Reproduction Parts for 1916-1964 Chevrolet Passenger Cars & 1918-1987 Chevrolet & GMC Trucks


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Recently awakened from a 6 year nap the generator was charging at idle 10 amps (dash amp gauge) and would peg at 20 with increased engine speed. It would drop down below 20 with the tail and dash lights on. Searched the threads on this and thought I may have switched the 3rd brush wire with the field wire but they were in the correct spot. The fixed brush holders were very hot.

Put my spare 943J in and it charged at about 5 at idle and would go to 10 with speed but very quickly drop down to about 3.

Both were polarized and it is a brand new battery I charged all night.

Any ideas what might be going on with the two generators? Could the second one be normal operation?

Thanks

Dave

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Hi Dave,

Seems like there are lots of variables to look at in this situation. I assume this is for your 1939 Master Deluxe. Based on the specs I can see that original generator is a 1100004 with a rated output of around 30 amps maximum.

Remember that the ammeter reads the net amperage going to the battery, not the output at the generator. Your car does have voltage regulation but no regulation of amperage. That is a set value and is controlled by the offset between the third brush in relation to the fixed brushes. The ammeter reading will drop as you add more load such as lights and a fan or radio. It will not drop as a function of battery state of charge.

I could not find the output specs for the 943J.

I suggest you look at these 2 items.
- Are the commutators oxidized? it might be as simple as cleaning them.
- What is the position of the third brush on both of them?


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Thanks for the reply. The 943J is for my 31 Chevrolet. The commutators are clean but the brushes maybe new. They look new , so I might try seating them butnot sure that would cause these issues.

The 3rd brush is at the lowest rate spot and the second is very close to that position.

Dave

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polarize the generator, seat the brushes, then use a meter and see if the output changes when you adjust the third brush. i use my inline meter to validate amps, and the voltage before and after the cutout

can also check to make sure that NONE of the wires are grounding out to case or to anything else. had that issue as well


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Thanks Jerry I will give them a try.

I should add I used the same diode converted cutout on each generator.

Dave

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I guess I need to focus on the second generator.

What would cause amps on the gauge to momentarily go from 5 to 10 and then drop below five when increasing the engine rpms?

Volts are at 8 and increase slightly with engine speed increases.

Is this normal?

Thanks

Dave

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yeah that is odd

try the meter on the generator output without it connected to the car. this will let you know what the generator is doing in terms of output and isolate generator or cutout or something downstream (errant ground)

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Thanks I will report back.

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It was 6.93 v at a slow idle, 7.95 v at a fast idle, and when increased to approx 1500 it it hit 8.05 but did drop down (slower than the amps) to about 7.95.

I ordered one of the brush seating stones you mentioned in an old thread so I am going to try that.

Thanks

Dave

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That will ruin your battery.
The regulator needs adjustment.
WL


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I apologize for my confusing response. I mixed up Dave’s cars and the work he is doing.

I now understand that both generators are 943J’s on your 1931.

I suggest that you compare the third brush position on the two generators. That offset is what controls the amperage going out to the car. It does not take much change in position to change the output noticeably.

Some of the owner’s manuals for these 30’s Chevy’s had instructions about moving the 3rd brush to change the output. The recommendation was to increase the output in cold weather or if the car was only driven short distances.

I agree that the best way to properly set the third brush is to connect an ammeter directly to the generator output.

I am a little surprised that either generator is providing current to the system at idle. The difference might be that your diode based cutout is more sensitive and will not cutout until voltage drops quite a bit. On my ‘37 the conventional cutout disconnects at between 6.5 and 7 volts. So at engine idle it is open. It will not close again until there is over 7 volts. Based on the specs the generator in my car has to be running at at least 800 rpm for the cutout to close. Typically I see the ammeter go to + when I get up to about 5 mph.

With respect to voltage, your readings are normal of these old systems. There is no voltage regulation. Like you, I was shocked the first time I saw a sustained 8 volt reading. The voltage is a function of the design parameters of the generator.

1938 was the first year that Chevy installed a voltage regulator on cars. That system still used a third brush generator so the output was a fixed value set by the position of that third brush. The voltage regulator was only 2 coils, one for the cutout and one for the voltage control. There was no third coil to regulate amperage. As a side note I chuckle when we refer to the 3 coil box as a voltage regulator. It is really a generator output regulator.

With respect to amperage output, you will see a drop in output at higher temperature as well as higher speeds. Once again, it is the nature of the design. The drop due to temperature is due to the increased resistance in the wire in the field coils and armature. I’m not sure I completely understand the physics of the higher speed reduction. Based on what I can determine at the higher speeds the magnetic fields from the coils are so saturated (strong) that running the armature faster actually creates its own self canceling current. On my ‘37 the generator output is less at 55-60 mph than at 40-45 mph.

I agree with the statement in one of the posts about letting new brushes seat so they fit the commutator better. Your idea to clean the commutators is a good one.

