Dealing with a very frustrating ABC problem...2003 S600TT.

One month ago, after parking, noticed a pool of fluid under my front left wheel area. Maybe about 1L of fluid. Seems it flowed out of the reservoir vent hole. Got red ABC warning.

Towed car to indy. Pump was apparently putting out 0 pressure.

Changed pump (rebuilt), bled system, etc. Passed rodeo no problem. Took the car home, it drove very poorly. Felt 'floaty' on road undulations, and very harsh on bumps and dips.

I get the white 'visit workshop' message when I hit the first serious bump of any drive. Turning the car off and on resets it. The code indicates "PSI pressure supply too low". (1526-016)

Level adjustment works perfectly. No sagging, no leaking that I can tell. System passes rodeos no problem, and pump puts out 184+ bar of pressure at idle.

Changed front and rear accumulators as well last week, drives a bit better but still similar issues with harshness on bumps and some floatiness. The white error comes on during most drives, sometimes while passing over bumps, sometimes not. It can come on 2 minutes in, or 20 minutes in, or never.


Not sure what to attack next, any ideas?

 

Does your indy have DAS (the STAR Diagnostic Assistance System), the complex and powerful computerized dealer maintenance and repair system? If so, does he know how to use it well?

DAS includes guided diagnostic tests for virtually every part of the ABC suspension system, and should quickly and easily identify your problem.

 

Yes, he has it, and I've watched them use it. There is no fault manifesting itself during testing, so it's hard to pin down. Basically DAS says to run rodeo and monitor pressure, which it passes fine.

I'm personally thinking wither one of the two remaining accumulators (which shouldn't really be affecting the ride that much I'd imagine); that the replacement pump isn't putting out enough pressure; or that there somehow is a bubble in the system somewhere.

 

A 'flat' accumulator, one which has lost all/most of the nitrogen gas charge, cannot hold very much high-pressure oil from the ABC pump. When the ABC system call for oil to be quickly added to a strut, a flat accumulator quickly runs out--and the pressure collapses, triggering a 'low pressure' warning. The pump cannot deliver enough high pressure oil to instantly supply enough oil for the system request.
And, YES, a bad accumulator DEFINITELY adversely affects the ride quality. The flat accumulator can no longer absorb the many road surface irregularities.

 

Makes sense. Given that the primary/supply accumulators were replaced, though, would a failed return accumulator alone or pressure dampener cause such symptoms?

 

The pressure damper is merely a length of hose included to 'absorb pump pulsations.' It is not an accumulator and is liquid filled. Any 'air' in the system quickly dissolves into the oil (and will foam out if pressure is released.)
As I said, an accumulator that is supposed to store high pressure oil to feed to the strut when needed--cannot do that if it has lost its nitrogen charge--like a deflated balloon. When the System calls for HP oil to add to a a strut, and it is not there, the system pressure instantly drops to that of the strut and triggers a Low Pressure warning. The pump eventually catches up (in a few seconds) but that might be 300' down the road. A flat 'hp return' accumulator is the opposite--it can not receive and store HP Oil as a 'reserve supply' as it is completely filled with oil when the nitrogen pre-charge is gone.

I used to be 'the guy' supporting the Infiniti Q45a the Active Suspension worldwide, manufacturing replacement accumulators and rebuilding other parts for them.
I also manufactured replacement accumulators for the height control and suspension system of the Lexus LX 470 and 570 luxo SUV's and provided lots of technical advice and support. Lexus techs never got training on the system and 'soccer Moms' complaining of a harsh ride usually got hit with $7000 or more as Dealers simply 'threw new parts everywhere.
I also support the Porsche 959 Super-cars and their full active suspension and the MBZ '100 series' vehicles such as the 450 6.9 ones and other cars with hydro-dynamic suspensions. Citroen's are well sorted out and Citroen Techs well trained in Europe and accumulators are inexpensive for them.

The ABC system licenses the patents Nissan holds. The Nissan system is superior to the ABC in many ways, including a very high capacity pump (17 L/min) that never failed.

 

Yes, that all agrees with what I've been thinking--that's why I replaced the supply accumulators after our pump replacement still left us with errors. That's why I'm so stumped--it's not the pump, and not the accumulators, so what can it be?

 

"Changed front and rear accumulators as well last week, ..."
"I'm personally thinking wither one of the two remaining accumulators (which shouldn't really be affecting the ride that much I'd imagine)..."
"Given that the primary/supply accumulators were replaced, though, would a failed return accumulator alone..."

How many accumulators - and which ones - have you changed? Are you changing the parts, or is a tech telling you what he has done?

 

The techs, under my direct supervision, have changed the two supply/primary accumulators (both part no. 2203270115), located at the front and rear.

We have NOT changed the other two (parts 2203200415 and 2203270215)

 

Based on my years of experience with hydraulic suspension systems and their accumulators, when one accumulator fails of 'old age', the others are near death also. Some systems have units in hotter locations and those fail first. ANY of the ABC accumulators in 'the hot zone' near the exhaust pipes and NOT shielded and insulated are highly likely to have failed.

