Guys,

My 1938 3/4 panel truck is coming along fairly well, and I'm now in a position to order a wiring harness. I used YnZ when I re-wired my 38 coupe and was very pleased. In my recent correspondence with YnZ, their rep told me that their model harness for a 38 panel was wired for a cut-out relay on the generator and not a voltage regulator. He wants to know which I want on the harness for the panel, seeing that they would obviously require different wiring.

My 38 coupe, 37 1/2 ton, and 40 1/2 ton all have voltage regulators; I told him that I thought the voltage regulator was the original way to go. He responded that Chevy did them both ways (something I didn't know).

Since the original engine in the panel had been removed and scrapped by the PO, it did not have an engine when I got it. I have since secured an original 1938 216 and have rebuilt it. The generator I have does not have a cut-out on it. Given that, and given my experience with my other vehicles, I was intending to go with the voltage regulator.

Is it true that Chevrolet used both cut-outs and voltage regulators in 1938? If so, which should I use? Is one more "original" than the other, or more proper for a panel truck? I'm attempting to bring this back to as close to original as possible. Any advice or insight would be welcomed.

Thanks in advance.

Jim

I'm not sure whether or not a Chevy Truck had a cut out in '38, but at sometime they did. The cutout will work. The problem with cutouts are that when they fail and when they do, they can either damage the generator or the battery goes dead, maybe both. The cut out is just a relay that monitors the generator output voltage. When the voltage is greater than 6.8 volts (or what ever it is set at) it's contacts close and the generator will start to charge the battery. And it will charge and charge and charge. If you do a lot of driving you can cook the battery. Then when the engine slows down where the generator output reduces to a valve lower than the battery, current will flow from the battery back into the generator. The cutout has a coil that senses that and opens the contacts, disconnecting the battery from the generator. The cutout is crude control device. It doesn't regulate anything, just on and off. A voltage regulator would be a much better choice. Your lights will not flare, your electrical devices will work better and your battery will last longer. My 2¢. Mike

1938 and up cars had a voltage control...not reallly a voltage regulator as they used a three brush generator.
Trucks through 1939 came with a three brush generator and a cut-out relay.
1940 and up both came with a voltage regulator and a two brush generator.

Often the 1939 trucks were converted to a later three brush gen. and a voltage control or a two brush and a regulator....and they also had some options that included these units.

Mike and Gene,

Thanks for your replies.

Gene, excuse my ignorance on the fine points of these generators. I checked the shop manual and see on page 244 ff. information about the Third Brush Regulated Generator. So, as it states there, trucks in 1938 used this system, as you have confirmed.

Question: Were all 38 generators three brush? I have a good 38 generator (from a car, I think)--would it be 3 brush? How do I determine that definitively?

What else do I need in order to wire it in the third brush regulated generator configuration, as opposed to using a voltage regulator? (in addition to the proper wiring configuration in the harness).

Thanks,
Jim

Gene,

Sometimes I'm a little slow on the uptake. In thinking about this, I just realized that this is the same generating system (i.e., with the cutout relay) that is operational on my antique John Deere tractors. The cut out is mounted on the generator.

Can I be certain that the 1938 generator that I have is a three brush?

Jmmmn37,

Wouldn't counting the brushes answer the question?

Remove the generator cover band and count the brushes.

OK, if you use the original truck (1939 and older) three brush you can not use a volt. control. The 1938-1939 car three brush is designed to put out up to 28 amps. The front and rear end frames are cut out to allow the air to pass through and the pulley has a fan on it. The original truck Gn. was made to put out 12 amps or so.
You should consider using a 1940 and up two brush and the 1940 and up three unit volt. reg. The volt. control is two units, a cut-out and volt. regulator. the amps is goverened by the third brush. The two brush volt. reg. has three units. A cut-out, volt. control and amps.

The basic charging system for 1937 - 39 Martin Parry trucks was 3 brush cutout (6 volt, 17 Amp), like 1937 Fisher cars. But there were many options.
see page <http://home.znet.com/c1937/act.htm>
38 - 39 Fisher cars had 6 volt voltage regulator.
1937 Flxible had 6 volt Alternator and 2 batteries in parallel.
I would love to see a photo of your 3/4 ton panel.
Lou

The 3rd brush generator for passenger vehicles has a fixed 3rd brush which limits the current output to a set value(in this case 28 amperes)the voltage being controlled by a current compensated voltage control unit in the same unit with the cut-out.Commercials,ie;trucks,would have used a std 3rd brush generator with a normal cut-out on the casing,the current output being entirely controlled by the position of the 3rd brush.