How do you test an alternator?

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take the hot wire off the batt while it is running....if it shuts off the alt is bad

Don't do this, not only is it an unreliable test of an alternator, it can damage other components when the battery is removed from the system. The battery serves as a cushion to protect against spikes.

i disagree. its not the best test but it would work. and these cars are pretty stout when it comes to electronics susceptable to voltage spikes.

If youre really concerned, then dont reconnect the battery if the car stays running with the ignition on. have someone turn off the ignition before reconnecting it. then there will be no spike so no possible damage

but a voltmeter would be best. check battery voltage at the alt outpput with the car off and then with it running. voltage should be a few volts higher with the car running

!!! DO NOT EVER DO THIS !!!

I've actually checked this years ago with a "test pig" that had no electronic anything. Disconnecting a charging unit from the battery generates OVER A HUNDRED volts of spike. I don't have it anymore, but I once had a 'scope photo of this spike, and used to haul it out for guys like you

One modern cars, with electronics of all kinds, solid state controls, radios, regulators and ignition, this is a GUARANTEED way of frying something. Yep---I know you "got away with it"---you might get by over a hundred times, and the 103rd time you do it, "the guy" might say, "Ya know, it's funny, when YOU worked on my alternator 'thu udder day' the RADIO quit working."

Disconnecting the battery on any vehicle while the vehicle is running is nothing but stupid. Its just as you say, it generates a huge spike, thats capable of serious damage. Oh, it might not be immediate, but it damaged something!

Ya know, I argued back and forth with a few know-it-all hacks here a few years ago over this very topic. Some people just dont want to learn.

The good news is most people get away with it because their alternator is actually bad (disconnecting it also can damage it too) and there is no spike. However, if its working....its a different story.

Besides taking the chance of creating a spark near the batt.It's obvious some people have never seen a battery explode,not pretty!! If you have a Batteries Plus or even Auto Zone should test it for you,or follow the instructions above and use your meter!!!

Posted By: scratchnfotraction

Re: How do you test an alternator? - 10/29/08 12:33 PM

ok,what ever

,call me lucky when it comes to me doin it

I am just a shade tree rabid parts changer and should keep my comments to myself

dont do it that way

the all mighty knows all has spokin

Posted By: stumpy

Re: How do you test an alternator? - 10/29/08 01:01 PM

Guess I've been lucky for the last 50 years. I never had one spike enough to hurt the electrical system or a battery blow up either. But we must listen to the "experts".

Posted By: denfireguy

Re: How do you test an alternator? - 10/29/08 01:28 PM

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I wrote a long version a month ago, and don't want to again. Here you go. There are others on the net, too.
http://autorepair.about.com/od/electricaltroubleshooting/a/battery_check.htm

You are right, thought we had beaten this horse to death already.
No wonder there are so many bad regulators on the rallye dashes out there.
Craig

Posted By: 5537SG

Re: How do you test an alternator? - 10/29/08 01:45 PM

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Guess I've been lucky for the last 50 years. I never had one spike enough to hurt the electrical system or a battery blow up either. But we must listen to the "experts".

sometimes the damage isnt obvious or immediately apparent.

I dumped alot of Freon into the atmosphere too, but I never saw anything happen

As for the blown up battery, tell that to the guy at the garage down the road from me with one eye. It does happen.

But theres no need to be cranky about it, just accept the new found knowledge and move on. The day a mechanic stops learning is the day they are no longer a mechanic.

Keep an open mind, and aim to learn, cause it never stops.

the guy who started this thread sought knowledge, now he has some.

Posted By: Fastback67

Re: How do you test an alternator? - 10/29/08 01:50 PM

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if you take the alternator off, Autozone will test it for free.

Most shops will test it for FREE

Posted By: Anonymous

Re: How do you test an alternator? - 10/29/08 02:14 PM

i wouldnt doubt that there could be momentary spikes but the wuation is , are they harmful to our cars.

The spike is probably a result of and instaneous change in the cirucit whether it be a lag in the VR compensation or some sort os charging effect. But even if there were 100v spikes, they are of what duration??? milliseconds? microseconds? how many amps? microamps? certainly not amps.

