# Bench Testing an Alternator



## ron45 (Feb 25, 2014)

What year is the alternator.?

The old way would be to leave installed.
With motor running pull the positive battery cable off
If still running replace positive battery cable
While running turn on all lights, air conditioner, radio, etc.,
With motor running pull the positive battery cable off.

If motor is still running the alternator is good.


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## phuz (Nov 1, 2012)

Brand new. I would’ve expected it to produce voltage on its own while being spun. 


Sent from my iPhone using Tapatalk


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## Rough Rooster (Feb 7, 2015)

Are you applying 12V DC to the rotor windings while spinning the alternator?
It needs the rotor energized to make magnetism for the stator to produce electricity.
Please don't do "the old way" if your car has a computer. The voltage can go higher than the electronics can tolerate.
It would help to identify the make and year model to get the best help.


RR :smile::smile:


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## de-nagorg (Feb 23, 2014)

As RR said, it's going to need a voltage applied to it's input, so it can actually begin it's function.

A 12 volt battery, and 2 alligator clip wires, will do it.

One to ground, one to + terminal of battery, and the correct post on the Alternator.


ED


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## Bigplanz (Apr 10, 2009)

ron45 said:


> What year is the alternator.?
> 
> The old way would be to leave installed.
> With motor running pull the positive battery cable off
> ...


This works if the car is 45 years old. Doing this on a car with a computer will result in the purchase of a new computer.


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## phuz (Nov 1, 2012)

This isn't on a car, although the same concept applies. For that very reason, I haven't tested removing the battery from the equation. It's a single-wire alternator for a powersport (aftermarket acccessory). I would've expect it to produce some voltage on its own while on the bench.


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## chandler48 (Jun 5, 2017)

What is the output of the alternator at the battery with the engine running? Good advice from all, but gee, why not just uninstall it and take to Auto Zone? You could buy a new one if needed in one trip.

I bought one for my Ram, and it failed after a couple of years. Went to buy a new one, and since AZ registers you via phone number, they showed it to be in warranty (lifetime) and gave me a new one.


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## Oso954 (Jun 23, 2012)

Your drill isn’t spinning the alternator fast enough to get full voltage. 

On a typical engine, your alternator is usually driven at about 3 times the engine speed. (Or at least that used to be a common drive ratio) So if you are assuming a 1500 rpm “engine speed”, you need to spin that alternator about 3500 rpm.

Take it to an auto parts store where they have a test bench.


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## Brainbucket (Mar 30, 2015)

ron45 said:


> What year is the alternator.?
> 
> The old way would be to leave installed.
> With motor running pull the positive battery cable off
> ...



Not wanting to step on toes. Only do this for generators. You can start a engine that has a generator and throw the battery on the ground and drive all day until you shut it down. Alternators need a 12 volt supply/storage to operate. You remove the battery connection and it full fields the alt and it hurts it. Not a good test. The excite terminal and the power terminal need to have 12 volts before it will 'turn on'. 13.5-14.5 is the rule of thumb. If it is a computer controlled alt, it has to be installed. Removing the pos post sends voltage spikes all through the system and if there is a computer or modules, it can kill them. Negative off first and installed last.:vs_cool:


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## HandyAndyInNC (Jun 4, 2018)

You can use a screwdriver and touch the rear bearing of the alternator. If there is a magnetic pull, the alternator is good. But the best way is to take it to an Auto Parts store and have them test it. That will do that for free. They can even remove it for you and test it.


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## ron45 (Feb 25, 2014)

Yep.
Just like they claim,

"" You probably keep jumper cables in your vehicle so you can solicit a jump if your battery dies or offer a jump to a fellow driver. But jumping batteries on cars built after 2000 might not be so smart. That’s because newer vehicles contain as many as a dozen computers and even more digital devices. Jump-starting with cables connected to a running vehicle can create a voltage surge large enough to fry expensive computers in either vehicle. And, since most of these components communicate on a shared data bus, surge damage to just one computer or digital device (even a radio) can disable the entire data bus, preventing the vehicle from starting and costing hundreds to diagnose and repair. Think about that: You can cause expensive damage to your own car simply by providing a jump to someone else.

Here’s the bottom line: Dealing with a dead battery in a modern vehicle requires new equipment and techniques. We’ll get you up to speed on the latest jump-starting methods and show you what new equipment, like a jump start car battery charger, you need to safely jump-start and replace an automotive battery.""


I say BS. 

And I have done it hundreds of times.


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## ron45 (Feb 25, 2014)

https://www.maniacelectricmotors.com/will-a-high-output-alternator-damage-the-ecu-pcm-ecm.html


"" This is the most common misconception about how alternators work. This question is by far the most asked. The ECU probably draws about 7-8 amps to operate. It will not try to draw any more than necessary. If you have an alternator capable of 80 amps or 1000 amps the ECU will still only need and draw it's required current. Any electrical component in the vehicle will only draw the current required to operate. The ECU in some cars adjust the field input to the alternator based on the given load of all electronics. On some cars this is all sampled and done 100% in the alternator's internal voltage regulator. And some cars like Chrysler, Dodge, or Jeep the ECU completely controls the voltage and field input. But with all of these systems the loads of the car will be powered by the alternator. All these systems will continue to adjust field input to power loads, no matter the given load. If the alternator doesn't have the capabilities to power the given loads then voltage will not be stable and the alternator will burn out attempting to work above capacity. It is never the amperage capabilities of the alternator that causes damage.

What can cause damage is too low voltage. The voltage in a HO Alternator is regulated exactly the same as the stock unit, keeping ECU and electronics safe. What happens when your stock unit is not capable of powering the demands is the voltage starts to drop as the alternator is attempting to operate above maximum capacity. It is an alternator that cannot handle required loads that can cause damage, not a better more efficient, more capable alternator. Unfortunately, yes, dealers and forums are misinforming you. Of course the forums will probably have those that know and those that repeat what they have heard.

The only way amperage can damage a circuit is if the circuit is not fuse protected and a power wire grounds or shorts out. This causes the massive amount of stored battery current to travel the circuit burning everything in the path. This scenario can happen with or without the vehicle even on. Fortunately most all circuits are protected with fuses in case an accidental short to ground causes battery current (amps) to flow through the circuit.

If bad voltage, too high or too low, causes damage it will normally be the battery that takes the brunt of it. Batteries will buffer most damage caused by out of range voltage issues. Even a stock alternator is capable of massive voltage output, but is regulated to around 14V through the voltage regulator. High Output alternators are regulated in the same manner. You won’t experience anything but an improvement with a more efficient, better capable alternator.

Alternators from the factory are designed to power most all stock electronics and not run at sustained capacity above 80%. If what they say is true, what is keeping your ECU from exploding with the stock 80 Amp alternator when most of your loads are off (blower motor, lights, radio, etc) and your alternator is only required to power a 35 amp demand? What happens to the extra 45 amps? It just doesn't work that way. Alternators work on demand. If only 35 amps is required that is all they are pushing. No circuit, including the ECU circuit is going to draw, or 'get' more current than needed by having a more capable alternator.

Hopefully, We have made a better understanding of this misinformation and one day those that know will drown-out the ones that keep repeating this.


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