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Old 01-02-2010, 03:28 PM
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Join Date: Mar 2009
Location: San Diego
Posts: 2,985
Question 300SDL door locks fail to lock/unlock now and then.

My 300SDL door lock does not lock/unlock 100% all the time.

Front Pass side door fails to lock now and then
Driver door fails to lock now and then
Rear Driver side door fails to unlock now and then

They do not happen all 3 at once, it is kind of random. Probably 1 door at a time. It is unlikely to have 3 sticking actuators?? I suspect it is the pump. Any ideas?


Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details.

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.
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Old 01-02-2010, 03:54 PM
Crazy_Nate's Avatar
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Join Date: Jan 2008
Location: Hampton Roads
Posts: 567
The 300SDL has an electric pump below the rear seat for the central locking, right? It seems that you'll have disassembly no matter where you start troubleshooting.

Since I don't have a W126 manual, I can try to provide some useful info for the similar system in the W124. The W124 (300D) spec for maximum time to unlock or lock for any actuator is 3 seconds from any point being unlocked (driver's door, passenger's door, trunk). During this testing, the operation of the supply pump should be listened to. If the pump is operating, first, test the system for leaks, second, test the pneumatic function of the actuator. If the pump isn't operating, the electrical portion of the system is tested first, then the electrical operation of the actuators.

Again, for the W124 300D, the spec is > 420 millibar after 3 seconds from unlocking (pressure) or locking (vacuum). There also is a spec for "safety switching time" after the pump is disconnected from the "line" system (off after 25-150 seconds) upon operation of the central locking system.

Hope this helps. It sounds like it could be the pump...but you'll have to find the tests for the SDL to really figure out what it could be (the specs might be similar ). I'm curious to see how close the systems between the W126 and W124 are...

1982 240D, sold 9/17/2008
1987 300D Turbo
W124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting
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