12 Volt Conversion, One Question

[JohnnyR]

Looks like a 12 volt conversion is going to be required on my SA in the not too distant future. The economics of getting a generator rebuilt is pretty much driving it.

I've been doing some research, and it looks pretty straightforward. The electrical systems on these old tractors are pretty simple. The one questions I have is with the "excite" line on pin 1 on the alternator. Most car installations used this line to drive the "alternator" idiot light.

The wiring diagrams I've seen for the conversion are very similar and consistent. All seem to agree on everything except for how to tie into Pin 1 on the alternator. Here's an example.

The wiring diagram shows the use of a lamp (much like a car idiot light). But it also says a 1N4001 diode can be substituted for the lamp. I've also seen that a 10 ohm resistor can be used in lieu of the lamp/diode. I could see a small resistive load being required in the line, but the option of just inserting a diode seems to eliminate that a requirement. Using a diode with the cathode toward the alternator would only block current flow from the battery.

I know the 10SI alternators are supposed to be self-exciting, but it usually requires a healthy bump on the throttle to get it going. I want to use the excite line so I can just start the tractor at an idle and not have to worry about remembering to jiggle the throttle.

Really hoping you guys who have been through this might give me some insight..

Thanks in advance.

[Jim Becker]

As you stated, any of the three (lamp, resistor, diode) will work equally well. The need is for electricity to get from the ignition circuit to the excite lug of the alternator at startup but not have enough electricity to power ignition get back from the alternator to the ignition circuit after the ignition is shut off.

Of course, depending on your skill level, the cheapest fix might be to put 3 new brushes in the generator.

[JohnnyR]

Thanks Jim, I appreciate the quick response. I kind of suspected that was the purpose based on the directionality of the diode as shown in the wiring diagram.

Chalk it up to more than one way to skin a cat I guess.

Thanks again,
Johnny

[Eugene]

I know the 10SI alternators are supposed to be self-exciting, but it usually requires a healthy bump on the throttle to get it going.

I have a 3 wire 10SI on one of my WD Allis Chalmers. Half throttle during engine start up will excite the alternator.

I use a toggle switch to excite the alternator, not the lamp or diode.

A Delco 10SI single wire alternator is available from your auto parts store. Simple to wire up and eliminates the diode, lamp, switch, or resistor. I have one on another WD Allis and my standard Farmall Cub.

[Gary Dotson]

I now have diodes on all my 1 wire alternators. As soon as the engine pops off, it's charging. Don't even have to think about it.

[Jim Becker]

A minor note, whenever General Motors designed a circuit for using a warning light with an alternator, they put a resistor in parallel with the light. This way, the charging system would still work if the bulb burned out.

[JohnnyR]

All good points.

Running a wire for the one "excite" line (pin 1 on the alternator through some small load) is not a big deal. But it pretty much guarantees the alternator will charge when starting at idle. Just takes away one more thing to remember (bumping the throttle), and I'm getting too old to remember anything else.

Adding a resistor in parallel is a good idea.

[outdoors4evr]

Having the alternator exite after the RPM's were above a certain threshold was intentional in the design. The late 70's "new international cubs" and the 184 had alternators. Having it not charge at an idle reduces the load on the engine upon startup. On my 184, the alternator starts charging when the RPM's go above 900 - 1000 RPM (about 1/4 throttle). Once it starts charging, it continues to charge even at an idle.
The alternator takes about 1 minute to recharge the battery after a start.

[Jim Becker]

. . . The late 70's "new international cubs" and the 184 had alternators. . . .

The standard Cubs used a resistor in the excite line (25 ohm). I haven't checked the 184 circuit. I never watched my '79 Cub to see when it starts charging.

[JohnnyR]

Diodes, lights and resistors. Several different ways to accomplish the same thing.

Almost completed gathering up the odds and ends I need to do the deed. Most of what I needed was small stuff. I already had an alternator and coil sitting on the shelf in my workshop. When I get finished I'll post up my final wiring diagram and piece parts list. So far I've been able to pick up all the bits and pieced I need from the local NAPA (so those are the part numbers I'll include).

Think I'm going with an indicator light with a resistor in parallel. The lamp I'm about to settle on (I think) is a NAPA TGA10 (25 ohms). Thinking the parallel resistor needs to be around 100 ohms. That should result in a 20 ohm resistance as long as the indicator is working, and 100 ohms if/when its not.

[Eugene]

Think I'm going with an indicator light with a resistor in parallel.

Over kill. One method of exciting the alternator or the other. You have an amp gauge instead of a light bulb to tell you if the alternator is working.