6 Volt Alternator Question

chas036

Member
I put a brand new 6 volt, positive ground, one wire alternator on my 720D with a pony motor. After I get the tractor running, I see no output 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, I see 6 volts, so I know the alternator is connected to the battery good. When the tractor is running full throttle, I put a volt meter on the battery and I still see 6 volts, and not 7 or 8 like I would expect , so I know it is not charging.

My question is , do those alternators need to be at a certain rpm before they start charging?
 
Yes you can change either the pulley on the alternator or the tractor to change the speed the alternator spins. The RPM problem of the 1 wire alternators is why I use the 3 wire they start charging at a lower RPM due to be excited by the ignition being turned on
 
Where can I find a three wire 6 volt alternator? I am looking on the internet , and everything I see is for a one wire.
 
That I can not tell you but most 1 wire alternators are made form a 3 wire. If you have a good alternator repair place in your area they should be able to set one up for you
 
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
 
My 8N came with a "one wire" alternator. With it running low rpm and not charging, I momentarily jumpered from output post to #1 edge pin. This jump started it to charge. I connected it as a 3 wire with diode in the wire from #1 pin to output of ignition switch. In other words, some one wire can be connected as a 3 wire. Been running that way for 5 years.
 
That is not always true. All depends on how the one wire has been set up. I have had some that that works on and others that you can hook up the #1 wire and it does nothing to help. My 8N has a one wire on it and you can hook the #1 wire up and it does not cause it to excite
 
That's what I was talking about below and it can work fine as you observed. On the 3 wire units that were made to work as one wire by using an in the plug resistor you simply apply excitation voltage to pin 1R and you're good to go.

John T
 
Big problem is know which one is which. I have run into more then one where putting power to the #1 does not cause it to excite since it isa true 1 wire not an 3 wire that has been converted. The one on my 8N you can hook that #1 to power and it doesn't do any thing to help it charge
 
Actually its NOT hard to determine which one you have. Many typical three wires were converted to one wire by simply adding a resistor in the plug and they are so easy to convert to three wire operation. That's how most I've seen are configured, you must have a different unit.

Best wishes

John T
 
O ---K ---- Now I have heard something I never ever ran into in all 45 plus years of electronics!!
I have never seen or heard of an alternator that only puts out 6 volts!!!!
In my mind - the alternators were designed to produce 12 volts for operational purposes and the output was usually a nominal 13.8 volts.
SO ------ I can see how one of these could produce 6 volts with an in-line resistor installed but it wold be a wire wound resistor of say 1 - 2 ohms with a power handling capability of 50 watts or so.
Any one want to enlighten me???
 
Ya the 12SI is the true 1 wire and the converted ones are 10SI. On soem of the converted 10SI the resister in internal so you can not see it
 
It's REALLY simple... it's all in the voltage regulator. ANY 12 Volt alternator can produce 6 Volts (7.4 Volts) if paired with the appropriate voltage regulator.

And it's NIT to hard to find a 6 Volt VR on the net.

It's a little tougher to find a 6 Volt (+) ground alternator, but they're out there!
 
Its (regulation) like Bob said, it wouldn't be the best engineering speaking to use a huge honkin power wasting resistor to drop 6 volts. The more magnetizing current the greater the filed strength the higher the output, so regulating that to achieve 6 nominal volts isn't really hard.

John T
 
Al alternator produces by exciting the fields, you can regulate it anywhere you want within reason.
 
As other posters mention, any alternator's output voltage is determined by its regulator. So by swapping a 6 volt regulator for the stock 12 volt regulator, it's a simple matter to reduce any 12 volt alternator's output voltage from 12 to 6 volts.

Incidentally early production (1965-66) BMW model 2002 cars were factory equipped with 6 volt Bosch(?) alternators.
 

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