300sdl, 1987. I opened trunk w/ key the other morning b4 work. (5:30 am on driveway, neighbors love me) and set off the alarm. Tried to reset by locking and unlocking door and trunk to no avail. Had to raise hood and pull fuse. Drove to work with misc. electrical items not working, but overhead seatbelt red light flashing annoyingly. Searched forum for causes. Replaced vac pump in trunk as it was first suspect. no help. Ok back to square one. Trunk lock was suspect, along with door switches that set off alarm. I tested door switches..no help both good. Trunk lock vac. unit was not hard to remove. Tested all three wires w/ ohm meter and activated lock by vacumn and pressure..appeared to work. Hmmm. Went into garage and got spare vac pod that I had scavenged to compare. Three leads tested same, but vac unit was alot easier to activate..ie push/pull on actuating rod was alot easier. Hmmm. I had noticed that in the past the trunk lock was kind of stiff to operate, but it has been for a long time. Replaced vac pod in trunk w/ spare. YEA! trunk now locks/unlocks very easily AND the alarm is no longer a threat..working like factory!
Hope this helps others ...check all vac units for smooth operation B4 condeming alarm unit!

I'm glad I live a few miles away. Thanks for the tip. I guess you have the Memphis area spare trunk vacuum pump, now, correct?

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daw_two
Germantown, TN

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Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
Cluster Needles Paint
New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more