MTFCA Forum

1925 runabout - Does it make sense that there's continuity between the mag, coil, battery, and light terminals on the back of the switch when the key is on mag and the light switch is on (engine off)? I checked it all about a million times, but I want to make sure things are right to prevent a magneto discharge. Everything seems wired correctly, and all of the continuities make sense in various other configurations. Back to the question - it seems like this might be the case when the key and light switch are in these positions since the headlight terminals are grounded through the bulb filaments, the battery terminal is connected to the headlight terminals (light switch on), the coil terminal is connected to ground (I think?), and the mag terminal is connected to the coil terminal (key on mag). Does this make sense? With the light switch on and the key off, resistance between the light terminals are battery terminal are very low, and there is continuity to the coil terminal with a resistance of about 167 ohms. Turning the key on to mag adds the mag terminal into the circuit with the same roughly 167 ohms between the coil/mag and the lights/bat.

I guess another way to put the question would be, does it make sense that with the key off and the lights on, that there would be about 167 ohms of resistance between the bat/light terminals and the coil terminal?

I have the neg battery post unhooked, if it makes difference to your thinking.

Here's a question: If you have your lights on, the key off, and the battery negative disconnected....How do you have lights?

Compare your wiring to the diagram - you may have the jumper as described in the red box

BIll, I don't have lights during this test, just continuity though the filaments to ground. TRDxB2, I don't have the jumper. It does have something to do with the coils, as is only does it if the timer is at a contact point. It seems like everything would be tied to a common ground when the timer is grounded at any of the four locations. I'm guessing the resistance I'm seeing has something to do with the resistance of the primary winding in the coil and/or capacitor playing a part. I don't need to fully understand, as that might be a stretch, but I am hoping someone can confirm this logic. I don't quite understand how the battery doesn't affect the mag if this is truly the case, though.

It would help to know exactly how you wired your T. Is it like this? This diagram also implies that the headlights (dim & bright) only work off the battery. What do you get if you disconnect the wire from X to W - do you still get continuity as before. Frank

Thanks, Frank. Yes, it's like that with the exception that my generator is self-grounded with a jumper right now and "T" is unconnected and insulated. Disconnecting at "W" makes an open between KLOR and X. Also, taking the coils out does the dame thing. That's why I'm thinking there's continuity through the coils to ground when the timer rotor/roller happens to be grounded. That would make the headlamp wires and coils both tied to ground, and therefore show continuity between KLOR and X when the light switch is on and the timer rotor/roller is grounded. If this isn't supposed to be the case, perhaps I have a coil short problem. My scenario would indicate that there's continuity between "W" and the timer side (top row) of the "1 2 3 4" coil box terminals.

I confirmed with several coils that there is continuity across the side upper button and bottom button, so that confirms that "X" would be grounded if the timer rotor/roller is in contact. I also confirmed that disconnecting all three light bulbs made the continuity between "KLOR" and ""X" go away; of course, turning the light switch off makes this continuity go away, too. So, I do think "KLOR-X" are tied together through ground when the light switch is on. Then if you have the key on mag, the mag is tied in, too, because of "X-Z". Note that "L" and "O" aren't tied to each other in the switch, either, but rather by the same issue of continuity between then through a common ground.

Maybe you tried this already, With continuity as you described - pull one coil out at a time and test for continuity.

I think I did that, and the continuity went away if you pulled out the coil that was making contact through the timer. I've convinced myself that this all makes sense because of everything being tied together by a common ground. I was kind of hoping someone more electrical than me would confirm, though.

All systems on the T share the same ground/earth. There is no separate ground for the magneto (AC) and battery/generator (DC). I have wondered about this a time or two, but it works so didn't worry about it beyond that.

Thanks, Mark.