The Stovebolt Site Forums The Shop Area The Electrical Bay '49 GMC Starter Shorting

OK, so I've got a '49 GMC I bought in June, all stock. I'm starting to tinker with it, trying to get it to turn over, but getting nothing.

I take the starter off, and tear it apart, everything looks great. So, I bench test it; sparks! There seems to be a short in it somewhere. So I take it apart again, can't find a thing wrong. So I test for shorts as I put it back together, piece by piece, contact to frame.

To get to the point, there is only a short when I put the end frame (holds the brushes) on and push it in tight. Then it will short. If it's pulled out, even a 1/4 inch, it will not short. AND, it isn't the leads for the brushes, or the throughbolts holding everything together.

Help? Thanks!

You should have a black spot where the short is. Take it apart and examine it.

Thanks for the input, but the only burns are on the contacts, in the switch. Does that help? I'm starting to worry that it's the armature. I have the shop manual, and it gives a few tests that you can do with a test light. It says to test the armature I need a growler. Anyway I can get by without one?

You should have a leather (or maybe nylon) thrust pad on the commutator end frame. From your description, it sounds like the the ground brushes are shorting the commutator when you push the rear end frame fully on to the armature shaft.

Harvester

Could this be a negative ground starter and you are hooking it up positive ground? Don't know, just throwing thoughts around.

There was a rubber pad on the shaft, yes. It seemed ok, but I've replaced it anyway because I had a generic starter rebuild kit handy. Didn't make a difference unfortunately.

I'm not sure if there are any differences between Chevy and GMC starters; that's simple to test though, I'll try it tomorrow with a negative ground instead.

Thanks fellas. I have a sinking feeling the armature is grounded internally though. I need to find a way to verify that without one of these "growlers".

You can test the armature for a ground with an ohmmeter- check for continuity between the commutator and the shaft or core. A battery-powered test light will work in place of an ohmmeter. Continuity (or light) indicates a ground. A short in the windings (between commutator bars) is what shows up with a growler. There are possibilitess for testing for a winding short, but I don't think any of them are especially reliable, and probably more trouble than they are worth- so skip those. Any generator or motor rebuilder should have a growler.

Harvester

Thank you very much for the clear concise answer, it's greatly appreciated.

This afternoon I tested it out; No continuity from commutator. There is, however, from the armature windings, to the shaft. Is that alright?

I'm getting quite frustrated. Everything on this starter looks new and checks okay, well except once it's all together.

-Matt

The armature windings are attached to the commutator bars in what appears to be a parallel arrangement, but electrically it isn't that. But that doesn't matter right now. As the armature rotates, a pair of brushes connects one end of a specific armature coil to power and the other end to ground. With your starter, you have two sets of brushes, two sets of field coils, and maybe two separate sets of armature windings.

You should not have any continuity between the windings/bars and the shaft or core, but you should have continuity from one bar to any other. Although, if your armature has two separate winding circuits, and everything is normal, you would likely find no continuity between two specific bars, probably those about 90° apart from each other, which would indicate two separate windings- in effect, two motors on the same shaft, inside one housing. Measuring bar-to-bar with an ohmmeter, you will find very low resistance- I'm not sure how much, but probably a few tenths of an ohm, but increasing as you increase the number of windings (number of bars between connections)in the measurement.

Another possibility for your trouble is that the insulating bushing on the power connection is broken and the field windings and insulated brushes are grounded to the case that way. It may be that when you pull the end frame away from the case, the ground connection is broken, by virtue of moving things apart. You should be able to take nut and washer off the power terminal and see the bushing is completely intact. If not, that may be where your trouble lies- or at least part of it.

I don't know how much all this helps, but maybe it will.

Harvester

"As the armature rotates, a pair of brushes connects one end of a specific armature coil to power and the other end to ground."

The brushes on my starter look brand new; but are however, quite thick, and I believe, they overlap a couple bars at all times. This makes me wonder, is it possible the previous owner tried to rebuild the starter and put on an incorrect brush set? Is that possible? What is the correct width, or thickness of the original brushes?

-Matt

I don't know the dimensions of the brushes, but they are large enough to contact two commutator bars as the armature turns- one bar is "falling off' so the speak and the next one is "rising" in the magnetic field from the field coils. Without brushes to look at, memory says they are slightly narrower (or shorter) than the length of the commutator bars by about an eighth of an inch. They are about a quarter inch thick, and probably less than an inch in height- maybe closer to a half inch in height. Starter brushes are typically bronze rather than carbon, and the commutator should not be under-cut- that is the mica between bars should not be cut below the commutator circumference as is done with a generator commutator with carbon brushes.

Hope this helps.

Harvester

I've been too busy lately to have a look at the starter again, but I wanted to say thank you for the detailed information! It definitely helps knowing how and why these things go together the way they do. Once I make any progress, I'll post an update.