Could this be my problem? When I bought the car no matter which way you turn the switch it was always on battery. So I decided to see what might be the problem. If the magneto had been bad I see no reason to wire the switch the way they did. Now I will order a new magneto post and go from there.
Just make sure that you don't ever have the magneto and battery both connected together since that will harm your magneto in an instant.
Check the wiring in the coil box to be certain they did not wire the bottom contact strip directly to the battery contact. It should be wired so that the bottom strip is connected to whichever side you turn the key - MAG or BAT. As John said, you do not ever want to connect your battery directly to the magneto as that will demagnetize the magnets.
One reason to wire the switch that way is to make a sale easier to a non-suspecting new T owner that has heard he should always make sure the Magneto functions.
A second reason is if you have a transistor ignition system, such as TrueFire that can be destroyed by the higher Magneto AC Voltage, the system is totally protected from damage.
In that case you remove the red wire that provides the Mag voltage to the ignition switch from the wire bundle that goes through the firewall and move it over to the yellow wire that provides the battery voltage to the ignition and light switch. Then either way you position the switch, you are still running on Bat.
Thanks to everyone for responding to my problem. I'm in the process of putting in a new wiring harness. Hopefully when it is all back together everything will be fine. If not I will probably be back on here with more questions.
Maybe someone can help with my mag problem. Car runs fine on battery. When I switch to mag car runs just a few seconds and quits. When running on battery at fast idle mag puts out good 20 volts. Have been thru starter switch. Have continuity between mag post and coil post with switch turned to mag. Wiring is new and installed according to wiring chart. What do I check next? Stumped.
Did you check the voltage using the John Regan 1156 bulb test setup or just use a meter? You need to load the magneto with the 1156 bulb to get a true picture of the magneto output. Unloaded they may look good but collapse under load. Your magnets might need a recharge depending on the results.
Make sure you do not use a digital type meter when taking AC readings on your magneto and as Mr. Mullin said correctly - without a load there is no accurate measurement and you can be fooled into thinking that the mag is OK when it isn't. Make sure you have well over 20V under load at high RPM. Starter switch condition has nothing to do with running on magneto. When you say you have continuity between mag post and coil post when you are switched to Mag - I am not certain what you mean since that can have different meaning depending on where the switch is which depends on the year of your T. For the record it is always best when posting to tell us the exact year and body style of your T as a minimum. Exactly what do you mean by "coil post" as I am assuming you have an early car with a mag post and coil post both on the back of the coil box. Some folks refer to the internal magneto post connection as the magneto coil post.