6 volt alternator question

chas036 · Sep 2, 2016

I just put a brand new 6 volt, positive ground, one wire alternator on my John Deere 720D with a pony motor. After I get the tractor running, I see no charge coming from the alternator to the battery. I know the amp meter is good, because I see it go to minus amps when I run the pony motor.

When I put a volt meter on the terminal of the alternator before I start the tractor, I see 6 volts, so I know the alternator to battery connection is good.

When the tractor is running full throttle (800 rpm) , I put a volt meter on the battery and I still see only 6 volts, and not 7 or 8 like I would expect , so this indicates it is not charging.

My question is , do these type of 6 volt one wire alternators need to be at a certain rpm before they start charging?

Janicholson · Sep 2, 2016

Probably. Many need to be at 1400 or so on their pulley to self excite. That is the reason John T and I recommend a 3 wire setup. If the alt is a Delco 10si, determine from the company that rebuilt it if it can be rewired to be excited from the ignition, through a diode or lamp, to the #1 terminal. Jim

John T · Sep 2, 2016

Here's my post over on the Deere Forum, as usual Professor Jim and I agree, either cuz "great minds think alike" orrrrrrrrr "the blind leading the blind" lol take your pick:

As you already know, that engine is fairly low RPM so the alternator may not ever self excite????? which YES a smaller alternator pulley (which a shop could provide) might cure but the belt has to be good and has to be tight. Ive seen some so called one wire alternators that are nothing more then a 3 wire with a resistor built into the plug that goes over the 2 small side terminals. If that's the case it would be easy to convert it to a 3 wire and provide excitation voltage yourself to get her charging. But that alternator wasn't cheap I bet so before getting another one Id first see if you can put a smaller pulley on it but if not then see if its a 3 wire in disguise. Sounds like its wired right if there's battery voltage on it when she's not running, all it takes is its output terminal wired to the battery via an ammeter if so equipped and of course the case frame well grounded. Id be interested in what Bob has to say???????????

John T

rustred · Sep 2, 2016

if your only running 800 rpm that very well be yout problem as the rated rpm is 1125.

Ohio88 · Sep 2, 2016

On my 12 volt I just rev it to half throttle and then idle it back down. Works every time. Start tractor wait a minute for oil circulation and then rev and return to idle.

Steve@Advance · Sep 2, 2016

I think the RPM is too low for it to excite.

But looking at Tractor Data, the rated RPM is supposed to be 1125. That combined with a smaller pulley should do the trick.
RPM Rating

Elmo · Sep 2, 2016

(quoted from post at 00:54:08 09/03/16) On my 12 volt I just rev it to half throttle and then idle it back down. Works every time. Start tractor wait a minute for oil circulation and then rev and return to idle.

On my 8n I had the alternator shop put in a low rpm cut in regulater which works well. Also as stated in the above post you can just throttle up and go back to idle and it will charge.
Elmo

Owen Aaland · Sep 3, 2016

If your alternator is a Delco 10SI, and it most likely is, you will have a self exiting,1 wire regulator. Since there is very little difference in the positive ground alternator versus the negative ground one they both use the same regulator. that means the regulator is still a negative ground regulator but is now insulated from the case. The main difference between the positive ground and negative ground alternator if the rectifier bridge. The positive and negative diodes are swapped to create the rectifier bridge for the positive ground unit.

The alternator may need to be externally excited to get it started to charge the first time. this done the same way for either alternator. You run a jumper wire from the #1 terminal to the positive lead. What is different for the positive ground unit is the positive terminal is now the alternator case. Just short the #1 terminal to the case with the engine running and the alternator should start to charge.

1948CaseVAI · Sep 3, 2016

Speedway motors in Lincoln NE sells a slightly smaller pulley that makes her spin faster. Works terrific on my 12 volt one wire.

6 volt alternator question

chas036

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Janicholson

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John T

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rustred

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Ohio88

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Steve@Advance

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Elmo

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Owen Aaland

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1948CaseVAI

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