I have a spare push button unit (PBU) that was from a 190 I no longer have. So I took it apart for practice. I can see a couple cold solder joints in this unit.

It makes sense that the rapid clicking sound is a relay that doesn't fully engage, and is controlled by the PBU.

That means the problem is either the relay, or the PBU. I'd like to find the relay and test it before I remove and tear into the PBU.

Any suggestions on the relay(s) location?

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1st MBz: 1982 300SD
2nd MBz: 1987 300SDL
3rd MBz: 1995 S420
4th MBz: 1987 190DT
5th MBz: 1984 300SD w/1983 300DT engine
6th MBz: 1999 C230k I'm 3rd owner, got it w/57,235 miles.
and manages Mom's 2007 R320 CDI