CorAegean

2009 SL550. Revised, ABC2 system.
I have been noticing one symptom and want to resolve for ride quality and to avoid further damage. When going over a bump, the rear bounces slightly low. Second bump a few seconds later, the rear is now firm. If after a minute or two I go over another bump, the cycle repeats.
MY BELIEF is: the rear accumulator is weakened, and does not support the rear struts firmly. Then it fills with fluid, and so it supports for that "second bump." It later drains at least partially, and causes the cycle to repeat.

I'm asking if anyone with good knowledge of the system (my knowledge is not in-depth) thinks this is a reasonable conclusion. I am edging toward just going ahead and replacing this rear accumulator. Reason for not getting diagnostic at the dealer, in case anyone is curious: It is one of the "good" dealers, nevertheless, they either won't fully use the diagnostic system, or they don't know how.

Thanks for any suggestions.
Reid C.

imtheking

Here's everything you need to know about the ABC system

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

Signs of a bad accumulator are overflowing reservoir and abc messages on the dash for 5 seconds or so that goes away after hitting bumps

brucewane

How many miles are on the car? Any service history indication that any of the accumulators have been changed before?

NHSl550

Great blog. I have a 2007 SL65 with 88k miles. I have no issues with the system but was thinking of doing some preventative maint. Acculators, valves, pump, hoses, etc. Should I? Or if its not broken dont fix it?
Also Is most of this stuff DIY? I know of some differences between the ABC and ABC 2 Are there any specific differences?

brucewane

IMO, it would be a good idea to go ahead and install new accumulators and rebuild the valve blocks.

From what I've seen/heard, accumulators have two failure modes.

The internal rubber diaphragm can develop cracks/splits and lose the pressurized gas charge, so the accumulator no longer acts as a shock absorber for pressure spikes in the system. This means the hoses and the other accumulators are placed under increased stress which can caused them to fail.

Similar, but far worse, the internal rubber diaphragm can fail more catastrophically, sending bits of rubber throughout the system. Then you have to go through the whole system, basically disassemble every junction, every valve block, push a wire down every line, to check for obstructions. There's a guy on YouTube with a CL65 who went through this, Legit Street Cars.

The seals in the valve blocks deteriorate as well. They can just start to leak pressure while the car is parked, causing a corner to sag, but the seals can also shred and make their way into the system, similar to the accumulators. They're smaller bits of rubber so they're not likely to get stuck in the lines, but will get stuck at the entry to other valve blocks.

The rest of it can be replaced as needed. Hoses are most likely going to show signs - "sweating" hydraulic fluid - before they completely burst, so regular inspection is a good idea. Hoses can be rebuilt with new rubber by a local hydraulic shop far cheaper than buying new. The pump will usually get to where it doesn't produce adequate pressure, which will set off trouble codes. Or it will start to leak. But usually not a catastrophic failure.

If for some reason you have to have the driver side coil pack out any time soon, I'd probably go ahead and put a new pump in at that time. It's a much easier DIY with that coil pack out of the way. I don't think WIS specifies to remove the coil pack, but it's a lot easier to see what you're doing with it out of there. You'll also want to remove the ABC and power steering reservoirs and their brackets, which is pretty easy.

CorAegean

Right about at 90k. No, there is no service indication of previous r&r on either fr or rr accumulators. I bought the car at approx 53k and I did replace the pulsation damper at about 65k.

After reading through the excellent guide above, I think I'll do a fluid exchange and see if just some pm will improve things.