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Hello. I finally was able to spend some time and track down the source of my current draw through the voltage regulator. With everything connected as it should be, including the mounting bolts attached, I get a current draw, i.e. a short circuit, which shows up on my voltmeter. My current regulator only has three posts to which the lines are connected; Battery (Black), Generator Armature (Red) and Generator field (Brown). Am I correct in assuming that the Generator Ground line would now be attached to the bottom bolt which connects to the frame of the regulator? Also, am I correct in assuming that the ground line from the generator to the regulator is in no way supposed to allow current to flow back through the body? What is happening now is that when attaching the bottom bolt on the regulator to the body, even without this ground line attached, this bolt is actually touching the body and I get the current draw, which I don't think is supposed to happen. The nut that, I think, is supposed to be insulated from the body to attach this bolt is no longer there, thus causing the short. I have a couple of ideas on how to correct this, one of them being to bore out the hole on the body a bit more so that I can fix an insulating sleeve over the bolt and then place an insulating washer on this bolt so that the nut does not make contact. Or, am I barking up the wrong tree?
Again, I thank you for your expertise. I hope you have or have had a wonderful holiday!

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Is your charging system still mostly stock with the 3rd brush generator but using a different or replacement regulator? If not stock, what were changes to the charging components?

I cannot read the number of the regulator you show but all illustrations showing the called for unit that are in the 37 service manual and in the combination 37-38 manual show the regulator as having 4 terminals. It appears capacity is the main if not the only difference between 37 and 38 models so one question would be is that regulator you show compatible with the generator? At any rate, all regulators have springs that are supposed to open the cutout relay contacts when the engine is off thus preventing any feedback current from the battery going thru the regulator and generator. Like the regulator/generator wiring diagram shows, in most cars the generator case is grounded to the fender or frame and to avoid poor sheet metal connections, also directly to the regulator.

One interesting thing is there is a caution note in the combination service manual that the cutout relay can be damaged if polarity has been reversed but it does not specify what happens. Is there any chance a reversal has happened either accidentally or on purpose and that regulator has a problem?

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I can assure you that yes, the ground line needs to be run back from the generator to the body of the regulator and it does attach to the mounting screw that holds the regulator to the body/frame. This assures a solid ground for the regulator to work
You cannot get a short or current draw from attaching a ground to ground. Somewhere else it the circuit you are getting a hot to ground, this is the only way to have current. I guess the only way to find the culprit is to unhook all other circuit's hot lines and start reattaching them until you see a spike in the voltmeter, that will be the source of the short/closed circuit.
(Lick your lips first!!)
Bob J.

That's a fairly interesting arrangement to have a third brush AND voltage regulator. The third brush generators were often just used with a cutout relay, relying on the brush arrangement for the voltage control. In reality, the battery was very important for sinking excess voltage; however, this would also prematurely wear the battery. (In other words, this whole system wasn't very good at its job.) On the other hand, for most cars with voltage regulators, the regulator will control the entire field windings circuit. This one is like half-and-half.

EDIT: I misunderstood the diagram. The third brush powers the one field coil which connects to the other field coil which grounds through the regulator. So the regulator adjusts both field coils, limited to the amount of voltage off the third brush. When I first looked at this I thought one coil was powered with the brush and the other powered by the regulator (both coils grounded).

Bob I highly suspect the cutout relay is somehow stuck closed. If so, the generator will try to act as a motor and will draw current through both the field windings and armature windings. I would be curious to know the cause, if this is not it.

Thanks for you input. I have every line to everything disconnected including the generator and yes, I do see the relay between the battery connection and generator armature connection closed, which is what I originally thought was causing the short according to the schematic. I cannot connect the battery directly to polarize the regulator because I'm afraid the current draw will damage something else, i.e. burn something up. I can insert an insulator in the relay opening those contacts between these two connectors and not get the current draw, which is what I was pursuing first. I had to purchase a replacement voltage regulator which, I think, came off of a '38 or later model. I did not polarize that regulator at that time, but am pretty sure that that relay was closed with I received that particular regulator.
Thanks, again for your input. I'll investigate further in obtaining a regulator that is correct for this model.

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Unless I am not understanding something in your description, the cutout relay contacts should be OPEN at any time the engine is off. They will only close when the generator is producing a voltage. If your cutout contacts are closed with the regulator just sitting, there is an issue.

When the generator is polarized, a small residual magnetism of the field coil poles in the generator with the proper North-South magnetic orientation for the polarity of the battery is established. That magnetism is very low but still enough that when the generator starts spinning a voltage out but with a very low current is produced. The voltage and small current out is enough to bring in the cutout relay in the regulator which closes its contact. That closed contact then connects the battery to the charging system. The battery voltage will then power the charging system sufficiently to provide more magnetism and the proper current as determined by the third brush is produced. The other relay in the regulator is voltage control and it is a relay that is in the field coil circuit. It opens and closes or vibrates very rapidly to either connect one end of the field coils directly to ground or to ground thru a resistor. Voltage powering the field coils is provided by the other end of the coils being connected to the generator output and battery. The rapid opening and closing shunting the coils thru the resistor in and out to ground regulates voltage output.

Thanks, Howard. Which coil (relay) are you referring to as the cutout relay? Is it the top one where the bat and gen wires attach across from each other, as shown in the Upper Coil picture, or the lower one where the fld wire attaches, the lower coil? If it is the one on top, as you see that one is closed when there is no current applied to it and the lower one is open when there is no current. This is a VRR-4005A voltage regulator which I was told was compatible with my '37 120C. The parts book says that we need part number 316500, VRD-4005A, which I have not been able to find anywhere. I've also read that the VRD-4001A is compatible, but there is no place in the parts book that says that. When looking at the wiring schematic for the VRR-4005A, it shows the bat line attaching to the left terminal (Upper Coil pic) but when I do and the regulator is installed on the body as required, I get the current draw without anything other wiring being attached. I know I'm missing something here but have yet to figure out what.
Thanks again for all of your expertise and help.

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If you have only two relays the one with the thick wires will be the circuit breaker AKA cutout relay. Actually the cutout relay has two windings. The shunt (meaning the other side is connected to ground) winding of thin wire pulls the contact in when the requisite voltage is reached. The series winding of thick wire increases the strength of the magnetic field when the cutout relay is closed, to provide hysteresis.

The cutout relay should be open with the engine off or at low speeds. You should see it connect when the engine is revved up and disconnect at a slightly lower RPM. This can easily be checked with a voltmeter...

Thanks, then yes, the cutout relay is open when no voltage is applied. So. then I must have a short in the other coil somewhere or somewhere else I've not been able to track down.
Thanks.

Were you actually able to confirm the leakage was through the regulator assembly?