# Is there a DIY way to test alternator output?



## TREETOP (Feb 11, 2009)

Long story shorter- 
2004 Tahoe Z71.
I've got a 250A alternator with overdrive pulley, dual matching batteries up front, big 3 (4) done in 1/0.
I'm running a little less than 3kw RMS total with only half of that going to bass. 
1/0 running to sub amp with a 200A breaker. Sub amp is inside the rear side panel.
4awg running to a distro block to my other 3 amps and crossover, with a 120A breaker. These 3 amps are in my center console, so the 4ga run is pretty short.

I'm getting _ridiculous_ dimming on deep notes, and my voltage gauge dips below what I'm comfortable seeing.

I'm also getting a whining/whirring sound that's RPM dependent, sounds almost like alternator whine but it's coming from under the hood. This noise is present any time the vehicle is running, whether the stereo is connected or not. Fortunately there doesn't seem to be any whine through the speakers at all. A buddy asked me if I have a supercharger the other day, that's the kind of sound it's making.

The alternator was an eBay purchase, and I can't recall the vendor. I installed it long before I started installing the stereo (beefed up the electrical FIRST this time), and I sold my factory 145A alternator, so I don't really have a reference or another alternator to swap in to test.

I'm leaning right now towards the possibility that my alternator isn't in fact 250A capable, or that it has a defect. But I'm open to other suggestions also.

Is there any way to test what kind of amperage an alternator is able to produce?

Thanks DIYMA!
-Cory.


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## lostdaytomorrow (Jan 26, 2007)

DC Amperage Clamp Meter, like the expensive Fluke DMM's. 

You just clamp the positive wire coming off the alt. while running full tilt and it'll tell you the RMS Amperage going through that wire. 

Only reason most don't do this is because these DMM's are about $150 and above iirc.

This one would do it: 
http://us.fluke.com/usen/Products/F...FlukeUnitedStates&Category=CLMP(FlukeProducts)


This seems to be a good choice for the money:
http://www.amazon.com/Mastech-Manual-Range-1000A-Clamp/dp/B000LY2WFG/ref=pd_cp_hi_3


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## TREETOP (Feb 11, 2009)

Won't that only tell me what my draw is? Or will it actually tell me what the alternator is capable of? 
What I mean is, is the alternator actually putting out maximum amperage even if there's less than maximum load?


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## 94VG30DE (Nov 28, 2007)

lostdaytomorrow said:


> DC Amperage Clamp Meter, like the expensive Fluke DMM's.
> 
> You just clamp the positive wire coming off the alt. while running full tilt and it'll tell you the RMS Amperage going through that wire.
> 
> Only reason most don't do this is because these DMM's are about $150 and above iirc.


And that only tells you the amperage it is capable of producing to keep up with the demand of your existing setup. That does not show you performance on sudden spikes, and it doesn't show you the "headroom" on your alternator. You need to properly load-test the alternator using a load tester. You put as much load as the thing can handle, and THEN see what kind of amperage it puts out. 

Luckily your local auto parts store should have one of these testers, and probably will test it for free. I know the independent shop I worked at did these tests all the time for free. It takes like 1 minute with the correct tool. Without it, the test results are inconclusive at best.


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## 60ampfuse (May 18, 2008)

I just put an alternator on my car last week, I bought it from a small alternator shop and I really wanted to make sure it A. worked before I installed it, and B put out the 70 amps that it listed. The guy hooked it up to a machine that he was able to control the rpm speed on and cranked it up, it showed 69.4 amps. So if its real easy to take out maybe you can just take it up to an alternator shop or maybe they could even test it without taking it out.


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## TREETOP (Feb 11, 2009)

60ampfuse said:


> I just put an alternator on my car last week, I bought it from a small alternator shop and I really wanted to make sure it A. worked before I installed it, and B put out the 70 amps that it listed. The guy hooked it up to a machine that he was able to control the rpm speed on and cranked it up, it showed 69.4 amps. So if its real easy to take out maybe you can just take it up to an alternator shop or maybe they could even test it without taking it out.


That's awesome, it'd only take me 10 minutes or so to pull it out and half of that is taping up the 1/0 positives attached to the back. GM was nice to put it right on top of the motor. 
Then I could run it to a shop on my scooter or use my wife's car. 

I'll call around tomorrow and see if I can find a shop who can test it for me.

I kinda hope it's bad, because at least I'll know and it's a simple fix.
But I also hope it's fine because I can't really afford another high output alternator right now. 

[edit:] Looking through my eBay feedback history I was able to figure out who I bought the alternator from, but that was back in January so I don't expect I'd get any help that direction.


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## 60ampfuse (May 18, 2008)

Well at least GM made it easy for ya, It was a pain in the ass on a 2005 civic!! But the whole test took that guy about 60 seconds so i would be kind of surprised if they even charge you for it. This machine showed the voltage and the amperage output of the alternator. It will only produce the rated power and the max rpm of the alternator. He had to crank it up to my cars redline (7k) to get the 69 amps.


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## drabina (Sep 18, 2009)

Do you have any numbers on the alternator? You could easily lookup amp rating when you know the part #. I think newer alternators will put up full charge when revved above 2400 RPMs.


