VCCA Home Forums TECHNICAL - GENERAL Electrical - 6 volt 3rd brush adjustment - '34 Master.

My generator doesn't quite catch it with the headlights on. I want to increase the charging rate a bit by adjusting the third brush. They mention it in the manual but don't describe how to do it.

There is a round head slotted screw on the end plate of the generator. Loosen the screw and then move the third brush. Moving the third brush towards the fender of the car increases the generator output. Moving the third brush towards the engine decreases the output. Be careful not to increase the output too much or the generator will overcharge. 10 to 12 amps would be max. with the headlights off.

The US cars used a 935-B generator. The 1934 shop manual says do not adjust the third brush as it is set from the factory for the maximum safe charge. The 1934 Master uses a resistor on the back of the light swirch. This resistor can be changed for more or less chareing. The day time charging rate can be increased by pulling out the light switch 1/2 of a notch.
Then it states that the charging rate should be 9 to 13 amps at 15 MPH and 14 to 18 amps at 23 MPH.
The original head lamp bulbs were 21 CP for the low beams amd 32 for the high beams and the car came with only one tail lamps with a little 3 CP bulb. With the lights on the high beam the needle was just over the charge mark on my '34.Note that the chargeing rate drops to only 5 to 9 amps at 60 MPH.....normal for a 3 brush generator.

I'll check what it does at what speed and get back to you guys. My generator was completely taken apart so the factory setting isn't there anymore.

Went for a little spin. It never went over the middle of the first space, 5 amps?

Move the brush DOWN to INCREASE charging rate.
Move it UP to DECRESE the rate.



FULL SIZE

Wow a picture! With one of those I can do it. Thank you very, very much.

Gave it a little shot. Now charges closer to 10. Another nudge should get it to 12-13. Thanks guys.

A picture's worth a thousand amps.

I've had a problem with too much voltage being put out by the generator. I've attempted to adjust the 3rd brush but can only get the voltage down to 7.8v at the battery. (It goes closer to 9v when moved closer to the other brush.)

Can the size of the brush be causing the higher voltage? I've double checked all the connections in the generator which match the wiring diagram. I did read somewhere that the 3rd brush should be smaller, but not sure if that's accurate or any idea as to the size.

Any suggestions?

BTW, This happens to be on a Graham, but I believe the electrical principles apply across manufacturers 8-) This is the most useful discussion I've found on the web about 3 pole generators

Hi there and welcome to Chevy Chatter. Glad to have you with us.

On the 7.8 volts output from the generator, as the old saying goes, "if it ain't broke don't fix it". Your 7.8 volts output should be fine, and the specs. for different models of the early Chevrolet three brush Delco-Remy generators call for 7.5 volts to around 8.5 volts output.

Is it a Delco generator?

Hi,

Yes, it is a Delco generator (937Y). I brought it to a rebuilder and they rebuilt it (not sure exactly what they did, but looks like new brushes and cleaned internally, tested and adjusted). I put it back on the car last night but I'm still seeing 7.8 volts across the battery at idle. Bringing the engine up to about 30 mph speed (roughly, by ear) it was about 8.4v. It seems high to me, I was expecting it to max out around 7v. Maybe it's ok, but it sounds like the battery is bubbling after I shut the car off which is driving (pun!) my concern.

Thanks for the feedback so far, much appreciated.

Jst
37 Graham Crusader

As mentioned above.....on Delco generators of that vintage the specs. call for an output of between 7.5 volts and 8.5 volts. Since there is no voltage regulator the battery gets a constant charge from the generator.

What is the amperage reading on your amp meter? It should be reading around 10 to 12 amps max.

Ok, several months ago I started a thread about my '32 Chevy being positive ground and learned (as I had always thought) that all Chevrolets have always been negative ground. I decided to have my generator rebuilt and repolarized negative ground. At the same time I found a NOS cut-off switch and replaced it. My generator is now negative ground but is putting out 17-18 amps. I'm told I need to adjust the 3rd brush to get the amps down to 6-8 amps. I see here how to do that with help of the great picture. Couple of questions: I assume I make the adjustment with the engine off and start it to check the adjustment until I get it right. Do I need to disconnect the battery while making the adjustment?

If you are careful, you can make the third brush adjustment with the car running and with someone watching the ammeter on the instrument panel for you. If you prefer you can also do it without the car running.

Move the third brush back towards the engine until the third brush (the brush on top of the generator) is approx. 4 comm. bars apart from the main brush (the brush in front of the generator). You do not need to disconnect the battery while doing this procedure but you can if you feel more comfortable in doing that.

After the adjustment, start your car and read the output on your ammeter with the generator running about 1,700 RPM. If it is too low, then move the third brush towards the fender about 1/2 comm. bar and then recheck the output. If the output is too high then move the third brush back towards the engine. At 4 comm. bars apart you won't be able to move the third brush much further back. With 4 comm. bars between the third brush and the main brush you should have somewhere around 10 amps output with the generator running at speed. Be sure to tighten down the slotted screw that holds the third brush in place.

By the way, 17 to 18 amps is way too much output!!