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Discussion Starter · #1 ·
This isn't bike related but I'm hoping someone can explain what's going on. The battery in my son's diesel tractor is a few years old, and didn't seem to be charging. He would periodically remove it, put it on a charger, and reinstall. He thought the alternator was bad. Took it to a business that rewinds alternators and they tested it and told him it was working fine. So he charges the battery and it reads 12.35 volts following the charge and before installation in the tractor. Puts it in tractor and connects it to the alternator. Without the engine running, voltage is 12.54. He starts the engine and gets the same reading (12.35 again) at the battery and the alternator cables. Any thoughts?
 

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1986 rebel 2004 sportster
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Seems like the voltage should be 13-14with the tractor running….have you tried to disconnect the battery with the tractor running? Something is not adding up….
 

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Do you have a load tester to remove the "top charge". Then you can see actual power.
 

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Discussion Starter · #4 ·
Seems like the voltage should be 13-14with the tractor running….have you tried to disconnect the battery with the tractor running? Something is not adding up….
Yes, I thought the voltage should be higher with the engine running too. It's a diesel engine, shouldn't it run with the battery disconnected, even if the battery or alternator are no good?

Do you have a load tester to remove the "top charge". Then you can see actual power.
Don't have a load tester AFAIK. Have no idea what a "top charge" is.
 

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Top charge is what a voltmeter will show without any loading of the battery. Under the load test it shows available actual power.
 

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Discussion Starter · #6 ·
From looking online, it appears a fully charged battery should have 12.4-12.6 volts, leading me to suspect the battery may be weak/can't hold a full charge. If the battery is weak, would that explain why charging voltage when connected to the alternator, which should be 13.7-14.7 volts, is still low, or would that be due to an alternator that isn't performing well?
 

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Diesel engines I've driven and worked on will not run without electrical power..
There is an electrical fuel cutoff solenoid typically on or near the injection pump that when power lost fuel is cut off shutting off the motor..

sounds like the battery has an internal short
 

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Yes, I thought the voltage should be higher with the engine running too. It's a diesel engine, shouldn't it run with the battery disconnected, even if the battery or alternator are no good?


Don't have a load tester AFAIK. Have no idea what a "top charge" is.
You are right of course- diesel will run w/o power source. Never mind:)
 

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1986 rebel 2004 sportster
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Diesel engines I've driven and worked on will not run without electrical power..
There is an electrical fuel cutoff solenoid typically on or near the injection pump that when power lost fuel is cut off shutting off the motor..

sounds like the battery has an internal short
Ok, i did not know that….learning every day here!!
 

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Discussion Starter · #11 ·
I forgot to mention he has been rotating two batteries; one recharging while other is in tractor. He borrowed a load tester and the older battery of the two, made in 2016, tested weak. The readings given above were from the older battery.

He's taking the alternator to a different shop to get a second opinion as to whether it's charging properly or not. He'll ask if the voltage regulator is built in or not.
 

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Older Ford tractors are almost a sure bet to have separate mechanical voltage regulator.
bad alternator negative diode will cause battery voltage drain as well.
 

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Discussion Starter · #13 ·
It's a 2015 Mahindra tractor. I can remember when cars had separate voltage regulators.
 

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Discussion Starter · #14 ·
OK, this is second hand from my son and by me, an electrical dunce, to say it nicely. Second alternator shop said it could be that the wiring that "triggers" certain things could be bad and explained how to bypass it. Involves momentarily touching a wire(s?) to the alternator at a specific point. A neighbor who is a pretty good tractor mechanic said it "polarizes" something so the alternator knows to begin charging. Charging at 14+ volts now. Going to look for a corroded connection that may be the root cause, and if that doesn't pan out, install a momentary switch to bypass it. Pushing the switch once the engine cranks should cause the alternator to charge.

Thanks to everyone who shared their expertise!
 
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ah yes. it's been decades since i've had need to polarize an armature..
had occasion to do it a couple of times on foreign cars and once to an Onan RV generator.
Good that your son found a shop who was on the ball electrically, I would have never thought of that remedy.
 

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I remember my father doing that to a couple of cars. I've never had any problems to need to do it myself, but I remember it well. Sounds like a good call from an experienced mechanic.
 

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Discussion Starter · #17 ·
In further discussion with my son, he said that the owner of the second alternator shop had the alternator hooked to a machine. He explained that "This light isn't coming on to indicate it's charging, and it should." He then took a hot wire, touched it somewhere on the alternator, the light flashed momentarily, and the alternator began to charge. Son plans to install a horn button connected to the alternator until/if we can find the root cause.
 

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Have your son research "alternator exciter wire diagram"
It's been nearly 40 years since I've worked on an alternator and my memory is fuzzy.
 

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Motorcycle Magnetos use a permanent magnet rotor to generate electricity in the stationary (stator) coils. Alternators use an electromagnet rotor/armature instead, and have brushes at the end of the shaft to connect power to the electromagnet. Old school designs fed a full 12v to the rotor at all times and let the voltage regulator diode pack take care of controlling the output by itself. New school electronics vary the voltage going into the rotor to regulate how much power is coming out of the alternator. "Polarizing the Armature" means taking the full battery 12v straight to the positive brush bypassing the fancy-schmancy input voltage controls and forcing the alternator to generate maximum output.
 
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