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## kyheng (Jan 31, 2007)

I will check all the connections first. Sometimes upgraded big 3 = more resistance = more voltage drop.
First I will take measurements on the battery and amps without engine running.
Then with engine running, I will test again the voltage on amps and battery with audio still off.
When I'm happy with the voltage(depending on car), then only I will start my audio and measure again.
Got an amp meter is good, but with multimeter is more than enough to troubleshoot.


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## 94VG30DE (Nov 28, 2007)

kyheng said:


> I will check all the connections first. Sometimes upgraded big 3 = more resistance = more voltage drop.
> First I will take measurements on the battery and amps without engine running.
> Then with engine running, I will test again the voltage on amps and battery with audio still off.
> When I'm happy with the voltage(depending on car), then only I will start my audio and measure again.
> Got an amp meter is good, but with multimeter is more than enough to troubleshoot.


Nothing you just said tests the alternator output.


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

You could do a quick voltage test before you remove the alt and take it in.... 

Start the truck, let it warm up, maybe take a small drive... 

Then at idle, with EVERYTHING electrical possible shut OFF, test voltage... 

Then, with that number in head, turn on EVERYTHING possible electrical... test again... Can you stay above 11.5-11v? If not, you might have a voltage regulator problem... 

Just a pre-lim test... it's simple though..


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## kyheng (Jan 31, 2007)

^, my bad... But if an alternator is working properly, it should be giving full rated amperage as stated. As far as from my noob knowledge on alternator, it only have 2 state, either giving you current(when working) or not giving anything(failed).


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## WRX/Z28 (Feb 21, 2008)

60ampfuse said:


> Well at least GM made it easy for ya, It was a pain in the ass on a 2005 civic!! But the whole test took that guy about 60 seconds so i would be kind of surprised if they even charge you for it. This machine showed the voltage and the amperage output of the alternator. It will only produce the rated power and the max rpm of the alternator. He had to crank it up to my cars redline (7k) to get the 69 amps.


FYI:

Maybe it was running at your car's redline if your alt pulley and the crank pulley are the same diameter, but guarantee'd the alt pulley is smaller than the crank pulley which spins the alt faster than the motor. Your alt probably spins 7k when the car is revving 3k-ish.


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## jrgreene1968 (Jul 7, 2008)

94VG30DE said:


> Nothing you just said tests the alternator output.



X2, and at idle, it wont be even close to full output


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

WRX/Z28 said:


> FYI:
> 
> Maybe it was running at your car's redline if your alt pulley and the crank pulley are the same diameter, but guarantee'd the alt pulley is smaller than the crank pulley which spins the alt faster than the motor. Your alt probably spins 7k when the car is revving 3k-ish.





> I've got a 250A alternator with *overdrive pulley*


Maybe this could be adding to the problem, also?


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## WRX/Z28 (Feb 21, 2008)

Aaron'z 2.5RS/WRX said:


> Maybe this could be adding to the problem, also?



Very likely. The alt wants to be spun a certain rpm, if you exceed that, it won't make any more power. 


OP, i'm guessing you don't have the stock sized pulley to swap back on?


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## jrgreene1968 (Jul 7, 2008)

battery could also have a weak cell..


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## Rudeboy (Oct 16, 2005)

That whine you are describing is a classic symptom of an alternator about to croak.


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## WRX/Z28 (Feb 21, 2008)

Rudeboy said:


> That whine you are describing is a classic symptom of an alternator about to croak.


Or being overdriven to the point of making noise...


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## TREETOP (Feb 11, 2009)

I'm using the overdrive pulley because at first this alternator didn't seem to be putting out much at idle. I know with most alternators, in order to make more power up top, lower RPM power is sacrificed. All the overdrive pulley does is lowers the RPM that the alternator makes full power, it's not intended to make more power.

I do have the original pulley that came with this alternator, it's the same size as stock. The truck makes the whining sound at idle though so it's not like I'm spooling it up too fast.

Thanks for the ideas guys, I'l try to get the alternator looked at today and see if it's the problem.


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## cichlid_baby (Aug 13, 2009)

I have the same problem.. and I also have one of these EBAY alternators.. could be the alt.

I've got an older 97 Tahoe with the 350 Vortec.. might be the same alternator.



I'll stay tuned here..


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## MACS (Oct 3, 2007)

Rudeboy said:


> That whine you are describing is a classic symptom of an alternator about to croak.


X2. 

I had an HO alternator built by a local electric shop that had a RPM dependent whine, but did not come through the electrical system. It lasted less than a week and didn't even give me enough time to schedule a return trip. 

Then had another one built by a different shop that got so hot it melted part of my harness. My luck with HO alt's has sucked.


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## jrgreene1968 (Jul 7, 2008)

i had read a thread awhile back..cant remember where, where couple people bought ho alternators(from same company), they wasnt doing what they had expected, so they carried them to a local shop for testing, come to find out, they put out less than the factory alternator


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## Ziggy (Nov 29, 2007)

Rudeboy said:


> That whine you are describing is a classic symptom of an alternator about to croak.