So there affect would be small because the average power or current delivered to the system would be negliable when you look at it real time.

its like the shock you get when you toch a door knob after rubbing your feet on the carpet. its several thousand volts BUT the duration is short (milliseonds) and the amps a low. so you dont die. Up the amperage or lengthen the duration and youre a dead man. you build up 50v of static charge just scratching your head- but you dont feel it.

on older cars with healty stought resistors and cap and maybe some power transistors (in the ign box or radio) they dont see that much average power. plus the resistance in the wires and the bleed offs from vraious power users drop that stuff into the dirt.

just a different persective

Posted By: 5537SG

Re: How do you test an alternator? - 10/29/08 02:32 PM

Thats a fair perspective, however the duration of the shock isnt important, the shock itself is. A static electricity shock such as the door knob shock you mention is by far capable of doing serious damage to any intergrated circuits in the car.

Even though older cars are pretty safe to get away with that practice, it still should be avoided. A GM with an internal regulator is subject to damage from that effect by itself, not to mention if there was an aftermarket radio/cd player in the car. Dont forget aftermarket guages as well.

If anyone wants to try this practice with an AutoMeter tachometer, disconnect your battery with the car running and rev it up a few times, reconnect the battery with it running and see how well that tach works.

Its a practice that should have disappeared before leaded/regular gas.

Posted By: therocks

Re: How do you test an alternator? - 10/29/08 03:14 PM

Easiest way is get a test light.One end to ground and car off touch the large wire.If it lights it has batt volts.Now unplug the 2 wire connector and touch the large wire in it if the test lihjt lights its good.now turn on the key and touch the small wire if it lights its good.They have an internal reg.I used to rebuild those all the time at work.Super easy but for the prive of a reman not worth the effort.Id also check the actual volts with a volt meter also.But if you do the test Id say the alt is bad.That or just take it to a parts store and have them check it.Thats an easy alt to remove.Rocky

Posted By: stumpy

Re: How do you test an alternator? - 10/29/08 11:38 PM

"Its a practice that should have disappeared before leaded/regular gas."

Newer is not always the only way. The disconect trial has worked for a long time with no problems. I'm not saying your method is wrong but don't discount the old way either. Both work and I don't see one being any better than the other. The statement the "the spike didn't show any immediate damage but who konws what damage down the road" doesn't hold water. If it is going to cause damage it would do it right now not later. So I say let's just agree to do it our own way and go from there. I will continue to suggest the disconnect way and you can recomend your way.

Posted By: 5537SG

Re: How do you test an alternator? - 10/30/08 01:55 PM

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"Its a practice that should have disappeared before leaded/regular gas."

Newer is not always the only way. The disconect trial has worked for a long time with no problems. I'm not saying your method is wrong but don't discount the old way either. Both work and I don't see one being any better than the other. The statement the "the spike didn't show any immediate damage but who konws what damage down the road" doesn't hold water. If it is going to cause damage it would do it right now not later. So I say let's just agree to do it our own way and go from there. I will continue to suggest the disconnect way and you can recomend your way.

Stumpy, the disconnect method never was, and never will be an accepted nor reccomended method of diagnosing a charging system. Its very dangerous for the persons performing it, and it can do serious damage to the car.

Doing damage now or later certainly does hold water. If you think it doesnt it only solidifys your lack of knowledge regarding electronics.

What if I took every second stud out of the walls of your house? Would it still stand? Im sure it would, but what would happen if you got a foot of snow on your roof? What Im getting at is that the damage (although immediate) may not present itself until later.

Im sure in all your years as a mechanic youve learned alot, and Im sure theres probably a thing or two you could teach me too, but when it comes to diagnosing a charging system, you need to learn the proper way.

You also need to stop reccomending disconnecting the battery with the car running before your advice seriously hurts someone.

Use a multimeter. Instead of grabbing a wrench to disconnect the battery, grab a meter and check the proper way. Just think, no matter what you determine by disconnecting the battery, youre going to need a multimeter for the next step ( determing if the alternator has all the correct inputs)...so just use the meter in the first place.

