1974 164E BW35.
I replaced the ignition switch because the fuel pump wasn't priming. The buzzer then sounded normally and not intermittently as it had been and the accessory lights came on as before and the fuel pump now would prime but I got no crank in position III. Nothing.
I wasted a whole day scratching my head, testing and swapping components and searching for something called a starter cut-out relay.
The next day I found a section in the shop manual that showed the location of the starter cut-out relay and mentioned something nearby called a seat belt interlock. Hmm. What's that?
It described how, if the front seats were occupied and the seat belts weren't fastened the car would not start. Oh? Really?
Go to car, fasten front seat belts, car starts immediately. :[ Only later did I get the back story on this device and the NHSTA. It was the 1970s...it was a different time.
Anyway, I had 'fixed' a number of things that I didn't even know were broken when I replaced the ignition switch and now they are 'working'. The problem now is that the buzzer never goes off even after the car starts. Ugh.
Disconnecting the seat belt interlock after starting the car will stop the buzzer but the car will not start again after it has been turned off unless I reconnect the seat belt interlock. I doubt I can find an occupant sensor device for this car so if anyone knows the best way to disable the seat belt interlock let me know.
Thank you