Date registered: Aug 2009
Vehicle: '72 250C, '74 280C, '85 500SEL, '81 300CD.
Location: Tucson, Az.
Mentioned: 3 Post(s)
Quoted: 51 Post(s)
If this is on your '73 and its a late year date (manufactured after October or so) another possibility is the seat belt safety system. It is a relay which doesn't allow the power to flow to the solenoid if the seat belt sensor isn't engaged. It was only used one year which was late '73 though mid '74. Its located by the park brake mechanism under the dash.
I had this on my '74 W114 and it drove me bonkers. It would occur at random times with no way to predict it. Doing the neutral to park shift trick worked most of the time but not always. Pulling apart and cleaning the relay did the trick but I ended up by-passing the relay to remove it from the system.