The auto-closing trunk mechanism on my W140 Chassis 93 SEL500 isn't working any more. The trunk will close and stay shut, but gone is the ability to lightly close it, and then have the mechanism slowly suck it all the way shut.

I got in there thinking the electric motor went bad or something only to find out there is no electric motor, all pneumatically operated. I did find one little switch in the setup that I'm thinking may have gone bad. All the other doors work with the auto-shut function so I don't think the pneumatic vaccum/compressor system is failing which is why I think it may be the switch.

Can anybody confirm that the switch would be causing the problem or that maybe its something else with a history of problems?

Remove fuses in trunk and reinstall and see if it works . By doing this you reset the system. if it works get back to me as this is not the cure.
good luck

do a search for trunk closing,trunk vacuum motor.there is a retro-fit out there involving an upgrade to the motor in the trunk

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'95 s500 (bought for wife but can't bear to share!!!) 125kms
'92 legend 180kms
'88 tbirdturbo(fantastic car-only regular maint.)120kms
'87 mustang gt(gone)
'86 tbirdturbo(gone)
'85 mustang gt(gone-but not forgotten)

You might want to reset the Central locking pump..Its in the L/R quarter panel behing the carpet above the wheel well in a foam pouch..Unplug it and retest the operation.. Listen for a Hissing sound and or listen to hear the pump running for a extended length of time if so you have a leak!