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Old 07-08-2005, 10:31 AM
syljua's Avatar
C140 CL500
Join Date: Jan 2005
Location: Norway
Posts: 132
Question W140 - MCS (lumbar) - no pressure from PSE


I have a 1998 S320 where I cannot get the lumbar support to
function. Since this has not ever functioned in my possesion of
the car, I do not know if the rest of the system after the PSE pump is faulty
or not (bladders in seats, switches, lines and reservoir in trunk).

My problem is, that I do not get the PSE to pump on that line at all.

All other functions of the PSE, like Central locking, head rest release,
trunk handle retract, A/C Rest supply works fine.

I have even dismanteled the PSE pump (great device:-)!)
to see if there was any obvious faults like stuck piston, etc;
(click on it to get a larger one)

This is a more closeup of the switch for the MCS (lumbar) pressure.
When pressure is reached, the piston slide down and opens the contact
under det disc;

There seems to be nothing wrong with the mechanical operation
of that switch, but like I said, I cannot get the pump to operate
at all. I know that the PSE is connected to the diagnostics module,
but sould not the error code (if the problem is a error code) be gone
after I have disconnected the battery? Or is the diagnostic module
telling the PSE pump not to operate the lumbar? Do I have to get
a dealer to enable it?

I think the lumbar pump should be active 4 seconds after the engine is
turned on? Or after ignition is turned on? As I can see, no other electrical
switches regulate the lumbar?

W140 S320 1998

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Old 07-08-2005, 08:01 PM
Registered User
Join Date: Oct 2001
Location: around Charlotte NC
Posts: 578
try the fuse to reset


I believe that pump learns about each circuit, either vacuum or pressure. If it cannot acheive its goal in some time period ( not sure how long) it will mark that circuit as inactive. I have heard if you take out the fuse ( #9 in the trunk fusebox) for 30 or so seconds, it will reset the disable.

THe longer term question is why didn't it work in the allotted time. That is sometimes a weak pump, or it could be other causes like a leak.


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