Posted By: scratchnfotraction

Re: How do you test an alternator? - 10/30/08 02:23 PM

here is my "disclamer"

what works for me...might not work for you

so..is unplugging the vr on my dodge truck and shorting the one wire to make the alt go full charge to see if the alt IS capable of charging the wrong way too?

been doing that just as long myself

Posted By: stumpy

Re: How do you test an alternator? - 10/30/08 02:25 PM

As I said you are carried away with your method being the only one and I will still use and will tell others of my method. It has been an accepted method for a lot of years. Otherwise how come so many people know about it? I don't know where you got the idea that it is as dangerous as you claim it to be but experience tells me it doesn't happen like you want everyone to think. I won't argue with you any more because I can see you are too hard headed to allow someone else to have an opinion that disagrees with yours.

Posted By: 5537SG

Re: How do you test an alternator? - 10/30/08 02:44 PM

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here is my "disclamer"

what works for me...might not work for you

so..is unplugging the vr on my dodge truck and shorting the one wire to make the alt go full charge to see if the alt IS capable of charging the wrong way too?

been doing that just as long myself

thats "full fielding" thats not really dangerous unless you left it like that for sometime.

disconnecting a battery is completely different. It turns a DC circuit into an AC one. Completely different scenario.

Posted By: 5537SG

Re: How do you test an alternator? - 10/30/08 02:49 PM

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As I said you are carried away with your method being the only one and I will still use and will tell others of my method. It has been an accepted method for a lot of years. Otherwise how come so many people know about it? I don't know where you got the idea that it is as dangerous as you claim it to be but experience tells me it doesn't happen like you want everyone to think. I won't argue with you any more because I can see you are too hard headed to allow someone else to have an opinion that disagrees with yours.

Im not the one hard headed here. Heres how willing I am to accept other methods...

diagnose it ANY OTHER WAY YOU WANT....but NEVER, EVER, disconnect the battery with the engine running on ANYTHING.

So, any other way you want. just not that way.

It has NEVER been accepted by ANYONE other than hacks and butchers. It certainly is not reccomended or accepted by ANY manufacturers or SAE service procedures. Not now, not ever.

why? because they understand what theyre doing.

Posted By: stumpy

Re: How do you test an alternator? - 10/30/08 03:22 PM

Name calling, now isn't that the adult way to try and settle

a discussion.

Posted By: Anonymous

Re: How do you test an alternator? - 10/30/08 04:01 PM

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Quote:

here is my "disclamer"

what works for me...might not work for you

so..is unplugging the vr on my dodge truck and shorting the one wire to make the alt go full charge to see if the alt IS capable of charging the wrong way too?

been doing that just as long myself

thats "full fielding" thats not really dangerous unless you left it like that for sometime.

disconnecting a battery is completely different. It turns a DC circuit into an AC one. Completely different scenario.

explain the dc into ac situation? im unsure how that happens. are the diodes in the alt bypassed? is the current cyanging directions ever cycle? i cant envision this happening

Posted By: HealthServices

Re: How do you test an alternator? - 10/30/08 04:28 PM

Stumpy, the biggest problem with giving advice on the internet is you never know the situation on the other end. For example we do not know the condition of the battery, if it is deteriorated or sulfating excessively. Although highly unlikely things do happen! You never know how mechanically inclined or aware the person you are giving advice is.

I will have to agree with 5537SG. I'm never was a supporter of the remove the battery lead. If it does work for you great (don�t do it though), what I am more concern with is the safety aspect of it. I�ve been a working shop foreman/mechanic at small auto dealership for 15 or so years. In that time I can remember three times where an inexperienced tech have a battery explode in their face, and one time when an experienced tech had one blow up not in his face. So experienced or not it when there is a spark, explosions can happen!

