New guy alternator help.

Hey folks, I've found myself ending up here many times while searching tractor issues in the past so I decided to join today to see if I can get some help and explanations with my charging set up.
The tractor is a super H I've owned since I was 18 it has been a great machine with no troubles other than a couple small annoyances. It is a 12v system with a 3 wire delco alternator. Ever since ive owned it the alt has drawn power even when the switch is off, to the point where within 30 minutes the battery is dead. Not a huge issue just unhook the battery. Fast forward to now. The previous alt finally gave out so I replaced it but while I'm in there and have a little time I want to solve this problem.
Ive found previous posts on this forum about putting a diode or "idiot light" in line from terminal 1 out of the alt to solve this but alot of those guys were having trouble with the tractor continuing to run even after the switch was cut off which isn't happening to me. If this will cure my drainage problem I would probably go the indicator light route as those are alot easier to find in my area vs a diode but being a system I don't fully understand I can't see how a light in line will stop power flow so I guess I could use that explained in simple non engineering terms. Also while I have a little ambition and time I want to swap my old non working amperage gauge to a volt gauge and was curious what that entails.
Sorry for the long drawn out post I hope it comes across as understandable. I appreciate any replies and help.

Ign system needs 3 or 4 amps to work. The small bulb won't allow that much currant but
enough to excite the alt. You can also put a switch in an oil gallery that closes when
the oil pressure comes up. That is what I have on mine.

The diode, light, or resistor in line from #1 pin to the on/off switch is to prevent engine from continuing to run when switch is turned off.

They have no effect in preventing battery discharge when the switch is turned off.

Check to see that the wire from the #1 pin goes to the switched (coil side) of the ignition switch. If it is on the always hot side it will keep the alternator turned on when engine is off and cause battery drain. #2 pin can connect to alternator output or other hot battery connection.

If switch is wired correct, suspect a shorted diode or other problem in the alternator. Take to a parts house and have it checked.

To change from Ammeter to volt meter, connect wires from both sides of the ammeter together. Install the volt meter, connect ground post of meter to tractor frame and positive side of volt meter to output of the ignition switch

I've done this alot. if you go to a parts store and get a sending unit that lets current thru an oil pressure unit when the tractor is running and cuts off
current when no pressure is present. then run all the wires thru this unit. tractor not running, no way it can drain the battery; tractor running it charges
the battery!

Delco 10SI, and other common alternators used used as upgrades in vehicles, rely on magnetism in the spinning rotor. The
magnetism is created by electricity that is turned on by the internal voltage regulator. when the alternator is stopped
and the ignition is off, the regulator shuts off all current to the rotor, and there is no drain. The regulator's #1
terminal in this condition appears as a ground. When the ignition is turned on, but not started, electricity from the key
ignition goes to a small filament type light bulb. The ground for this bulb is the #1 terminal, so the bulb lights up (the
same as when the car key is on and the dash light (ALT) lights up). When the regulator senses this new found voltage
applied to its #1 terminal, it is ready to start giving the rotor electricity to make magnetism. When the alternator
begins to turn, as the engine starts, the regulator turns on the voltage to the rotor, and changes the #1 terminal from a
ground to 12v positive. Now there is 12v from the ignition on one side of the bulb, and 12v from the regulator on the
other, and the bulb goes out. There is positive 12v on both terminals of the bulb, so nothing flows through it. Inside
the alternator is a complex array of diodes (one way check valves for electricity) this array changes the alternating
current from the windings in the alternator housing, to direct current to charge the battery, and feed the regulator some
charging information. The #2 terminal on the alternator is connected to the output so the regulator "understands" how much
voltage is being created, and regulates it correctly.
When the ignition is shut off, the light bulb has no voltage on the ignition side, and that now appears to the light bulb
as a ground because it is connected to the coil through the ignition wiring. The light cannot flow enough electricity to
make the coil function, so the engine stops running. If the light was not there (or a diode or resistor) there would be
enough electricity coming from the #1 terminal to keep the ignition on, and the engine would not shut off.

