Protecting the magneto

Discuss all things Model T related.
Forum rules
If you need help logging in, or have question about how something works, use the Support forum located here Support Forum
Complete set of Forum Rules Forum Rules

Topic author
Jim Eubanks
Posts: 101
Joined: Mon Jan 07, 2019 6:04 pm
First Name: Jim
Last Name: Eubanks
Location: Powell, TN
MTFCA Number: 49847
MTFCI Number: 8340

Protecting the magneto

Post by Jim Eubanks » Thu Jul 23, 2020 6:47 pm

I would like to put a one way diode in the circuit from my mag to the switch. Has anyone done this and if so, what model and brand is recommended. My mag got shorted and killed and as of yet I have not found the cause, have checked the switch and wiring.

George Hand
Posts: 147
Joined: Sun Jan 06, 2019 12:02 pm
First Name: George
Last Name: Hand
Location: Preble NY
MTFCA Number: 28114
MTFCI Number: 21834

Re: Protecting the magneto

Post by George Hand » Thu Jul 23, 2020 6:58 pm

Jim, Well for starters the output voltage of the Model T Flywheel magneto is of the AC Varity, the rest of the Ford electrical system is DC. George

User avatar

Posts: 103
Joined: Sun Jan 06, 2019 6:57 pm
First Name: John
Last Name: Hunter
Location: Blue Mountains, Australia
Board Member Since: 2002

Re: Protecting the magneto

Post by JohnH » Thu Jul 23, 2020 7:28 pm

A diode would work in that manner....except that the magneto output would now be half wave rectified DC. I'd expect rough running with each negative half cycle of the AC waveform missing. Perhaps a low current fuse might be a better option.

User avatar

Posts: 199
Joined: Sun Jan 06, 2019 7:30 pm
First Name: Mike
Last Name: Kossor
Location: Kenilworth, NJ 07033
MTFCI Number: 22706

Re: Protecting the magneto

Post by MKossor » Fri Jul 24, 2020 2:18 am

Sorry to learn of your magneto woes. By shorted I presume you mean the magneto output post got shorted to ground and then there is no magneto output.

I'm not sure if shorting the magneto output to engine ground will result in demagnitization of the magnets but doubt it will damage the windings. Others may be able to comment on that further. If so, adding a fuse to limit the current to a safe value would be the solution.

Adding a diode will not limit the current to a safe value and may prevent optimal magneto operation as John pointed out; here is why. A diode in series with the magneto output will only limit the polarity of the magneto output to one of the two voltage polarities the magneto puts out. The magneto polarity changes direction every 22.5 degrees of crank shaft rotation. The magneto voltage polarity selected to operate the coils changes as you rotate the timer housing orientation with respect to the CAM by adjusting the spark rod. These iterations of magneto voltage polarity are sometimes referred to as nodes or pulses. The first pulse occurs when the spark rod is fully retarded. Lets assume it is a positive voltage pulse. The positive voltage pulse powering the coils from this timer position is too far retarded for good power. Advancing the spark lever to the next magneto voltage pulse (a negative polarity pulse) improves performance but still not opimum. Further advancing the spark lever advances the timer to utilize the third magneto voltage pulse (a positive pulse) which is is properly timed with piston position for optimal engine performance. Had you installed a diode with cathode (banded) lead connecting to the magneto post, you would not be able to hand crank the car on magneto (positive pulse spark lever retarded would be blocked by the diode). Once started, the engine would run mediocare advancing the spark rod to the second (Negative) magneto pulse but advancing the spark rod further to the third magneto pulse would not improve performance because it would be a Positive magneto pulse.blocked by the diode. The engine would stall at that spark rod position. Advancing the spark rod further still would again advance spark to use the next Negative magneto pulse the diode will conduct to operate the coils, however, this 4th magneto pulse is too far advanced for optimal engine performance. In summary, adding a diode in series with the magneto output restricts coil operation to one of the two magneto output polarities which may it be properly timed with piston position for optimal enginge performance.
I-Timer + ECCT Adjusted Coils = Best Model T Engine Performance Possible!

Posts: 772
Joined: Sun Jan 06, 2019 11:57 am
First Name: Adam
Last Name: Doleshal
Location: Wisconsin
MTFCA Number: 23809
MTFCI Number: 1
Board Member Since: 2000

Re: Protecting the magneto

Post by Adam » Fri Jul 24, 2020 10:09 am

What may discharge your magnets is if DC current from the battery is allowed to flow into the field coil. Whether the engine is running or not. This can be caused by a wiring fault, improperly connected wiring, poorly insulated wire harness ends touching each other on the ignition switch, or something as simple as removing the ignition switch back with the battery connected. One pulse of DC current to the field coil if the magnets are in the right particular orientation may be enough to substantially weaken the magneto output. This is a good reason why extreme care should be taken to insure that all the wiring is absolutely correct before the battery is connected. And to NEVER make any wiring adjustments or connections unless the battery is disconnected.

Post Reply Previous topicNext topic