M electrical issues

appraiser

Member
My 51 M is typical in the fact it has 12V battery on a 6V system. It uses the usual rectifier hooked into wiring. It will start and run fine usually. If I don't run it for several months it will not start. If I hold the starter a little too long I will start to see smoke coming from the wiring near battery. What am I missing here? Do I need to put in a complete wiring harness? Why does it start easy and run fine if started frequently but act up if sits for several months? Usually you have to replace the rectifier.
 
I will address the items as best I can. (Not a flame at all) The M Farmall 12v or 6v has no rectifier in the wiring. If it has an alternator replacing the original 6v generator, it will have diodes (rectifiers) built into the alternator and its (usually internal) regulator. If it has a generator, it will have no rectifiers. If it has a "one wire" type alternator, it can cause battery drain as they are always "on".
The smoke issue is a bad connection in the thick wires from the battery to the starter, or their end connection, or a bad ground connection.
So::: We need to know what the generator or alternator looks like. An alternator is aluminum looking and usually not painted. A generator is more like a 3.5"X 7 inch cylinder. Painted, not bare metal. A one wire alternator will have only one wire (10 gauge) hooked to it. A generator will be hooked to a regulator with 2 wires. (old alternators may have external regulators as well, but less likely. Tell us more about what you have. Jim
 
A "typical" rectifier is an AC to DC converter, to chop the sine wave down to a more constant voltage.

So with all DC wiring, the tractor would not need it as a discrete component.

The alternator, as a self-contained device, however, generates AC "internally" and then uses a diode pack to bring this down to a DC voltage that's outputted thru the large stud.

I agree that any smoking issue is caused by a bad conductor or a bad connection. Typically with 12V the battery cables can be downsized from the 6v originals if you have to buy new.

I used to have issues with "green stuff" - aka some corrosion building up underneath the distributor cap on the metal poles and sometimes the rotor's flat bar when I let my tractor sit for many months. Usually scraping this away got it starting easier.
 

The "rectifier" is more correctly called a "ballast resistor". The resistor is used to reduce 12 volts to 6 volts for the ignition system, and is usually located somewhere close to the ignition coil.
 
So what do you mean by "will not start?" Does the engine crank when you push the button?

What do you have to do to make the tractor start after it's been sitting?

All the large battery connections must be kept clean and tight, and they tend to corrode when left unattended for months at a time.
 
"If I don't run it for several months it will not start."

Check for an invisible film on the points contacts. Take a folded dollar bill or piece of cereal box and draw it through the closed points. That will often clean them and restore contact.

I have a couple of tractors I have to do that with if they set awhile
 

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