Just completing restoration on my 1915 Roadster. Got a new battery, all new wiring, etc. Car starts and runs on magneto but will not start on battery. I checked my ground and all is good. When I turn it on battery at the switch the coils buzz as they should. Checked for voltage at the spark plugs and the voltmeter jumps all over the place. It has to be something simple that I am missing. I can't find a diagram that shows the wiring with the 12 volt battery on a non starter car. Any ideas?
What voltage do you get at the primary winding of the coils? If they buzz, they should fire. The voltage you need to test is the buss which goes across the coil box and is connected to all the coils. If you are on a 12 volt battery, that voltage should be 12 volts. Since they do work on magneto, there is nothing wrong with your secondary wiring. Note, be careful not to connect the battery to the magneto. The switch should connect to either the battery or the magneto, but not both at the same switch position.
Here is the wiring diagram for your 1915. It shows the battery plus (+) connection marked "BATTERY TERMINAL" right next to the coil box. It assumes that you know when installing the battery you need to hook the negative (-) terminal to the frame.
You need to measure magneto voltage, and have the coils adjusted properly on a hand cranked coil tester.
I'd be looking for a loose connection somewhere from the battery to the coil box, or perhaps a wire that is just terminated to the wrong place. I imagine another potential culprit could be the coil box switch itself.
What happens if you start on mag and then switch to battery while the engines running ?
If the coil box switch is bad would I still have coils buzzing when I switch it over ? Is the neg from the battery is to the frame and I checked continuity from the neg on the battery to the base of the spark plugs and I am O.K.