Meade,
If the bounce is SOFT and/or the car is sagging, your base pressure
is low. When the internal spring weakens in the base pressure
component of the levelling valve, base pressure is allowed to escape,
which in turn leads to a SOFT bounce. The relevant, poorly translated
passage from Chassis job 32-501:
"Oil pressure in the spring struts should not drop below a given
basic pressure, so that the spring struts can fully meet their
additional jobs of shock absorbers acting independent of positions of
level controller. Making sure of that basic pressure is attained by
means of a compression spring which pushes the return flow valve in
level controller out of reach of control disc when the available
pressure is below a given mark."
Essentially, the damping effect that a shock absorber usually
provides is achieved by constraining the movement of hydraulic fluid
within the STRUT. This can only be achieved if there is sufficient
base pressure to ensure that there's no cavitation around the flow
restrictors. The good news is that the restrictors don't wear out!
If (and only if) the bounce is HARD or DIAGONAL (one side harder) do
you need new spheres/accumulators. New spheres do not (and cannot)
solve sag or soft bounce, except by pure coincidence. Random
component replacement occasionally disturbs and thereby corrects the
actual cause of the trouble, but it can be an expensive methodology!
In case someone suggests replacing the struts: the failure mode for a
strut is external fluid leakage (MB gives a permissible leak rate);
they don't "wear out" or cause sagging. Cars sag because of a bad
base pressure valve OR worn subframe mounts, the latter causing far
less sag than the former. If the base pressure valve is not holding,
the reason is probably that the spheres (MB calls them
"accumulators") were low on nitrogen, causing frequent system shock
and leading to component failure. Spheres without gas are like using
a solid bar instead of a spring. Something has to give when you hit a
bump, and it will be the strut mounts, strut seals, and level control
valve components that take the hit.
To quote from 32-520, "Checking pressure reservoirs":
"Checking the gas filling pressure of pressure reservoir is required
whenever suspension is getting harder.
"The gas filling pressure decreases during operation, the time factor
is evaluated higher than the mileage. Under normal conditions, the
minimum pressure is attained after approx. 2 to 3 years or 80,000 to
120,000 km. Under extreme climatic conditions or in countries with
very high outside temperatures, the gas pressure may decrease faster
under given operating conditions, e.g. repeated slow bumper to bumper
driving.
Five years seems a little early for total sphere failure, but perhaps
the PO installed used ones or wasn't clear that they'd been done a
few years earlier. In any case, if they're shot and you do not
replace the spheres, you'll be replacing struts, strut mounts, and
levelling valves fairly regularly. When one side sags this can also
be an indicator that a sphere has failed (because of the differing
rate at which nitrogen's density changes under pressure), although it
could also be a collapsed rubber strut top mount.
Since you've already noted that one side is harder than the other,
I'd bet you're due for spheres. In that case it makes sense that,
when loaded, the bounce seemed even worse. The trailer weight would
have caused the system to add pressure to the struts to raise the
car. The added pressure in a system with less "give" because of low
gas pressure, plus hitting a bump, might have triggered the
overpressure release valve, causing you to lose any damping (and
height!) in the suspension.
Since you say the car sags overnight, your levelling valve definitely
needs to be rebuilt. If the car seems harder on one side than the
other, it's probably time to replace the spheres too (don't try to do
just one side; the pressures will not be the same), but I'd start
with the valve and see what happens.
Does the system react when you add load (200+lbs) to the back of the
car? What happens when you manually activate the level control valve
(disconnect the ball rod and move the lever)?
D.
Meade Dillon wrote:
Dieselvolk,
Well, this past weekend I towed my sailboat back and forth from
storage to boat shop to house to boat ramp, and found the experience
most unpleasant. Lots of bouncing, tongue of trailer yanking on the
hitch like a wild animal trying to escape, having to drive real slow
on the rougher sections of road, stopping several times to try to
shift more weight onto the tongue to see if that would fix it.
So, it took me three days to realize it, but I've finally decided
that there must be a problem with the Self Leveling System (SLS) for
the rear axle. I did the bounce test, and the RR corner seems to
bounce easier than the LR. Most of the time the rear of the car is
at the correct height when driving, but when loaded with the trailer
that only occcured some of the time. Also, she leaks down and sags
a little most of the time.
My first guess is that it's time to rebuild the control valve. The
valve in the car now came from a '79 donor, cost was all of $10 last
summer. It worked well until recently, when the car started sagging
when parked, not always but enough to bug me. Does it make sense
that a failed control valve would cause one side to be weaker than
the other? Supposedly the nitrogen spheres were replaced by the PO
about 5 years ago, I wonder if one of those has failed, which in
turn pushed the control valve over the edge? Hmmmm, last rebuild
of the original control valve also failed after ~year or less.
Hmmmm....
Very respectfully,
/s/
LCDR Meade M. Dillon, USNR
Digest Lurker since 2001
'85 300TD 322k miles (Euro 5spd)
'96 Infiniti I30 149k miles (wife's 5spd)
'73 Balboa 20 'Sanctification'
Charleston SC