One suggestion is to “play” with the third brush location and watch how it changes the output. Mark the position before you start. Loosen the holding screw and move the third brush closer to the closest fixed brush. That should reduce output at a given speed. Moving it further away increases output. A good starting point is to have 2 commutator segments between the 2 brushes.

I don’t think things are too far off. It might be good to locate the performance specs for the 943J. That will help you set the output. You want to avoid setting things for too much output. With the third brush you can do that. That will overheat things pretty quickly. I have heard stories about people setting the third brush so far from the fixed one that the generator got hot enough for the solder on the armature to melt.


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Thanks Rusty and Wilson. I will play around with it and report back.

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I take it this is for a 1931 after all?

As Rusty already brought up, there is no voltage regulation, only current regulation.

How many wires connect to the generator? Some third brush Chevrolets have a way for the headlight switch to change the rate of charge. I think 31 is too old for that, but it would be good to know.

Assuming a simple third brush system, one wire out of generator going to the cutout, and one wire out of the cutout heading toward the harness, you just have to use an ammeter to check it. The one on the dash could do if it is really accurate, and all lights and accessories are off, and you add whatever the ignition draws to the reading. If not, hook an ammeter in series between the cutout and the wire heading into the harness.

The current must flow THROUGH the ammeter, so cutout terminal >> one ammeter terminal >> ammeter >> other ammeter terminal >>wire going to harness.

There is a setting in the manuals for testing, but that is just for testing and is too much for running. Also the generator will melt if run that high on the car. See if you can find a default spec for running (not testing) and start there. I'm thinking about 7 amps. Rev the engine up.

The troubles with these systems are twofold. First, it only has the maximum set output (amps) at some particular RPM, and falls off if you drive faster or slower. The second thing it that it will charge at whatever it can do (amps) always, even if the battery is already full, and even if the voltage is getting too high. That is why the current cannot be set very high. It has to be a rate of charge (amps) the battery can take when full, without swelling up and exploding, even if it isn't happy about it.

At night however, if you are running with the lights on, that set rate from the daytime won't be enough to carry them, and if you drive long distance with the lights on, eventually the battery will go dead. Such is life with a third brush and a cutout.

The vintage way of setting it is to start out with the default (probably 7 amps or something like that), drive it for a few days or a week, and check the battery with a hydrometer. If it is overcharged, adjust lower. If undercharged, adjust higher.

The goal is to have the battery just getting full as you get home at the end of the day.

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Thanks Bloo.

Yes a simple 1931 one wire system with a diode converted cutout. The dash amp gauge seems to be working well but I will try my meter.

What do you think of the system at 5 amps at idle and when rpms are increased the amps momentarily go to 10 and then drop back down to slightly less than 5? It seems it would/should stay at the higher amperage.

Thanks

Dave

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is the battery fully charged ?? would make sense.

remember the cutout will open up if the battery is putting more power back then the generator is putting out. that is how it operates. if generator puts out more power then the battery the mag field created in the coils closes the points and flow power to the battery, battery charges up and then the coils open up.

the diode conversion does the same but electronically. since they are a one way electric valve.

gotta use the meter and see what is coming out, directly from the generator before the cutout.


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Hi Dave,

The output of this design generator is definitely speed sensitive and not linear. As Bloo noted output increases with speed up to a point and then drops off as the speed increases beyond that point.

However you should see a constant output at a constant speed. You noted that the output increased from 5 to 10 amps and then dropped back to 5. Was the drop at a constant speed?

I think your cutout is functioning just fine. It is a simple on-off switch. So if you are getting any + reading on the ammeter it is closed and the generator is providing more current than the load of the vehicle. That excess is going to the battery.

As Bloo also noted, that is one of the limitations of the non-regulated 3rd brush generator. It always produces the same output at a given speed whether the car and battery need that amount of current. So you can overcharge a battery.


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Originally Posted by Dave39MD
Thanks Bloo.

Yes a simple 1931 one wire system with a diode converted cutout. The dash amp gauge seems to be working well but I will try my meter.

What do you think of the system at 5 amps at idle and when rpms are increased the amps momentarily go to 10 and then drop back down to slightly less than 5? It seems it would/should stay at the higher amperage.

Thanks

Dave

I'm not sure. I would be sure the battery is fully charged and retest. That does not sound quite right, but I wouldn't be in a hurry to condemn it, Typically you dont get anything at idle. After starting, on a regulated system the current (when revved up or driving) would fall off as the battery gets back full and demands less, but on these systems with no voltage regulator, they tend to keep pushing no matter how the battery feels about it.

Be very cautious about not having the third brush (current) set too high, These early generators can really hurt themselves.

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The generator on the car that was going up to 10 amps and then dropping off is doing fine after the brush seating.

The one that was going full output I took to an old friend that has been in the automotive electrical business for years. The early generator guy had just retired but I and a good tech were able to use their test bench and it was still pegging their meter. We tried everything and tested everything nothing would change the wide open output. We moved the third brush again and for what ever reason it settled down and we could set it to 8 amps. I have no idea what was causing it but am guessing something was grounding the third brush.

Dave

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That is great news! And maybe you have helped the tech learn some new skills.


Rusty

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