I thought I'd pass this along in case it happened to somebody else.On my 86 300sl with a m-103 motor. The tach quit working but only when the hood was closed! I finally discovered that the driver's side hood spring was rubbing on the wires going to the round unscrewable cap that has alot of female pin connectors in it[don't know what it's called] and somehow shorting it out.I isolated the wires and now it's working again even with the hood closed. Greg.

Good detective work!

It is always something simple, it is finding that simple thing which takes so much time.