I need some advice in troubleshooting a magneto problem on my TT. I notice when reading the different threads that many people talk about running their T using the magneto. I have never tried this on my truck and thought I would give it a try this weekend. Well, it doesn't work at all when using the magneto. It won't start or run once I switch it over to mag. It run just fine using the battery. I searched the past forumns for magneto problems to see if I could get an answer to problem. I read a forumn from 2008 that indicated that I should read 1/4 to 1/2 Ohm of resistance between the magneto post to ground with the engine off. I'm reading approximately 3000 Ohms. Does this mean the magneto is bad? What would be the next step in troubleshooting this problem? I didn't brother checking the AC output of the magneto since I have high resistance with the engine off. Should I check to see if I have AC output? Is it worthing fixing the problem right now or am I better off just running of the battery?
If you have 3000 ohms, your loaded output will be nada - zip - next to nothing.
First check the magneto post for lint. While you have the mag post out, check the solder button on the field coil to ground.
If still high resistance, it sounds to me like the field coil is close to open-circuit.
I did take the magneto post out and found some lint on the end of it but not that much. I measure the resistance from the bottom of the spring/coil that touches the solder button. It looked good as the resistance was only .1 Ohm. I measured from the solder button to ground and it was still around 3000 Ohms. Though, I had no idea what it was called but I figured by the way the magmeto posted made contact with it that it should be reading the same. If I measure the side of the opening for the magneto post to ground it reads .2 Ohms. I'm assuming if the magneto circuit is open that will require a new magneto?
Most likely, your field coil has a broken wire or a bad solder joint, or perhaps both. The reading from the button to ground should be very low resistance, almost but not quite a dead short.
Randall Anderson is correct.
You have a defective magneto field winding!
The next step is to rebuild the magneto which is a big task. You may want to power the coils (only)with a 12 volt motorcycle battery so you can enjoy the car till you completely rebuild the engine.
Ron the Coilman
Seth was correct, also.
R.V. - Thanks, unfortunately that is what I thought might be the problem. Looks like I now have a reason to rebuild the spare motor in the garage. Not sure if the wife will agree but it's a reason none the less...
Ron - quick question for you in regards to the 12 volt motorcycle battery. I'm guessing that the 12 volts from the motorcycle battery will provide a better "spark" to the motor than the 6 volts from the battery. Would it running better than using the 6 V battery? Also, is it better for the coils to see the higher voltage?
From what i have read on the internet it is not good for the coils to see higher voltage for longer than a few seconds at a time as they get hot and after a minute or so of continous sparking the tar will melt so the engine should be running or at least turning over at the time
We prepared an article on this subject a few years ago. More On Model T Spark Timing
If you operate Model T coils on 12 volts you do have to be careful not to leave the coil buzzing continually with the engine not running to avoid overheating. This can also happen on 6 volts, but on 12 volts because of the significantly increased power it can happen much faster. Worse yet if this happens tar will boil out the top of the coil and jam the points closed and then the coil has 6 or 12 volts directly connected to the coil. In the later case 40 amps. Obviously a very dangerous situation.
Running the engine on 12 volts will not cause any coil trouble because the intermittent timer connection significantly reduces the duty cycle and the coil will not overheat.
Hope this helps.
Ron the Coilman