KABOOM! my rear suspension HURTS. - PeachParts Mercedes-Benz Forum

blankenship · #1 08-05-2007, 04:50 PM

so, i was driving my just-purchased '85 300td back from Texas, and had my wagon REALLY loaded down and was noticing suspension fluid leaking out from the valve at the rear of the car...i climbed under, and found that the 'suspension pull rod' pivots were totally disintegrated and it was just kinda hanging there. for lack of parts availability, i was in a tough situation. i tried to keep the fluid level full on the trip, but fluid wasn't readily available either and i was having tons of other simultaneous problems, so she ran dry with the pull rod 'hanging' for quite a while.

when i got her (towed) home, all the fluid in the reservoir was gone, so i filled it up and replaced the pull rod with a new one. she didn't seem to "take" any fluid. (i filled the reservoir up to the top with the motor off, and then started the motor and the fluid level stayed the same). i checked for self-levelling by pushing up on the actuator, and it still works! (the back end will raise and lower).

i've been hoping for the best, as she's never sagged noticably, but since then, i've noticed a huge KA-BOOM! in the rear suspension every time i hit a bump. she doesn't "bounce," it's just a really harsh hit and resulting sound...stiff as a board.

now what?

mobetta · #2 08-05-2007, 05:14 PM

the pump is mounted to the head at the front of the engine.

use hydrualic fluid, not motor oil.

have you checked the sway bar links?

Bio300TDTdriver · #3 08-05-2007, 05:33 PM

As someone on here says: "It's always the accumulators."

Chris

blankenship · #4 08-05-2007, 06:06 PM

re: MOBETTA: yeah, i used FEBI hydraulic fluid, not motor oil. by 'sway bar links' you mean the little plastic ball-linkages coming up from the wheels? if so, yeah, they appear to be intact, and will rotate.

re: Chris: is that the answer? am i doing an accumulator job?

JimmyL · #5 08-05-2007, 06:16 PM

The accumulators seem to be the only part in the system that wears out regularly. Other things may have issues from time to time, but the accumulators are the regular culprit.
And they will fail both ways, as I have found out.
First wagon was super bouncy and soft, bobbing along like a car with only springs, no shocks. Was saggy in the rear, and would not raise via the linkage from the valve.
Current wagon was super harsh, with the requisite "whamming" that you speak of. However, it would still raise and lower via the linkage some.
Accumulators took care of both issues.
As a side note, on BOTH wagons, not long after replacing the accumulators, the hydraulic high pressure line blew out! Within a month or two. So, if you see oil slung on the underside of hood, or somewhere in the engine bay, you might want to have that line repaired. They will cut the ends off and braise them on new hydraulic hose. Others I believe have had removable fittings, but mine weren't like that. Cost was around $60 bucks, so you might want to do it anyway. That fluid ain't that cheap........

lietuviai · #6 08-05-2007, 06:33 PM

I can't help but chuckle when reading the title of the thread but it's no laughing matter. Even if I don't own a wagon, I like learning about how the suspension works and what problems are most common.

#7 08-05-2007, 06:43 PM

Both the pump on the head and the valve in back depend on the suspension fluid for lubrication. If you ran for long with no fluid, you may have destroyed the pump.

blankenship · #8 08-05-2007, 06:58 PM

doesn't the fact that she doesn't sag overnight and will also raise up when the lever's moved mean that the pump's good?

Bio300TDTdriver · #9 08-05-2007, 07:16 PM

It sounds like your pump and valve are still OK. Good tip on the high pressure line JimmyL. I need to do my valve and accumulators, so I'll be watching the line.

Chris

Douglas.Sherida · #10 08-05-2007, 07:56 PM

The SLS system is not such a big mystery.

In the front you have a fluid resevoir, a high pressure and low pressure hose and a pump. The resevoir and hoses either leak or not. If the system goes up and down with the lever (with the engine running) then your pump works.

In the rear you have springs, which support the weight of the car when unloaded. If they are good the car won't sag even when the SLS is disconnected. If the car sags, then the springs are worn (or broken).

You also have a sway bar attached to the rear hubs by plastic "z-links", if they are loose or broken replace them. The middle of the sway bar has an adjusting rod (pull rod) that connects to the lever arm on the SLS valve, just behind the differential. Shortening the pull rod sets the SLS ride height higher, lengthening it makes the ride height lower. The valve itself modulates how much hydraulic fluid is in the actuators (rear struts). If the valve is leaking, you can rebuild it by replacing the o-rings. The actuators (rear struts) are just hydraulic rams, they either leak or not (hope that they don't because they are expensive). The shock dampening quality of the actuators is controlled by the accumulators. The accumulators are gas filled spheres with a diaphragm in them that separates the hydraulic fluid (from the actuators) from the gas, since the gas is compressible (vs the hydraulic fluid) it "absorbs" the shocks of bumps in th road. When the diaphragm in the accumulators ruptures, you have no shock dampening, if you're unloaded and riding on good springs its pretty bouncy, if you're loaded (and/or have bad springs) you are riding on the hydraulic fluid in the actuators (WHAM!).

So, it sounds like you have bad accumulators, and a leaking valve. both pretty easy and cheap to fix.

Do not drive around on just the actuators, for fear of ruining them. They cost a small fortune and are a PIA to replace.

Crawl under the car and see what is leaking and/or loose, then rebuild or replace them.