 

Sure, but am I just changing parts for the sake of changing parts at that point, or would the return and dampener accumulators (which is an actual accumulator, not just a length of pipe) actually make a difference in the ride quality?

 

Took it for a drive today with DAS running to monitor pressures. Even on the region's worst, bumpiest road, it never dropped below 135 bar. The warning light came on after a few minutes of driving along this road, and the pressure at that time read 138 bar. Did a few more passes to test, very similar result--sometimes the light would come on (showing pressures around 140 when that happened), sometimes it wouldn't come on at all.

Stumped again.

 

Could it be a low pressure switch failing and 'turning on' at too high a pressure? It could also be a flow restriction between HP accumulator and demand.

 

It could be. I don't notice any difference in driveability before and after the error message, so whatever is throwing the fault is, in my opinion, occurring 100% of the time. Is there only the one switch near the pump, or others?

As for the flow restriction, that seems like a very possible thing as well. I'll try to do a more thorough inspection this week to see if we can detect anything of that sort. It seems all four corners of the car are equally affected, so I'll pay particular attention to the main lines by the pump. Thanks!

 

I do not know where the DAS pressure sensor is located in the ABC system. For DYNAMIC pressure readings, the type and location of the 'sensor' makes a big difference. Some sensors will 'trigger a warning' almost instantly (micro-seconds) while others have a natural 'time delay.' Mechanical switches usually have a restriction orifice in them to damp out (smooth) the pressure surges to keep brief spikes from damaging/wearing out the sensor. Some pressure sensing systems are 'extremely fast' using electronic sensors that respond essentially instantly to detect explosions etc. Those are not usually found in automotive systems as the sensors and display readouts get costly.
With no performance symptoms (other than alarm caused) is the 'low pressure' real or an unusual pressure transient or 'overly sensitive' pressure detection system. It seems the car DOES have good pressure delivery everywhere but the 'low pressure' switch function. Is there any way to easily add a mechanical gauge? Or just replace the 'suspect' low pressure switch?

Good Hunting!

 

I'll try to change the pressure sensor, but all DAS tests show it to be operating correctly.

I'm debating whether to change the return accumulator (due to the initial fluid spill on the first day of the problem), and possiblty pulsation dampener at the same time.

A MB tech at the dealer (I was there for unrelated business) suggested the pressure supply valve, but everything I read leads me to believe a failure there would have given more codes.

 

Some updates:

Started to get the fault more and more often. Ran a couple of rodeos, and the car stopped many times with a corner down, running about 90 bar.

I'm thinking either the pump is bad, or the pressure supply valve.

 

Replaced the return line accumulator and the pressure supply valve (including the pressure dampener). It was better for about 15 minutes, and then back to the same problem

To recap--the car drives as if it had blown shocks: it floats around a bit on road undulations, but every bump is extremely harsh. I intermittently get a white ABC warning message indicating "malfunction in pressure supply" (1526-16), which doesn't affect the ride. Pump puts out 185-189 bar, passes rodeos, no leaks, can raise car quickly and evenly.

 

Have you replaced the filter? How old is the fluid?

 

The fluid was flushed and filled twice this summer so far. Filters changed at the same time.

 

The weird thing is 'floaty' feeling together with hard/poor bump absorption. Struts full of air would feel floaty, but would absorb bumps, Flat accumulators would be harsh riding but not floaty, It is almost like the struts have been set to super firm like a 'track' setting, I suggest you have this possibility checked out

 

Thanks!
I do get a message on my icarsoft scanner saying that the control lock is on, whatever that means. I'll see if I can find anything on that.

 

I solved part of the problem: the PSI supply fault was caused by the pump which, every 30 seconds or so, would momentarily drop pressure to about 40 bar (lasting about 1/2 second), before rising again. This happened too fast to register on a digital gauge, but was detected by an analogue gauge we hooked up to the pressure sensor.

Changed the pump again, that problem is now gone. However, the harsh ride remains, as does the "ABC control lock on" message on the Icarsoft. No fault codes whatsoever anymore though.

Any thoughts appreciated.

 

I've been advised to try changing the ABC control module. Anyone know if I can use any control module (from another model CL/SL/S) or does it have to be from an S600?

 

If the part has the same part number it will work. . If only the last few digits are the same it MIGHT work. Mercedes is famous for having 'different' almost identical 'modules' for the same model vehicles with different 'equipment' installed. Sometimes they are completely incompatible. For example the several 'fiber Optical' network versions. Why is the second COMMAND version 1.0 while the first is 2.xx and then there is MOST following the D2B versions....etc. Add in the many 'cell/mobile' telephone versions and features and the vehicle electronics are almost UNIQUE to each vehicle.
Chrysler Corp learned many years ago working with Motorola and used a common (identical) 'module' for every function but different computer instructions (PROMS) easily adapted(programed) for purpose.. Toyota has only two (2) radiator caps/filler necks GM had hundreds of different and incompatible ones.