Farmall A convertion to 12V negative ground

dhabeck · Feb 2, 2015

I am really in need of some help with a conversion to 12 volt on a Farmall A. I am struggling with the coil and distributor. The 6 volt system had a coil which had the + going to the condenser connection of the magneto. The magneto's output is not being used. Now, with the 12 volt coil, I have the + going to a power source and the - going to the condenser connection. Do I need to switch out to a completely new distributor, or can I get this to work.

Hayfarmer · Feb 2, 2015

There is a really good wiring diagram in archives by Bob M. I am a little confused by your talking about a magneto and also a distributor must be one or the other.

Bob · Feb 2, 2015

What you are doing has been done many times and does work.

The primary of the coil INSIDE the mag needs to be disconnected from the points and condenser (which presumably has already been done).

And you are correct in the power being connected to the (+) side of the coil.

You also need a "true" 12 Volt coil, or a ballast resistor ahead of a 6 Volt coil.

dhabeck · Feb 2, 2015

I think that is was is confusing me also. There is a distributor cap on the unit, but the center connection came from the coil. The magneto center connection is there, but not connected. When I got the conversion kit, they said to connect the - of the 12 volt coil to the distributor. I have no wire coming out of the distributor part.

dhabeck · Feb 2, 2015

Thank you Bob. I thought that was correct. I was hesitant so I just turned the switch on and touched the battery and I had a bad draw. Does that sound correct?

dhabeck · Feb 2, 2015

I have added a picture for clarification.

Look right?

Bob · Feb 2, 2015

The ignition current through the coil (with the breaker points closed) should be in the 3 to 4 Amp range.

More draw than that would indicate you are using a coil with low primary resistance that needs to be used with a ballast resistor.

Bob · Feb 2, 2015

The "wire coming out of the distributor part", in this case, is the screw terminal to which you have correctly attached the primary wire from the coil.

Bob · Feb 2, 2015

Also, once again, did you verify that the original, now abandoned INTERNAL coil is disconnected from the condenser terminal?

dhabeck · Feb 2, 2015

Is there an easy way to check to see if that was done?

Bob · Feb 2, 2015

Take the upper bakelite coil cover off, and find the two coil primary wires. One was originally grounded, and the other went over to the condenser. The lead going to the condenser should be disconnected (or even cut).

dhabeck · Feb 2, 2015

I will check. Also I seem to have much more than 3 or 4 amps with just the switch on. Be right back.

dhabeck · Feb 2, 2015

I took a picture. am I in the right place I don't see any wires.

teddy52food · Feb 2, 2015

Looks good. Just add a resistor to the wire between the switch and the coil.

dhabeck · Feb 2, 2015

Any recommendations on a resistor. I did get a 12 volt coil with my conversion kit. Does it sound right I should need a resistor? My multimeter climbed to like 7 - 8 amps before I took it off.

Bob · Feb 2, 2015

The magneto's coil has been totally removed, hence no wires to bother with. Teddy's got you covered on the rest.

David G · Feb 2, 2015

Would this unit have spark advance, being gutted like it was?

Bob · Feb 2, 2015

Yes, the same as a magneto, TDC at cranking with impulse operating, full advance as it comes up to idle speed.

dhabeck · Feb 2, 2015

Bob,

Do I just need to get a standard ballast resistor. The coil was supposed to have an internal resistor.

Janicholson · Feb 2, 2015

Coils do not have internal resistors (there are examples, but not normal parts store reality) 12v coils are just wound with more and finer wire to run on the 14.2 volts normally found in 12v systema when running. If yours is marked "no external resistor required, it should have about 3.5 to 5 ohms of resistence ((disconnected from the points)
if lower, like 2.5 ohms or so, it is in need of a resistor. A 66 chevy ballast resistor will be fine. Wire it into the supply side of the coil from the ignition switch. Jim

Farmall A convertion to 12V negative ground

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