I have seen several "Pin" back type Model T key switches with seven wire connection terminals on the rear as opposed to the usual six.
The additional seventh connection is marked "GRND"
Investigating the electrical internals of these switches reveals that when the key is in the "OFF" (center) position their is a connection between "COIL" terminal (blue with yellow tracer wire to the coilbox) and the "GRND" terminal. When the key is switched to the "BAT" or "MAG" position this connection is removed.
I have never seen a Model T wiring diagram showing a wire connected to the switch "GRND" terminal. I have ran the usual Model T electrical trap lines, some have seen this but no one has a clue about this function.
I was wondering if anyone had researched this at the BFRC and can share some knowledge as to the purpose of the "GRND terminal and its function?
Ron the Coilman
Could it be for an aftermarket magneto? That would have to be grounded out to kill the ignition, I would think.
Ron my take would be that the switches were being made by outside company's and the same switch could be used on several makes of cars, some of witch may have been setup from the factory to run a magneto. I wonder if the ground terminal is more prevalent in the pin or tab type switch?
This is a new area of interest for me.
Much work on the chronology of Model T switches (not to mention wiring looms)obviously need to be undertaken.
If I understand Ben Martin correctly, you only see the "GRND" connection on the "PIN" type switch backs? Larry (Original) Smith's reproductions also have this connection, but I have yet to speak with him.
To me, the magneto accommodation and the same switch being used on other cars sounds implausible? But, I have learned never say never with Model T's.
Ron the Coilman
The same method of grounding the ignition magneto is still used today in lawn tractor switches. When the key is moved to OFF, the ignition is grounded and the engine stops. A like method is used for lawn mower engines. The throttle lever moves in contact with an insulated tab that's wired to the magneto. When the throttle is moved to STOP, the lever makes contact with the tab and grounds the magneto.
The Model T switch's ground may have been designed as an antitheft device. It would keep (slow down) someone from hot-wiring the coil box.
We also discussed the "anti-theft" aspect.
Till we know more, that is the only explanation that makes any sense.
Ron the Coilman
It would be worth a little bit of investigation FWIW.
I'd tend to agree that the switch was probably originally generic.
I have also seen switches where the small buss link jumper on the back are different lengths...and then noticed that the labeled 'mag' and 'batt' screws were also reversed!
I recently upgraded my ammeter. It has a back lit Ford Script that requires a ground wire. Could the GRND be needed or used for this?
Ron, I believe that Original Smith still makes a reproduction Pin Type switch back with that Ground connection.
It was required to have a wire attached to the frame when running the engine with an external separate Magneto, as most of those units required the Ground to kill the Magneto output and stop the engine.
I have original Briggs and Stratton Pin Type back plates with the Ground Pin installed.
Scott - No. The GRND terminal on the switch is meant to have a wire going to ground--It is not a ground source.
Use the gauge or panel mounting screws for a ground source.
We are going to remove the ground terminal from any new pin type switch backs we make, since they never used them anyway, and it will save screws!
Larry, some people do use them yet and you are the only source for the authentic back plate.