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Old 02-02-2019, 10:11 AM   #1
Zephyr4
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Question Bench testing a generator cutout

I have a 31 model A generator that isn't generating. The car has been in a barn for decades and the generator has never worked for me. I suspect all kinds of issues within the generator itself, but thought I'd start with the cutout.

I followed the "Restorer's Model A Shop Manual" by Jim Schild. The heavy coil read zero resistance and the fine coil 51 ohms. Good according to the book. I manually operated the armature and found that the contacts didn't contact. So I undid the two screws and closed the gap to .015". Now they would make contact. While I was at it, I cleaned the contacts. The book says the contact points should have a clearance of 0.015" to 0.020", so that's good.

It then says the "core gap should be 0.10 with the contact points closed." Is this the gap between the armature and the iron core? If it is, then I have 0.010" with the points open and pretty much 0 with them closed. How do I adjust that one?

The book next says that connecting a trickle charger or battery across the base and generator terminal should cause the points to close. I used a variable power supply and dialed up 6v. Nothing happened. I manually operated the points and found they would stay closed until I disconnected the power. So I wound up the voltage and found the points close at a tad over 8v.

I've read this page http://modelabasics.com/Cutout.htm and found it very helpful too. I'm guessing Schild's "core gap" is the "air gap" on this page.

So here's my uncertainty:

1) The air/core gap should be 0.010" with the points closed? Can anyone reassure me on that one? To adjust, do I loosen the two screws that go through the spring retainer? The exploded diagram on the webpage shows circular holes, nothing slotted, implying no adjustment that way.

2) The webpage says 6.1 to 6.3 volts should be enough to close the points, but that's on the generator with the engine running. I'm doing a bench test, so does the 6.1 to 6.3 volts still hold true?

Any advice would be greatly welcome. I'm aware I can easily fix this with a diode conversion, but I'm a sucker for punishment.

Apologies if this has been covered before, I did search to forum for it but didn't quite get what I was looking for.
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Old 02-02-2019, 12:04 PM   #2
Tom Endy
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Default Re: Bench testing a generator cutout

The attached article does not show how to bench check a cut-out, but offers an understanding of how it works. Looking at the simple electrical sketch a test could be rigged up with two batteries.


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Old 02-02-2019, 12:14 PM   #3
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Default Re: Bench testing a generator cutout

If it's been sitting for decades you should polarize the generator.


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Old 02-02-2019, 12:32 PM   #4
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Default Re: Bench testing a generator cutout

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Originally Posted by Tom Endy View Post
The attached article does not show how to bench check a cut-out, but offers an understanding of how it works. Looking at the simple electrical sketch a test could be rigged up with two batteries.


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Old 02-02-2019, 12:38 PM   #5
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Default Re: Bench testing a generator cutout

Air gap is unimportant so long as the armature doesn't touch the iron core. .010" is typical. 8 volts is too high to prevent points arcing. Adjustment is made with a screwdriver as shown on the web page, but you're already minimum, so no help there. The small gap suggests the points have been filed before, perhaps many times. I'd conclude the inner shunt coil is somehow compromised - possibly too few turns or internal short, but more likely bad connection(s) to generator and ground. I've never opened up a cutout, so you're on your own there.

Best to you as well, Tom.
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Old 02-02-2019, 12:54 PM   #6
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Default Re: Bench testing a generator cutout

You could possibly elongate the holes for the lower contact point mounting and then move the point up enough to give the desired air gap at the core.
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Old 02-02-2019, 08:00 PM   #7
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Default Re: Bench testing a generator cutout

As to your second question - yes, 6.1-6.3V on the bench or in the car. And forget the resistor mentioned on the web page. If it clicks closed, it works. Also check your power supply output with a voltmeter, it could be off.

Oh, and buy a diode.
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Old 02-15-2019, 04:50 AM   #8
Zephyr4
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Default Re: Bench testing a generator cutout

Thanks for the replies, guys. I appreciate it. Already checked my power supply with a volt meter and it's spot on the voltage it says it is.

I've been busy with other stuff which means I've only had a short time to have a play with the cut out. It turns out that undoing the two horizontal screws on the spring side of the armature allows the armature to slide up giving me the 0.010" core gap with the points closed.

The downside is that it now takes 10v to close the points. And that's where time ran out. I'll look to adjust the spring when I next get a chance and get down to having it operate in the 6.3v ball park.
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