X 3... I know this sound as well... fist bumps from the Dalai Lama, yo!


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## Austin (Mar 12, 2009)

kyheng said:


> Sometimes upgraded big 3 = more resistance = more voltage drop.




It should be the other way around no? less resistance = less voltage drop?


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## 94VG30DE (Nov 28, 2007)

kyheng said:


> ^, my bad... But if an alternator is working properly, it should be giving full rated amperage as stated. As far as from my noob knowledge on alternator, it only have 2 state, either giving you current(when working) or not giving anything(failed).


Wrong on both accounts. 

Just as an amp is not delivering it's max power to your speakers at all times, neither is your alternator working its hardest at all times. They both respond to demand in the system. 

HowStuffWorks "How Alternators Work"


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## kyheng (Jan 31, 2007)

By theory, yes. Actual depending on type(and quality) of ring terminal that we are using. Then follow by connections between alternator and the ring terminal. I've experienced this before because of dirty connections. Initial thought was my amp got problem but after follow the list of my troubleshooting, the cause was the connections problem. It may not be the cause, but this will be my first step as it is cheapest and easiest method to know if the alternator is not working properly.


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## 5Speed (Sep 23, 2009)

TREETOP said:


> I'm using the overdrive pulley because at first this alternator didn't seem to be putting out much at idle. I know with most alternators, in order to make more power up top, lower RPM power is sacrificed. All the overdrive pulley does is lowers the RPM that the alternator makes full power, it's not intended to make more power.
> 
> I do have the original pulley that came with this alternator, it's the same size as stock. The truck makes the whining sound at idle though so it's not like I'm spooling it up too fast.
> 
> Thanks for the ideas guys, I'l try to get the alternator looked at today and see if it's the problem.


Thanks for the information, I always wondered if an overdrive pulley made more amps or adjusted the power curve of the altenator.


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## TREETOP (Feb 11, 2009)

I contacted the seller, and although the alternator was purchased back in January he's stepping up in an impressive way:


> hi cory, i'm sorry to hear this. it sounds like a diode in the rectifier possibly went bad. if at all possible you could send that alternator back to me i will test and repair at no charge. the problem is that your vehicle would be down for a few days. let me know...


Problem is, this is my daily driver and I can't be without transportation for the week+ that my alternator would be making the round-trip journey. Even a cheap autozone 105A alternator is $130 plus $35 core charge, Rockauto wants $108 plus shipping, and I wouldn't consider buying one and returning it in a week when mine gets back either. No luck on Craigslist either. I'm gonna try some junkyards for a temporary I guess..


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## 94VG30DE (Nov 28, 2007)

Did your OEM one go bad or something? Why not just swap that back in?


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## TREETOP (Feb 11, 2009)

94VG30DE said:


> Did your OEM one go bad or something? Why not just swap that back in?


What a great idea!  Oh wait...



TREETOP said:


> ...and I sold my factory 145A alternator, so I don't really have a reference or another alternator to swap in to test....


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## MACS (Oct 3, 2007)

After my experiences with HO's, I decided to keep a spare working OEM alternator for back up. 

Look on Ebay for good used OEM's. They can be picked up pretty cheap there if you have time to wait for one to get shipped in. You might check to see if your model interchanges with any of the OEM severe duty alts like those used for ambulance duty .

Another good source: http://www.car-part.com/index.htm
Pretty much every major auto salvage is on this site.


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## TREETOP (Feb 11, 2009)

The guy I bought the alternator from has been awesome about this, he's shipping me a 270A stator at no charge. 
Now... Anyone ever change a stator in an alternator? Is it pretty straightforward?


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## TREETOP (Feb 11, 2009)

TREETOP said:


> The guy I bought the alternator from has been awesome about this, he's shipping me a 270A stator at no charge.
> Now... Anyone ever change a stator in an alternator? Is it pretty straightforward?


I received the new 270A stator today. This should be fun. 
Anyone have any tips?


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## Dryseals (Sep 7, 2008)

Yep, take it to an alternator shop and let them do it for you. That way they'll test it at the same time.

In the real world of electricity, it moves at the speed of light. Too bad the components that regulate and create the power don't do the same. We call it slew rate. If the demand for power jumps suddenly, the current and voltage will not jump at the same rate. More so on a device like an alternator. So there will always be a dip in the power no matter what you do.
In the old days, we learned, ELI the ICE man. E=voltage I= current L=inductive C=capacitive. ELI voltage leads current in an inductive system. ICE current leads voltage in a capacitive system. Your system is more of an inductive system or in other words the voltage has to be there first before the current will follow. So if your amp demands a sudden draw of current, to produce the current, your system will have to create the voltage first, thus the dip you see.
Diodes and regulators all have certian slew rates. The faster the slew rate, the more expensive the components. Just testing your alternator for load will tell you what it can produce, but it won't tell you how fast it can produce the power. We use gear at work for detecting dips and slew rates in power. Plotting the current and voltage together is the only real way see what's going on.
My guess is that your alternator is slow and the only fix is a bank of caps in the trunk or a better alt.


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