Light and Ammeter Question

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Tractors Owned: 1948 Farmall Cub
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Light and Ammeter Question

Postby cecildew » Sat Oct 17, 2015 10:24 pm

I have a 1948 Cub with 6 volt w/cutout and magneto ignition that I acquired last year. Got it running. It plowed snow and ran all summer. I then decided to do a more complete overhaul. The tractor is running fine, but even after I got it running last year I had an issue with the Ammeter reading and the headlights only having one brightness. Now that I have it apart again, I figured that I should switch these issues.

The battery is charging I believe as I did not have it on a battery charger all summer and it kept starting, but the Ammeter reads at 0 whether the knob is turned to low charge or high charge. If I turn the headlights on it shows discharging at 10. Does this mean the ammeter is broken or could it be another issue? Also, I have a 4 position switch. However, when on Dim or Bright, the light output is exactly the same, no change. Is this an issue with the switch, or could it be the headlight dimming resistor, or field resistor? Any help would be greatly appreciated. :D
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Re: Light and Ammeter Question

Postby Scrivet » Sun Oct 18, 2015 10:01 am

First thing, check directly on the battery with a voltmeter. Then start the Cub and see if the voltage increases. If it does it's charging, if it doesn't it isn't. I wouldn't be surprised that it isn't charging, that the cutout is stuck open, and if your Cub starts easy that the battery would last all summer with a magneto.

For the lights not dimming, the dimming resistor has most likely been replaced with a wire jumper or the lights are wired wrong on the back of the switch. If the dimming resistor fails it usually opens and you wouldn't have voltage thus no lights on dim.

As for the ammeter not showing charge, cutout as previously mentioned, or if someone has been monkeying around with the wiring (dimming resistor) it may be hooked up wrong. I had one of my Cubs that started life with a magneto, cutout, and LHDB switch, doing the same thing. It was long ago changed to a regulator, and distributor. Somewhere along the line something got hooked up wrong and it did everything fine but show charge.

Also keep in mind that the light switch and charging switch are two completely separate circuits. Their only connection is the knob and where it is turned.

Now I just have to ask......Who actually uses the dim setting? Personally I think the settings should be labeled "Can't See" and "Really Can't See" :lol:

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Re: Light and Ammeter Question

Postby Eugene » Sun Oct 18, 2015 11:12 am

Depending on which generator is installed the output is very low; 12 - 15 amps and 7.7 - 8 volts.

When my Cub had a generator and cutout installed, I checked the voltage as Scrivet describes above. There was only a bit less than 1/2 volt difference between the high and low charge switch setting. The amp meter would hardly move to the charge side.

My Cubs are work tractors. When I converted the standard Cub to 12 volt alternator, I removed the lights from the tractor. I never used the lights, ever.

One thing you could do is install a volt meter. That way you can observe the generator output from the tractor seat.
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Re: Light and Ammeter Question

Postby Puffie40 » Sun Oct 18, 2015 2:55 pm

First step I would do is look over the wiring harness and confirm the wiring is correct. If you need to replace wires, 6 volt systems need thicker wire than 12 volt- don't go lower than 12 AWG.

The fact the ammeter shows a discharge indicates the ammeter is working - I suspect you might have an issue with the generator as you should get a charge indication equal to the draw of the headlights when the switch is in the second position.

A rough test you can do with the generator is to take off the belt, disconnect the cutoff and feed voltage to the A and F terminals. If the generator spins, then the generator is fine and should work.

The cutout is simply a relay that closes when the generator output equals 7-8 volts (current /voltage control is done by a third brush on the generator and statically set when it was assembled).

I used a variable power supply to confirm the operation of my cutout but you might be able to do it with some D-cell batteries in series.

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