Fried Chicken

Background:

I just had an ABC hose pop on me in the rear on the CL. I haven't looked but I'm 99% sure it's the hose going from the rear valve block to the rear accumulator (from the rear valve block, Y36/2, to the rear axle pressure reservoir, #4). The same thing happened on my W220 a few months ago, even though the hose had been relatively recently replaced and looked brand new. The accumulator was in working order and had not burst. Additionally another high pressure ABC hose had failed on the w220 just a few weeks prior. Taken together, I suspected an additional failure of the ABC system causing an overpressure situation in the high pressure ("working pressure") hydraulic circuit as the only possibility for such a failure.

Looking at the ABC diagram:



The only failure in the ABC system that can cause an overpressure in the system is in component #52 in the diagram:


I've spent the past half hour or so staring at this very valve block to decipher exactly how it works, and I believe I have figured it out. The high pressure hydraulic fluid form the pump enters the valve block where, via a series of internal channels, the pulsations are dampened, the pressure is measured, and excess pressure is bled off via two relief valves. Why there are two I do not know. The smaller one appears in the diagram, however is not labeled.




As the pressure ABC pump comes in, the fluid flows into these regulator/bleed valves which feed into the return circuit and go back to the reservoir via the cooler. This is how the ABC system regulats its pressure, and as far as I can tell is the only point of failure that can result in an excess pressure in the ABC system.





These pressure regulating valves are passive devices and should be extremely reliable, but in the case of the w220 it's the only way I could find that would cause the pressure to spike too high in that hose. The Part # for this block W220 V8 is: 220-320-08-58

Possibly these could be found separately. The larger of the two has inscriptions of "GE" and "146". The smaller has inscriptions of "DJ" and "146". Interestingly, I believe the hex nut adapter for unscrewing them is in Imperial units, but I could be wrong. Unfortunately I can't think of an easy way to test these. Maybe the smaller one by inserting a pick or other cylindrical tool and testing movement in what I presume is a sprung cylinder. I'll ask at a hydraulic shop if they've seen something like this before. It's much cheaper than a brand new $700 valve block from Mercedes.

Fried Chicken

Here are additional pictures of #52b, "Valve pressure limiting valve":