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29 DeSoto charging question


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29 Desoto should have a cut out that will charge same amount no mater what the battery charge is. The only adjustment is the 3rd brush. 25 Amps is probably the max rate of charge. You must have something touching elsewhere because the 3rd brush regulates the arc of the commutator that changes the amp output. I have had several cars with cut outs and never ran into this problem. Your commutator may have accumulated some metal filings and are shorting out so that the 3rd brush is not cutting the arc. You may want to pull the armature and undercut the commutator. Basically clean out between the copper bars.

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A cut out has one set of points. When the engine reaches a certain rpm the points close and you get the charging amps that were set by the 3rd brush. The regular can monitor the battery and adjust itself.

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If the generator cutout switch points are stuck closed when you turn the ignition off, the ammeter will be pegged at maximum discharge. Always watch the ammeter when you are turning off the ignition. If this happens to you, you need to either manually open the points in the cutout switch or disconnect the current from the battery to the generator cutout switch in order to stop the battery from discharging through the closed points in the cutout switch. When you see this situation occuring, you should look at replacing the cutout switch. At the very least you will have a dead battery when you least expect it. At the most you can have an electrical short leading to a fire.

Not to confuse the situation but some of us may not realize that if you reverse the wires on an ammeter when you are connecting it, it will show a dicharge when it is actually charging and vice versa. It can be a little un-nerving when you start the car and you see a discharge instead of a charge on the ammeter. Thanks, Terry

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Guest DodgeKCL

This is exactly why I have a battery disconnect switch on all my vehicles. Wiring fires were very common with our old cars and trucks.

I don't think you've lost your ammeter forever. Any I've ever taken apart are just a very thick loop of brass or copper between the 2 terminals. The actual movement is suspended inside or nearby the loop and responds to the varying magnetic flux as the current rises and falls in the thick loop. There is no actual connection between the 2 parts. If they've had an over current rush it's usually fixable if you're a tinkerer.

I agree with everybody and I don't think the 25 amps came from the generator. It sounds like it came from the battery and went backwards into the generator through the stuck cut-out points.

This is why I have changed all my cut-outs to 50 amp stud mounted silicon diodes. They've been on the cars for about 25 years now and my 6 volt batteries last an average of 12 years. I was checking one last week for water and my note on it says it was installed in May 2002.

This problem you're having becomes a thing of the past. (Is that a pun?) The diode puts any current from the generator that's available over the terminal voltage of the battery into the battery and car buss and no current EVER goes backwards into the generator windings. With the original cut-out atleast 3 or more amps MUST flow backwards into the generator circuit to break the hold down force of the cut-out switch. And of course you get that annoying flash and drop in brightness every time you slow down to stop or even shift gears, if you drag out the shift. With a diode in there all that stuff stops. I also believe the reverse current uses up generator brushes earlier and cooks the windings earlier. But I'm not sure why. Just a hunch. (Something to do with upsetting the polarization everytime the points have to pass reverse current to open??)

It's not hard to install a diode by DIYers. They can be bought at any electronics supply store but not Radio Shack any more. They are descibed as 50 amp 1/4"-20 stud mounted diodes with a break down voltage of no less than 50 volts. You just simply strip out the cut-out and use the remaining mounting for the diode. And it can't be seen because of the cover so no authenticity issues arise.

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