How to test a High Amperage Alternator

I was once asked why can a rebuilder or store like Autozone not properly test an alternator?..
I figured this would be a good place to post..

Here was the question

"Dom
If you don't mind, I'd like to ask your explanation of why it is difficult to test alternators and how one can go about getting one properly tested? I know you're busy with work, but your contributions in the electrical arena could greatly help increase the knowledge base on this forum.

Thanks!"


No problem, I would love to answer this question..

Here are the main reasons for each party that tries to test an alternator.

Your hometown rebuilder.

1. They are just that, a rebuilder. They know very little about the design and function of an alternator. They just repair factory alternators by replacing parts. They also will tell you your crazy when you tell them an alternator can do 200 amps. They dont understand what they dont know.

2. When they do test an alternator,(on their bench), they almost always use small aligator clips to the battery post of the alternator. Probably 8 or 6 gage. Would you connect your 2000 watt amplifier to your battery with 8 gage? I dont think so....for obvious reasons.

3.On their bench test, they more than likely will never go over 3500 to 4000 rpm. Dont freak out, this is alternator rpm, not engine rpm. An alternator should spin atleast 3 times faster than your motor. The industry standard for max output is to be tested at 6000 rpm. Approximately 1500-2000 engine rpm. 3500-4000 is just above idle.

4.When a load is applied, it should be gradual. 75% of the test stations that are common to rebuild shops are either on.... or off when it comes to loading. (typically 300 amps) There is no way to ramp up the load presented to the alternator. When you load an alternator with an instantaneous 300 amps, the regulator does not have time to react and the reading will be substantially lower. It also depends on the type of regulator used. Some vehicles must have a time delay type regulator due to the car either being so new or aftermarket not available yet. When you load the alternator down, you keep going until the voltage level falls to that of the battery. Which is typically 13.0-13.5 This way all current being pulled is is being produced by the alternator, not the battery. You then take the current reading off the battery post of the alternator.


Your local "Autozone" or "Advanced"

I have taken my personal vehicle to both of these parts stores just to see why I get so many calls and emails stating "I had my alternator tested and it doesnt work or only does 20 amps". Both companies are improperly trained in how to test your alternator.

1.The first thing they do is roll out their tester and connect it wrong. They put the current clamp on the battery negative. Why they want to know how much current is going through your negative battery terminal is beyond me..... I personally like to know how much current is coming out the alternator OUTPUT terminal...which is the positive one.
While your sitting there at idle, before the test even begins, the tester is telling you how much current is going through the negative battery terminal. This is how much charging current your battery is pulling. They all confuse this with how much current your alternator is doing at idle. You need to first load the alternator down before you can determine how much current your alternator is capable of producing.
Just for example....I have 3-300 amp alternators on my Suburban. They told me I was doing 55 amps at idle and 168 amps max. He did admit tho after seeing under the hood, he didnt really know anything about setups like mine.

I would type more, but I gotta go. If anybody would like some explanation of other issues, let me know.
Hope this post helps.

Thanks, Dominick Iraggi

Ok, I guess I forgot to actually explain how to test an alternator.

I will explain it two different ways. One is how to test your own alternator and make sure its producing current and the other is how an alternator shop should test your alternator.


1. Put a voltmeter as close to the alternator as possible. (Positive and Negative both.) If you can put the meter leads directly on or in the wire coming off the alternator that would be optimum.

2. Now start the car and turn all accessories off and check the voltage level. It should be between 13.5 and 15.1 volts. (depends on your make and model of car) It is a good idea to write down all voltage readings your taking. The main thing your looking for is a drop in voltage. Once you hit around 12.5, your alternator is not going to produce any additional amperage.

3. Now turn on one accessory at a time. Write down that accessories fuse rating. Keep turning on accessories and adding fuse ratings until the voltage falls to 12.5. Now add 15 amps if your electric cooling fans are on and 10 amps per battery in your system. Your numbers may look like this.

2 batteries- 20 amps
Cooling fan- 15 amps
Headlights - 10 amps
A/C blower - 10 amps
Rear Dfrst - 15 amps
High Beams - 10 amps
Fuel Pump - 10 amps
---------------------
Total not including stereo system 90 amps.

If you can turn on all these accessories and your voltage does not fall below 12.8 your alternator is capable of producing more than 90 amps of current.
You can now turn on your system and start slow on the volume. Its hard to say how much current your system pulls unless you know how much power your amps are producing at a given volume level.

You will need to do all these measurments at idle and at 2000 rpm. This way you can test idle output and max.

NOTE:
When testing output at idle your numbers may appear a little low. It is probably because your alternator is making your motor bog down and when you spin an alternator slower it can not produce as much amperage.

now that is a cool read thanks for taking the time to share this!