Conversion wiring for 12 volt alternator

I'm looking at the installation of an alternator on a Ferguson TO-20 tractor. There are 2 wires coming off the alternator. I would assume 1 is going to the battery for power or amp meter. the second one might
go to the ignition switch. I don't know if this is correct. Does anyone know the correct wiring for this system?

Depends on the make and model of the alternator. If a common Delco 10SI with no charge light and a standard internal regulator - two wire hook-up is correct. One to battery positive and one to a switched source of power that in not capable of backfeeding into the ignition system if the tractor has battery ignition.

Thanks, The tractor has a ignition switch with the start position. I'm told on of the wires from the alternator is hooked up to the assy terminal of the switch.

It all depends on which alternator you have. The most popular is a 10-SI Delco.

Looking at the back of the alternator, the plug where the #1 and #2 wires plug in... If the plug is about 1 inch long and 1/4" wide, it is an internally regulated alternator. If the plug is 1/2" square, it is externally regulated and will need a regulator.

Or it could be an entirely different alternator, just guessing here.

When installing the alternator, you will need to fabricate a good mount system. It is critical the pulleys line up, the alternator is parallel, and the pulley match the wide belt. Sometimes you can actually use the old generator pulley.

And, the polarity will need to be reversed, alternator requires a negative ground. The wiring on the coil will need to be reversed, and a resistor added or replace with a 12v coil.
10 SI Wiring

Kermit, when you say two wires coming off the alternator, do you mean the 2 small side terminals?? On some those are the small low current Voltage Sense and Excitation terminals. There are also what people call ONE WIRE units on which there's only the big main high current output stud while the excitation and sense are controlled and often there's a plug that fits over those two small terminals and all you have to wire is the big output up to the ammeter or battery.

In the event you use a Delco 10SI family of what are sometime referred to by some people as Three Wire Alternators, and for Negative Ground systems (as most alternators are for Neg Ground) here is how I wire them.


The big rear main alternator output terminal goes to and splices into the wire that used to be connected to the BAT terminal on a Voltage Regulator or Cutout Relay, but basically, the alternator output wires to the Load (NOT battery/starter) side of the ammeter (if it has one) which takes place if you use the wire that used to connect to the BAT terminal on a Voltage Regulator.. HOWEVER if that wire wasn't already upgraded, when I convert from a Genny to an Alternator, I replace that wire with a 10 or 8 Gauge and it runs from the alternators main output up to the LOAD (NOT to battery/starter) side of the ammeter or battery. I also replace/use 10 or 8 Gauge from the ammeters (if it has one) other SUPPLY terminal to the battery/starter.

The alternators (if a GM 10 SI) little side number 2/F "Voltage Sense" Terminal gets connected (use a short 12 gauge or so jumper wire) right up to and with the big main output terminal mentioned above. It can wire direct to the battery and sense voltage there but often people just sense the voltage right at the alternator as that big wire runs up to the battery anyway. It can make a slight difference in voltage regulation if you sense voltage right at the battery or the big alternator output stud terminal.

For the GM 10 SI's little 1/R side Excitation Terminal, run a 16 or so gauge wire from the ignition switches IGN (which is hot ONLY when ignition is turned on) output terminal ORRRRRR the coils high input (NOT to distributor) terminal, and in series with it place a 10 ohm 5 watt or so resistor. Then in the rare event (likely dont even need a diode if the resistor which I prefer is used) the tractor continues to run cuz of alternator backfeed when the ignition is switched OFF, I add in series or just use instead a 5 amp 50 PIV minimum Diode. The diode must be wired with negative cathode end to the alternator and positive anode end to the switch. Again, the ratings are not that exact but about any diode over a few amps and 50 volts or so works. I have also used a small incandescent idiot light charge indicator light instead of a diode or resistor. The purpose of a diode or resistor or idiot light is to prevent backfeed so when you turn the engine off the tractor don't keep running but you are still giving the alternator its initial excitation is the purpose of that wire.

NOTE there are other methods to initially excite the alternator, the above is only one, others can work.

This is for a Three Wire Delco 10SI, if you have something else it can wire different. This is for a gas tractor DUH

John T

That is an excellent way to wire a Deco SI series. The ACC terminal is isolated from the IGN terminal.