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Discussion Starter · #1 ·
I know it sounds like a stupid question but I have searched and not really found a clear
explanation of how to do this. I have checked battery terminals with my multimeter while the truck is running and the reading is good, around 14.2-14.4 volts. But isnt there another way of testing directly at the alternator itself, from the red wire connection at the rear of it?
 

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What you're doing is the way I would test the alternator. Unless you have a DC clamp-on ammeter, then you could use that to see what amperage it's putting out as well as the voltage test.

Are you having battery issues? If so, you should get the batteries load tested, as your alternator is likely just fine.
 

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Or you can test at the big post on the alternator.
If you want to test amps, alternatively ( see what I did there by choosing the word: alternatively ?) to Mr. Hansen’s suggestion, you can use a shunt with your multimeter. You can get one from Amazon. They are not much.
 

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Discussion Starter · #4 ·
What you're doing is the way I would test the alternator. Unless you have a DC clamp-on ammeter, then you could use that to see what amperage it's putting out as well as the voltage test.

Are you having battery issues? If so, you should get the batteries load tested, as your alternator is likely just fine.
I do have a clamp multimeter. Do i just clamp around the main red alternator cable while truck is running and measure amps?
 

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Discussion Starter · #5 ·
Or you can test at the big post on the alternator.
If you want to test amps, alternatively ( see what I did there by choosing the word: alternatively ?) to Mr. Hansen’s suggestion, you can use a shunt with your multimeter. You can get one from Amazon. They are not much.
How exactly do i use the shunt to test amps?
 

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What you're doing is the way I would test the alternator. Unless you have a DC clamp-on ammeter, then you could use that to see what amperage it's putting out as well as the voltage test.

Are you having battery issues? If so, you should get the batteries load tested, as your alternator is likely just fine.
Raise and I had a conversation about this...... so if the alternator has a bad diode causing a parasitic drain to the batteries while the truck is turned off could the battery still throw the 13.7 + volts needed when running? If so, then as mentioned here before - with the truck off and disconnecting the accessory belt from the alternator one would be able to see the alternator pulley turning by itself freely (?)
 

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I’m not sure what your meter can do with its clamp. Contact manufacturer. I was referring to an old school shunt. You disconnect the battery cable. You attach it to the shunt. You connect the shunt to the battery. The shunt has two smaller test posts on it. You take the reading there. I have not checked for amps in well over a decade. The need never seems to rise.
You can’t just bridge the gap between the battery cable or alternator and battery with your meter. The probe wires and meter can’t handle it. Now maybe a meter that has a directional clamp could but you need to contact the manufacturer on that.
 

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Discussion Starter · #8 ·
I just replaced both batteries, but i want to go ahead and test the alternator before I really put the truck back into full operation, cuz I dont want to ruin the new batteries with a bad alternator.
 

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If you're concerned about bad diodes, and your clamp on ammeter does both AC and DC (not all do DC amps), you can measure amps with the engine off (should be zero). Or you can use Big Horn's trick of removing the belt to see if the alternator motors.
It takes about 10 minutes tops to remove your alternator, and most parts stores will test them for you for free. Drive over there, remove the alternator in the parking lot, take it inside and have them test it. You'll know if it's good or not then.
 

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Yes, with a burnt out (shorted) diode, it lets battery current back into the alternator windings and the alternator acts like a motor (but a very inefficient one). An alternator with burnt out diodes will drain batteries quickly.
 

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But isnt there another way of testing directly at the alternator itself...?
This page explains how - it's where the (Ford/MC) voltage regulator says "GROUND HERE TO TEST":

(click this text)
...with a burnt out (shorted) diode, it lets battery current back into the alternator windings and the alternator acts like a motor...
That's physically impossible because the commutator on the alternator (generator) is 2 full rings; not segments like a motor has. And being wound for DC output with 3 groups of diodes, the stator cannot convert a DC input into a rotating magnetic field. So at most, the stator would VERY weakly make ~1/40 of a revolution (~9°) and stop. But even that is unlikely.

And when a diode burns out, it doesn't short - it goes open. The one at the bottom center of this pic exploded so hard that it knocked its cathode off to the side.

(click this text)
 

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Quote: “In addition, bad diodes can also allow current to leak from the battery back through the alternator to ground.”

 

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Bad diodes ( one way valves for electricity ) cause juice to run back.
How did we get to a spinning pulley?
Im not ruling it out! I haven’t researched that far yet, but plan to. But that wasn’t the only quote made:

“And when a diode burns out, it doesn't short - it goes open”


But! Following a father who was an industrial electrician and General Foreman as well as top in his game in electronics......for years until his passing- I learned that sometimes the improbable can happen.... I’m not really interesreted in hypotheticals..... of why something shouldn’t or couldn’t, I’ve seen it make liars of people over the years, time and time again.
 

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It’s nice to see your respect for your father. It
seems to me that there is less and less of that these days in our society.
I will say that I have never seen that happen in 37 years. I know of no one that has.
I am not an electrical “ expert”. Steve however seems to have a black belt on the subject and certainly made the case for it being an impossibility for this alternator.

To confirm… This is not something that you have personally witnessed but rather a theory?

I am all for research. Please keep us posted.
 

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Sure, it's conceivable for a diode to fail so that it passes current in both directions. But it's not likely for the failure to STOP there. Usually, the flowing current instantly overheats the diode, causing it to detonate. Sort of like throwing a pencil across a table and having it land on its end, standing up. Yes, it CAN happen & there are YouTube videos of people doing it. But it's not common enough to call "possible" in the context of everyday life. Those videos don't show the THOUSANDS of failed attempts.
 
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