedes code numbers it spit out but I can always go back and get them.

 

Your valve blocks needs rebuilding , best deal is the rebuild service on eBay they will do it for you

search eBay for abc valve block service

 

So this is the front valve block that's behind the front bumper, right? Thank you for the reply

 

At this age of the car it’s time to do both front and rear valve blocks

understand that the o rings are 20 years old and at end of life

 

-How could I test or isolate the valve blocks to be sure?

-Should they be leaking if they are failed?

-Can they fail without leaking any fluid/light seeping?

-How can I be sure it isn't actually the accumulators?

-Is there any other part that could fail and cause this?

 

All your accumulators are due for changing too. These are normal maintenance items on hydraulic systems . If you want to do accumulators first go ahead , but I’ve had several cars with failed accumulators and they don’t affect the system the way you describe .

valve blocks leak internally more often than externally , preventing proper pressure from
being achieved

 

Valve blocks leak internally. The most common system is the car sagging when it's parked. They're not hard to remove, a few trim pieces, three small rubber mounts, and then the hoses. You'll need a good 17mm line wrench. I did mine a few years ago with o-rings from a local parts supplier. There is a vendor that sells the exact sizes you need on eBay, you could go that route, or have them done.

 

IMHO, after more than 15 years of experience with the Nissan'Active Suspension system and decades with MBZ Hydro-pneumatic system and only a year with the 2001 S-600 system, I offer the following:
A): The pump is sucking in air from a leak in the inlet hose or connections
(B) A pressure control valve starts leaking internally
(C) A strut level valve starts leaking internally.

Due to age, I suggest renewing ALL the seals/O-rings in the system. One or more starts leaking as it warms up. You might try adding a viscosity-increasing additive (STP) to see if that will improve things. It won't hurt anything.

 

I honestly think it's the valve blocks leaking internally.

He posted over on BW and I answered in that thread.

https://www.benzworld.org/threads/de...#post-18213660