35 Amp Alternator issue

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WildFarmall
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35 Amp Alternator issue

Postby WildFarmall » Thu May 20, 2021 3:34 pm

Hello everyone. I have a 1949 Farmall Cub that was converted to a 12 volt negative ground. Last fall I bought a new 35 amp mini alternator to replace my large alternator. The new one fits fine but it does not seem to charge properly. I have a new battery and my wiring is correct. My questions are:
1. Has anyone else had a problem with them?
2. What voltage should the alternator put out under medium idle?
3. The literature on the alternator says that it will not start charging until it reaches 800-850 rpm. What rpm does the cub idle at? I understand that the top end is around 1,600 rpm, is this correct?
4. Any more advice or things I should check?
Thanks, everyone!

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Re: 35 Amp Alternator issue

Postby Eugene » Thu May 20, 2021 5:51 pm

14.5 volts _+/- a tad bit charging.
Cub idles about 450 rpms.
Alternator should start charging at a bit more than half throttle. 3/4 throttle for sure.

Which alternator brand/model did you purchase?
I have an excuse. CRS.

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Re: 35 Amp Alternator issue

Postby ntrenn » Thu May 20, 2021 8:18 pm

Mine just lets the battery go dead occasionally. They are really sensitive to battery terminal cleanliness. A little corrosion and they seem to hold 14.5 v on the circuit but no current flows to the battery. Clean the terminals and use an analog voltmeter to check battery terminal voltage.

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Re: 35 Amp Alternator issue

Postby tmays » Thu May 20, 2021 8:50 pm

Quite a few mini altenators on the market. Most of them are not a true one wire alternator. They need a diode installed, at the least. I bought one of db electric’s and so far so good. One wire.
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Re: 35 Amp Alternator issue

Postby outdoors4evr » Fri May 21, 2021 5:03 am

I'm guessing that the 800-850rpm spec is for the alternator rotations. Perhaps changing the pulley on the alternator to a smaller pulley would increase the alternator RPM's when compared to the tractor RPM's.
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WildFarmall
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Re: 35 Amp Alternator issue

Postby WildFarmall » Fri May 21, 2021 7:45 am

Thanks everyone. I do not know the brand. I bought it from TM Tractor and they are sending me a new one. A few things:
1. My battery read 12.9 volts unattached from the tractor and when I attached it and started the tractor running at medium throttle it read only 12.7 volts. Running at full throttle it read about the same at 12.7 or maybe 12.8.
2. I never saw any voltage readings coming back at anywhere near 15 volts. I am guessing that it is just not putting out the right amount of volts.
3. The alternator already has a small pulley. obviously it is a lot smaller than the tractor pulley but I don't know how to know if it is reaching the right rpms to engage the alternator. My guess is that I am overthinking this and that there are plenty of rpms and that I just have a bad alternator.
4. I have no idea about whether the alternator has an internal diode or adding a diode. Where would I add one and attach it? I assume it would be between the alternator and the ampmeter.
5. I cleaned all of my battery cable contacts prior to all of this.
6. Finally, as this is a mini alternator, I am hoping that I can loosen the bracket and slip the alternator off and replace it with the new one without removing the hood.

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tmays
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Re: 35 Amp Alternator issue

Postby tmays » Fri May 21, 2021 9:06 am

Sounds like bad alternator, as you say. Didn’t realize you got it from TM. My post was more to inform folks who get an altenator from Amazon. I bought mine off amazon(db electric). Researching the other altenators offered told me I didn’t want any of them. And just so happens I bought two at a reduced price because packaging was torn. They look perfect and work just fine. At half price. Score!
Last edited by tmays on Fri May 21, 2021 4:33 pm, edited 1 time in total.
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Re: 35 Amp Alternator issue

Postby Gary Dotson » Fri May 21, 2021 4:01 pm

The diode (or resister, warning light, etc.) is connected between a switched 12 volt source, and the vertical small terminal on the back of the alternator. It's purpose is to excite the alternator to begin charging, while not permitting a back feed that will cause the engine to continue running when the ignition switch is turned off. Currently, your alternator is not being excited, it's not producing any current.
My 1st question, do you have a magneto or battery ignition? If you have a magneto, it's a little more involved, adding the diode. If you have battery ignition, you simply install a diode or resister in series, between the positive post on your coil and the alternator, as mentioned above. If you have a mag, there are several methods that we'll cover later, if we need to. First thing, you said they are sending you another, give it a try and if you still have no charge, we'll get after it.

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Re: 35 Amp Alternator issue

Postby Don McCombs » Fri May 21, 2021 4:39 pm

I looked at the TM Tractor Parts website and I do not see an alternator listed.
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tmays
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Re: 35 Amp Alternator issue

Postby tmays » Fri May 21, 2021 6:13 pm

Don McCombs wrote:I looked at the TM Tractor Parts website and I do not see an alternator listed.

I don’t either :shock:
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Re: 35 Amp Alternator issue

Postby WildFarmall » Sat May 22, 2021 7:42 am

Don McCombs wrote:I looked at the TM Tractor Parts website and I do not see an alternator listed.


A few years ago I started to order almost exclusively from TM Tractor mainly because I could call Ron up and ask him questions and order over the phone on the spot. Ron is fantastic and patient. I ordered my alternator from them over the phone. I assume that they don't have everything on their website.

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Re: 35 Amp Alternator issue

Postby DRaymond » Wed Nov 10, 2021 7:13 pm

Make sure the alternator is grounded through the mountings. Mine did not charge so I checked the mounting bracket and found that I had painted the mating parts too well. Put a cut washer under nut to the mounting bracket, scrapped some paint from the surfaces that mated and low and behold the electrons floweth!
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Re: 35 Amp Alternator issue

Postby outdoors4evr » Mon Nov 22, 2021 7:32 am

Remove the alternator and take it to a car parts shop - like NAPA, Autozone, etc.
They usually have a setup that can bench test an alternator to see if it is basically working.
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