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Magneto Troubleshooting help

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1923 Depot Hack with a 1926 engine and who knows what else. 

  • Car starts and runs fine on battery
  • Switching to magneto yields unpredictable results. Sometimes the switch from battery is seamless and car runs fine albeit with a slight increase in RPMs. At other times, the switch to magneto power results in marked roughness in the motor. Missing and rough to the point of stalling. 
  • Voltage measured at magneto post yields about 8.5 VAC at low idle. Volts increase commensurate with engine speed.
  • Cleaned/Tightened all wiring connections at magneto post, terminal block and back of switch.
  • Removed and cleaned the magneto post.
  • While under magneto power, I have tried wriggling the key switch, wires and connections with no effect.


So as a Model T and magneto newbie, I have some questions:

  1. I have not been able to find a good reference regarding the post. Should the contact point be sharp and needle like?
  2. Is there any guidance for making sure that everything regarding the post is set up properly? 
  3. Assuming that the output of the magneto is good, what are the other areas that could possibly cause these symptoms? 
  4. Am I correct in thinking that since everything works fine on battery, the problem is somewhere between the magneto post and the switch?
  5. Other than checking voltage output of the magneto, is there anything else I can measure or observe without tearing things apart? 


Any help is much appreciated,



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If the mag voltage stays consistent while the engine is running you have a switch or coil box problem. You can watch the mag output while switching over from battery to see if it cuts out or anything out of the ordinary. 

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  • 1 month later...

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