BTW, the full system takes 3.5 liters of hydraulic fluid. Most of that stays in the resevoir.

blankenship · #11 09-10-2007, 08:05 PM

well, i'm the original poster, and i've got NEWS.

to compensate for the 'kaboom!', i replaced BOTH the sway bar 'z' end links AND the accumulators this past weekend (to the tune of $210), and i've still got the same 'kaboom!' as ever.

frustrated, i crawled underneath again today to confirm there's nothing leaking from the actuators/struts, and that the SLS pump was working...and i confirmed there's no leakage. i disconnected the SLS arm, and she will move up and down when you move the pivot...there's a pretty clear 'switch' feeling in the pivot when you change from 'up' to 'down', which i don't remember noticing on a previous wagon i owned...

anyway, i took in some slack on the arm, to get the ride height a little higher (as my euro hitch has been scraping coming in and out of the driveway), and i'd just "guessed" when i installed a new arm last month to replace the one that was totally missing. i tried to follow sherida's advice from another post

Quote:

"Normal" ride height is no passengers on level ground. You should be able to see the top of the rear tire when you look at it straight from the side (it should not be obscured by the fender).

and now when i measure from the ground to the wheel-center-point of the rear fender, i get 24.5" (as opposed to 26" for the front fenders), but am pretty close to the limit of what i can 'adjust out' of the arm...(as it's adjusted currently, it's pretty dang short).

would love to know WHY IN THE WORLD IS MY REAR SUSPENSION STILL SO STIFF AND PAINFUL? it seems to have NO shock absorbtion cababilities.
would also appreciate hearing measurements of all you wagon owners out there about the 'normal' measurements of your fender height!

Douglas.Sherida · #12 09-11-2007, 07:49 PM

Did you ever identify where the leak was?

The purpose of the SLS valve is to compensate for changes in ride height cause by increased cargo load by adjusting the overall length of the actuators. The vast majority of the time the SLS valve is in its closed position. It only opens when you increase or decrease the cargo load in the car (the valve opens far too slowly to respond to road bumps).

Having said that, you can look at the actuators and accumulators as a closed system while driving.

Hydraulic fluid is incompressible, nitrogen gas is compressible. Normally, the fluid in the actuators is able to flow out the top of the actuator through a hose into the accumulator. The accumulator is a metal sphere with a rubber diaphragm in the middle. The diaphragm separates the hydraulic fluid from the compressible gas (I believe it is nitrogen). When you hit a bump in the road, hydraulic fluid is forced out of the top of the ram (through the hose) and it pushes against the diaphragm, compressing the nitrogen. It is the compressible gas in the accumulator that absorbs the bump and acts as a shock dampener.

Harsh ride like you describe would seem to indicate that the actuators are "frozen" (i.e. they are unable to compress rapidly in response to road bumps). Off the top of my head I can think of three possible causes: 1) damage to the actuators that cause them to bind, 2) a kinked or blocked hose between the actuator and the accumulator and 3) no compressible gas in the accumulators (failed diaphragms or leaking cylinder).

kerry · #13 09-11-2007, 08:13 PM

Is it also possible that there is a problem in the control valve, not releasing the additional pressure put in the system from a heavy load? I'm not precisely sure how it works but it seems theoretically possible that the system thinks the load is still on the wagon and is producing too harsh a ride as a result.

blankenship · #14 09-11-2007, 08:15 PM

hey doug / all-
i never directly identified where the leak was (i only saw it do it once on that road trip), but i think it was coming directly from the valve. it hasn't dripped since, which makes me think the leak was due to the fact that there was a U-haul attached to the trailer hitch that was really loadin' her down, and may have forced valve leakage that wouldn't occur under any kind of normal conditions?

regarding the actuator / harsh ride condition: any ideas on how to TEST for damage / binding? the actuators are NOT leaking, and appear undamaged...i assume it can't be any fault of the accumulators, as it's behaving identically to it was prior to their replacement.

there's no visible damage to the lines. when i did the right side, a bunch of fluid drained out. when i rolled over and did the left side, there was none. i assumed it had all drained out when i did the first one?

Douglas.Sherida · #15 09-11-2007, 09:01 PM

First off, if it was leaking fluid from the valve it will be wet. Its an oil so it won't evaporate. If your valve body is damp, consider rebuilding it. I am hesitant to suggest rebuilding the valve because I'm the one selling valve rebuild kits, but if it leaks you should replace the o-rings.

Second, I'm not so sure about the assumption that cracking one accumulator line would drain both. Fairly easy to test: just open the lines in the opposite order than you did last time. If the one that didn't drain last time drains this time, and the one that drained last time doesn't drain this time then they are connected. If the one that didn't drain last time, still doesn't drain when you open it first, its a blocked line.

Finally, it has never been clear to me how the system bleeds itself. But you can facilitate the bleeding of air bubbles through the bleeder screw on the valve. With the wheels on the ground (or ramps) have an assistant start the car, flip the valve lever up to pump up the back end, turn off the car, crack the bleeder screw, repeat. This is the last step when I do the full system flush. Caution: the car should drop down to rest entirely on the springs when you open the bleeder, and if your springs are weak it could squish you! But the idea here (besides bleeding the system) is to see that you actuators respond smoothly allowing the rear to raise and lower.

Right now, my money would be on a blocked hose between actuator and accumulator.