Farmall M Electrical Issues

Braxton · Jul 25, 2009

We've had this 47 Farmall M for 20 years. It ran perfect until this year. The tractor does have the original 6 volt system I assume but without the generator and any lights. We run it with a 12 volt battery. As a result of some electrical issues, we replaced everything. It got a new starter, ignition switch, starter switch, battery cables, points, condenser, rotor, plug wires, coil, and new wiring. Ran great for a while until the coil went bad. Was very, very hot to the touch. Put the original coil back on and it ran fine again. Now it won't run yet again and the coil appears bad. I figure I must of rewired the system wrong.

My question is: What would the wiring schematic be for a 6 volt system running a 12v battery without a generator. Thanks guys.

Braxton · Jul 25, 2009

The tractor is 12v negative ground if that helps.

Anonymous-0 · Jul 25, 2009

The problem is that you are running a 6 volt coil with a 12 volt battery without a ballast resistor. If you are going to use a 12 volt battery you have two options: 1) A 6 volt coil with a ballast resistor or 2) A 12 volt coil without a resistor.

Dave S. · Jul 25, 2009

Get a 12 volt coil (make sure it is a 12 volt coil to be used without a ballast resistor, it should be printed on the coil) and hook the wire from the - side of coil to the distributor if you have negative ground.

Using the 12 volt coil with internal resistor will eliminate the external resistor and one more thing to go wrong.

John T · Jul 25, 2009

You say its now 12 volts, first thing that comes to mind is you are still using a 6 volt coil !!!!!! If so it will run very hot and eventually burn up and open becaase at 12 volts its drawing twice its intended current, in which case the points will also burn up prematurely.

The easiest thing would be to buy a full true 12 volt coil (NO ballast required) and use it. Of course, you could use a 6 volt coil plus an in line series voltage dropping (12 to 6) Ballast Resistor, but why do all that and have the extra expense plus extra connections to go bad UNLESS you rig a ballast by pass system for starting only which yields a good hot starting spark.

NOTE the coil is also somewhat polarity sensitive, so at Neg groudn wire its lil + terminal to the ignition switch and its other lil - to the distributor.

John T

Braxton · Jul 25, 2009

Thanks guys. I'm going to try out your suggestions. Will check back later and tell ya how it went.

Farmall M Electrical Issues

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