Need Help w/ 3 brush generator

Hello all, I have what I believe to be a Delco-Remy generator on my TO-20. The thing wasn't charging, turned out to be the voltage regulator field points were not grounding the field coils.

But, while I was at it, I got into the generator, which is a three brush generator, circuit A type. I dug up as many specs as I could find, according to them, the field coil is separated from the armature.

With a DMM I measured the resistance from the field post on the gen. to the armature brush on the commutator, expecting that if I did see some current flow, all I would have to do is raise the field brush and break the circuit.

Well, I did, and it didn't... :?

There was an increase in the ohms when I lifted the field brush, but still there was a connection between the field post and the armature brush. btw, there was around 7 ohms with the field brush connected to the commutator, and about 10 ohms when I lifted the field brush.

Now, this sounds like the field coil is close to shorted, and the voltage would run high. But the thing works....

Low RPMs the charging relay will not even close in the voltage regulator, medium RPMs, 14.0 volts, full throttle 16 volts output from the generator.

So, my question:
Is the field coil on some of these generators connected in some manner to the armature without going through the brushes?

Since it is working, and has been for years, should I leave it alone, employing the "if it ain't broke, don't fix it" strategy?

btw, I've looked the generator over, there are no numbers on it at all. But, I think it is a Delco.

Thanks in advance!

I can't answer any of your technical questions. My experience is with AC tractors. They used a three brush generator with a cut out, and later a two brush generator with a voltage regulator. I was surprised when helping a neighbor with his MF 65 that they used a three brush generator with a voltage regulator. Why? I don't know where the third brush should be set in this case. I assume you have to set the voltage high, and let the regulator do its job. I concur with your "if it ain't broke"......... statement.

Are you sure the third brush isnt adjustable they wer on the Gens used by I-H with cut-outs you just adjusted the third brush for current output. Check to see if there isnt a screw head on the brush end you loosen it then you can slide the third brush closer to the fixed one. You need to have the Delco-Remy model # on the tag to replace the cut-out. Car Quest will have a reg for that gen and they are America made. They should have a listing for that tractor problem is finding a sales rep that can look up what you need. Is the system pos or neg ground as the reg will be made for the correct ground factory. Too many just buy a one size fits all then thats where the trouble starts. I am guessing your system is pos gnd lots were in those days.

Useful info on Delco generators Old Document PDF Jim
Delco A circuit

Thanks for the pdf!

I should have mentioned, this generator is not the original for a TO-20, I am pretty sure someone back there, put a generator from a TO-35 which is a 12 volt, negative ground system. (and uses a 12 volt neg. gr. regulator)

All the literature I've read says the third brush should be adjustable, but if this one is, I can't find how... The third brush going to the field coil is just like the main brushes, no "lengthening adjustment," but offset from the main brushes.

The tag is long gone, no numbers or model number on the beast anywhere.

My main concern is that there is a connection between the field coil and the coils on the armature.
But nothing is shorting to ground, and it seems to be working. Although I am concerned about the 16 volt at full throttle.

Here is the schematic that seems to match this generator.

Thanks again for the help!

If the 3rd brush is lifted from the commutator (where it picks up a fixed % of the voltage generated on yours because it is non-adjustable)and a test light is hooked from
the positive battery source to that 3rd brush, it should light. If the Field terminal on the side of the generator is disconnected, the light (when touching the 3rd
brush should be out. I use a test light because the modern digital multimeter has such little internal resistance it will indicate voltage through very high resistance.
Where a light will load the circuit and little if any real current will flow. Copper dust and some grease types flow voltage. Jim

Interesting document! I didn't read anything about whether a third brush was adjustable or not. But you say this fellows third brush is non adjustable. How do you know this? With the three brush/regulated system referred to in the document is the third brush non adjustable?

As I recall, on the original three brush generator on the MF 65 I worked on, the third brush was adjustable. The original voltage regulator was long gone.

Anyone really interested in understanding a generator, cutout, regulator system would do well to read that entire download. I still have my old training manual from correspondence course that I took while in the Army showing very much same material. Good stuff.

(quoted from post at 07:20:38 04/13/16) If the 3rd brush is lifted from the commutator (where it picks up a fixed % of the voltage generated on yours because it is non-adjustable)and a test light is hooked from
the positive battery source to that 3rd brush, it should light. If the Field terminal on the side of the generator is disconnected, the light (when touching the 3rd
brush should be out. I use a test light because the modern digital multimeter has such little internal resistance it will indicate voltage through very high resistance.
Where a light will load the circuit and little if any real current will flow. Copper dust and some grease types flow voltage. Jim

Click to expand...

Thanks Jim!
I forgot all about the digital meters and their limitations. But this is a great suggestion! I'll try it! And, I think you are on to something with the copper dust. (or something that is providing a path for some kind of connection between the field coil and the armature)

Pete 23 is very correct about the download, I had already found and read the TM-9-1825 war department tech manual from Jan. '44, the Delco-Remy service manual though not as long, is very helpful!

To to reiterate, the third brush, which does pick up a lower pos. voltage going to the field coils on this beast is not adjustable. There is no difference in the brushes, same holder, mounting, everything, on all three brushes, the two main (pos. and ground.) and the field coil. (except the spacing)

I can try to get a picture as the gray ban, snaps off the back of the generator, if that would help.

I would give a lot to know the model number of this thing...

Again, Thanks for the help! I just don't want to burn up the battery!

He indicates this in the original post. Jim