My New Alternator Just Quit Charging All of a Sudden!!!

AL Farmall Boy · Nov 19, 2010

Hey guys, I recently re-did a 1949 Cub and wired the alternator per the diagram everyone on here is familiar with. I used the "warning light" method. I could only find a 12 volt light at Radio Shack that was 100 milliamps (I know thats half of what is recommended). It worked fine for a couple days, then the light burned out and now the alternator doesn't show a "charge" status on the ammeter. Does that little light affect the ammeter charging status, or does it just prevent "feedback power" and allow the engine to stop? Please help me with my dilemma. THanks.

AL Farmall Boy · Nov 19, 2010

Here is a "before" picture of when the system was working correctly. As the picture shows, the light comes on when the key is in the "ON" position. Once the tractor is started, the light does not burn and the ammeter reads approximately 14 amps.

Now, the light doesnt come on at all, and once the tractor is started the gauge only reads around 11 to 12 amps.

Anonymous-0 · Nov 19, 2010

It is possible that, since the light is not working,
the alternator isn't being "excited" and therefore
isn't charging.

Of course, it's also possible that, since that
ammeter is showing an 11-12 amp charge, the battery
doing OK and only NEEDS an 11 or 12 amp charge upon
startup.

AL Farmall Boy · Nov 19, 2010

My question is....since I couldn't find the 200 milliamp light per the diagram specs (mine is 100 mA), can I just use the glass fuse holder commonly found on the Farmall Cubs in place of the warning light?? Will it work right if I put the fuse in there?

AL Farmall Boy · Nov 19, 2010

Or should I just use the diode that is mentioned to use in place of the warning light?? I would rather keep/replace the light with another similar one that is the correct rating, OR use the 20 amp fuse. My reasoning is because there is an empty hole in my dash that I am wanting to use so it isn't an eyesore. If I used the diode, the hole would be visible. THANKS>

Anonymous-0 · Nov 20, 2010

A light puts a LOAD on the circuit that creates a
voltage drop; a FUSE is NOT a load which creates a
voltage drop.

Google "Ohm's Law" and learn the relationship
between volts, current, and resistance.

Anonymous-0 · Nov 20, 2010

Having a properly-operating circuit is a bit more
complicated than just filling a hole in the
dash...not a LOT more complicated, but a little.
The light or diode in the circuit has a purpose
OTHER THAN as a charging indicator--besides, you
already have an ammeter which should tell you
whether the alternator is charging or not.

So there is a PURPOSE to having a LIGHT in the
alternator circuit. Please go back and re-read
some of the explanations as to WHY you need that
LOAD on the circuit, and make sure you understand
that BEFORE you proceed.

Owen Aaland · Nov 20, 2010

(quoted from post at 23:36:22 11/19/10) Hey guys, I recently re-did a 1949 Cub and wired the alternator per the diagram everyone on here is familiar with. I used the "warning light" method. I could only find a 12 volt light at Radio Shack that was 100 milliamps (I know thats half of what is recommended). It worked fine for a couple days, then the light burned out and now the alternator doesn't show a "charge" status on the ammeter. Does that little light affect the ammeter charging status, or does it just prevent "feedback power" and allow the engine to stop? Please help me with my dilemma. THanks.

The way you have it wired the light serves two purposes. One is to provide power to excite the regulator so that the alternator will begin to charge. The other is to provide an indication of the system working when the light goes out. With the light not working the alternator never starts to charge.

Normally manufacturers would use a light along with a resistor so if the bulb burned out the resistor could provide the power to the alternator for it to begin to charge. On systems with lights for both oil pressure and charge indicator you will often notice that the charge indicator bulb does not burn quite as bright as the oil pressure indicator bulb when the switch is on with the engine off. The resistor in the indicator light assembly is the reason for the difference.

Janicholson · Nov 20, 2010

Put 10 OHM 1 Watt resistor across the leads of a new bulb. Get a bulb for automotive dash, or marker light use (not a diode light, bulb only). This will provide the needed resistance to start the charging. It also provides the needed current limiting to prevent the tractor from continuing to run (back feed from alternator to ignition).
The radio shack bulb may be designed for real 12 volt applications like in a computer. Automotive bulbs are rated for 16volts and live well on 14.6 to 15 all day long. Best of luck, Jim

AL Farmall Boy · Nov 20, 2010

Thank you so much Jim, I really appreciate and look forward to your answers because I can always understand what you are saying and they work everytime. Thanks to everyone else who provided info for me as well.

I originally wanted to use that type of light (automotive marker push-in type) but do not know where to find the "holder" for it with a colored lens that will fit the hole. I looked at the auto parts store and didn't see anything as far as dash warning lights except for LED lights. I went to Radio Shack and they had the closest thing which is sealed and doesn't have a bulb to change.

I want to use this type of light, but don't know where to get the holder and colored lens. I am assuming a generic holder for a marker light would work, but what about the lens that would fit in the dash. Thanks.

Janicholson · Nov 21, 2010

Digi Key, and Avnet, are two of many sources. Jim

My New Alternator Just Quit Charging All of a Sudden!!!

AL Farmall Boy

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