I cannot make this simpler, and actually explain it, so I will move on to what happened to your's.
First the fact that your tractor did shut off tells me that there is either a resistor, light bulb, or diode placed in the
circuit (called Exciter circuit). The fact that the battery went dead is probably related to failed diodes in the Array I
spoke of earlier. When a diode fails it has two options it can be like a switch that is off, or it can be turned on in
both directions (shorted). if turned off, the alternator can still work some with reduced total output. If shorted, it
lets battery voltage flow the wrong way through it to ground and discharges the battery when the tractor is shut off.
So it is possible that the alternator failed much earlier but had enough output to keep going, then failed radically and
died. It is also possible that there is a diode, resistor, or hidden light bulb in the exciter circuit. If you put a good
alternator in it (three wire same as it was) it will probably be fine. If it still discharges, there may be no bulb,
diode, or resistor, and it might not shut off. Replacing the amp meter is easiest, and at this point necessary to make
sure it is charging. Jim

A Delco series SI with a three-wire hookup has the main big output terminal,and the two small plug-in terminals #1 and #2. #1 must get
power only when the ignition is "on." If it it left hot all the time it will draw down your battery. #2 can be hot all the time.

The main output terminal is also "hot" all the time and has isolation diodes built into it so it cannot drain your battery. If those diodes
leak, it will cause a drain.

the backfeed issue you mention has nothing to do with the drain issue you mention. Backfeed is usually stopped by using a diode or an
automotive type ignition switch with separate IGN and ACC terminals. If you use the latter, no diode is needed to stop ignition backfeed.

The alternator does have residual magnetism and can excite by itself with no added field current, but it has to spin so fast to do it, #1
terminal is used to send exciter current to create an electric magnet inside the alternator. This way it can start to charge at 1600 RPM
instead of 4000 RPM.

As to your current drain when "off", check the voltage at terminals 1 and 2 with the engine stopped and ignition "off".

There should be NO voltage at terminal "1", terminal "2" typically will have battery voltage.

If there IS voltage on #1, pull the connector and check for voltage in the "harness side" #1 terminal.

If voltage is present there, the wiring is wrong, and you need to trace the wire and figure out what it's connected to and move it to a source that powers off when the ignition is switched "off", also to prevent engine run-on and protect the alternator there SHOULD be a resistor, diode, or lamp in that circuit.

One thing that I have seen many times... the system may have worked originally, but at some point the wires to terminal "1" and "2" get switched around.

Does yours have a connector body/plug that will only plug in one way, or two individual wires?

Even with a plug, it's possible that it's been replaced do to damage and/or someone has put the wires in the wrong cavities.

The use of a "diode" or a "resistor" or an "incandescent idiot light" in the alternators small 1R excitation terminal is to prevent backfeeding an ignition coil so the engine doesn't still run when the ignition is turned off in cases where that initial excitation is provided by wiring the small 1R to an ignition coils input (what ignition switch feeds when on).

If you use an idiot light in that circuit (a diode or resistor can also work) make sure its a small incandescent NOT an LED. The initial excitation circuit I'm talking about is one fed by say an ignitions switches terminal which is hot ONLY when switch is on/run. If you wired it to a voltage source that's always hot, that could eventually discharge a battery when the tractor isn't running.

A simple volt meter can be wired to the battery NEG and POS via a switch that's hot when a switch is turned on.

If the alternators internal diodes have failed, even if all else is wired right and per the above, that could cause your problem.

John T

Update.
I decided to mess with it a little more tonight. Turns out they had it wired to the wrong side of the switch. So I swapped that over but then had the wouldn't shut off issue. I then spliced in a makeshift indicator light into the #1 terminal and that cured my problem. Now to run something up to the operator station thats a little more permanent. Thanks for the help I look forward to being here with you

That is fantastic. Welcome to our addiction. Jim

I echo professor Jim's point. Congratulations and thanks for the feedback. Yeppers either an incandescent idiot light or a diode or a resistor in the 1R initial excitation circuit can prevent run on when ignition is turned off YOU DONE GOOD !!!!!!!!!!

John T

Or go to napa and buy their 1 wire altenator warrantied for life. Last one a couple weeks ago under $60