HYDROPNEUMATICSTRUTS:
Thefour-hydropnematicstruts that arepart of the450SEL’s 6.9 suspensionare an itemthathas a
lifespan of approximately10 years or100,000 miles. The strutskey failure areasare the nylon
sealsand the 70durometer rubber o-rings. Theseitems fail dueto heat andcontamination. As
thestrutmovesup and down tremendousheatis generated and therubbero-rings are literally
bakedandunbaked. Overtime these o-ringsloose there elasticityandstart to collapse,thus
causingtheoil leaks. Theoilleaks show upin two ways;externaland internal oilleaks. The
externaloil leaks arevisible at thebottom of thestrut. Internal oil leaksare distributed back
throughthereturn tube (D2) ndinto the reservoir tank. Thisleak is less apparentbutcan be
measuredbyremoving the D2 lineconnection, plastic tube, fromthe strut. A measurement of
2cc’s in a 4 hour period or 6cc’s in 12 hours. For simplicity, if you get a considerable amount of
fluidinashort time the strutsareleaking and should bereplacedto prevent further problems.
Contaminationinthestrut occurs from therubberboots being torn andexposedto the elements.
Thenylonseals(wipers) trap the roaddebristhat enters the strut. Sincethegap between the strut
cylinderisinfintesimal,very little dirt isexposedto the seal. The mainculpritis time and eventually
thesealswereout. The rubber bootsonthe struts should neverbecomprised. Inspect and wash
themregularly,replaceiftheyaretorn. This will improve the longevity of the strut. The struts can
berebuilt.
ACCUMULATORS:
Theaccumulatorsactas the coil springforthe car,inthatthey contain the nitrogenchargeto
supplementthestruts. Theaccumulator life span isthesame as the struts. Thereare2 types of
accumulators used: Front ones are charged at 75 bars, red color. Rear ones are at 60 bar and
coloredblue. Themain failure isthat the rubberbladderthat separates theoil and thenitrogen
collapseand the nitrogenis displaced in the oil. Theride quality becomeshard or bouncy and all
theaccumulatorsshouldbechanged. I do not recommendtestingtheaccumulatorssince they
canbepurchasedinexpensively. The accumulators can notberebuilt.
HYDRAULICVALVESANDPRESSUREREGULATOR.
Theslefleveling valves again havethe similar fate ofthe struts, in thattheo rings faildueto age.
Longevitycan be upto 10-20 yearssince the o~ringsused are verysmall. Heat isnot a factorin
therear valve sinceit is adequatelycooled but thefront valve isexposed to engineheat and fails
moreoftenthantherear valve. The valves canbecheckedforexternal leaks by avisualcheckof
hydraulicoilleaking from theshaft area. As forinternalleaks Mercedes recommendsattaching a
plasticlineto the (d1)or marked Ronthe valve andmeasuring the fluidloss. Ifthe leaks are8cc’s
in4hours or 24cc’s in 12hoursthen the valveshould be replaced. Thisisonly true ifis leaking
onlythroughthereturnport.Anyother internal leaks can not bediagnosedandifthevalve is giving
problemthenit should be replacedaccordingly. The pressure regulator,which sits onthetop of
thesupplytank,canonlybechecked for external leaks. The valves canberebuilt.
HYDRAULICPUMP:
Thehydraulicpump is a radialpiston pump. The maximumdelivery pressure for thepumpis 200
barsat 2000 rpm. Thepump volume can be checkedand should yield.2 liters in 30 secondsand
1000rpm. Veryrarely do the pistons fail since the bore and pistons are lubricated with hydraulic
fluid. Ifthetankruns dry these componentscanbe damaged. The main failureinthis pump is the
rearradial seal. When thisfails the pumpleaks hydraulic oilback into theengine oil. This can
dilutethe engine oiland cause damage.Mercedes has addeda suspension warninglight in the
dashand strict attentionmust be paidto this. 6