Posted By: Scott Carl

Re: How do you test an alternator? - 10/30/08 04:33 PM

Don't mean to

,
but here are just a few documents that should warn folks to be very cautious when dealing with batteries.

http://www.aa1car.com/library/battery_safety.htm

"WARNING: NEVER disconnect a battery when the ignition is ON, or while the engine is idling or running.
Doing so can damage electrical and electronic components."

http://ca.answers.yahoo.com/question/index?qid=20070222191231AAmSB9T

"Do not disconnect the battery with the engine running, the resulting voltage spike from the alternator can fry all your electronics."

http://www.engadineautoelectrics.com.au/battery%20care%20and%20maintenance.html

"NEVER disconnect any battery terminals from a car's battery while the engine is running (to test if your alternator is charging, for instance). This could destroy your cars charging system and it's onboard electronics!"

http://mypage.direct.ca/k/kdomries/safety.html

"Batteries can explode. I didn't believe this until one blew up while I was connecting jumper cables to it. It made a very loud noise, and splattered battery acid all over the place, not to mention flinging jagged bits of plastic in all directions at very high velocity."

http://www.loudouncountytraffic.com/2008/02/helpful-hints-for-changing-your-car.html

"Don't disconnect battery cables while engine is running (your battery acts as a filter)."

And for those that don't believe "harmless" static electricity won't do any damage, put on yer Hushpuppies, shuffle across the carpet and go work on your PC.

Posted By: Anonymous

Re: How do you test an alternator? - 10/30/08 05:05 PM

apples and oranges scott.

microciruits in NEW cars (note the word, NEW) can easily be damaged by the spikes. just like your PC can. but pre 1980's cars do NOT have those in them. Different result.

try blowing a tube from a 1950's TV by unplugging it. wont happen. too stout. they run on HIGH VOLTAGE HIGH CURRENT. do that to a microprocessor and you can kiss it goodbye.

DIFFERENT TECHNOLOGY

BTW, the batteries exploded because of the build up of flamable gasses which eminate from the battery. one spark and POOF. now, a car that is RUNNING typically have a FAN blowing which DISPERSES that gas so it DOESNT IGNITE.

Much like how gas stored near a water heater will explode- gas fumes are concentrated and not being blown and diluted but fart next to a bbq and no explosion cause the gas is diluted

Posted By: Scott Carl

Re: How do you test an alternator? - 10/30/08 05:29 PM

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apples and oranges scott.

microciruits in NEW cars (note the word, NEW) can easily be damaged by the spikes. just like your PC can. but pre 1980's cars do NOT have those in them. Different result.

try blowing a tube from a 1950's TV by unplugging it. wont happen. too stout. they run on HIGH VOLTAGE HIGH CURRENT. do that to a microprocessor and you can kiss it goodbye.

DIFFERENT TECHNOLOGY

BTW, the batteries exploded because of the build up of flamable gasses which eminate from the battery. one spark and POOF. now, a car that is RUNNING typically have a FAN blowing which DISPERSES that gas so it DOESNT IGNITE.

Much like how gas stored near a water heater will explode- gas fumes are concentrated and not being blown and diluted but fart next to a bbq and no explosion cause the gas is diluted

You do have a point. Most of the hazzards relating to battery disconnection do pertain to newer cars with onboard electronics.

Posted By: scratchnfotraction

Re: How do you test an alternator? - 10/30/08 06:46 PM

man I must be dumb... I have jumped cars,with # 12 wire and touching the bumpers together on the side the road,when every one said that ol hick trick wont work...was that bad for the car?

undo the batt,swap them in out with the engine running in the rain,sure that was bad too!

drove them around with no batt and never had any probs with them,then or later,that was bad also, right?

so now I am a rabid redneck hack shade tree parts changer..got it

it a dumb people method...best left to us dumb people

to the OP,sorry your post got hijacked,hope you get it fixed,no matter which way you do it,like said...just trying to help... should have left that chivy post alone

...my bad

Posted By: Scott Carl

Re: How do you test an alternator? - 10/30/08 06:58 PM

Just for the record

and

I have done the bumper to bumper trick a time or two. No ill effects that I know of, but they were pre-computer cars. What with the rediculous cost of todays electronical wizardry, I don't think I would take the chance. Probably wouldn't work anyway with all the plastic in the way

Also, my apologies as well, StationWagonGuy, for stepping on your thread. Hope someone was helpful here.

Posted By: Anonymous

Re: How do you test an alternator? - 10/30/08 07:04 PM

(note to self- dont buy used cars from scratchin or scott)

Posted By: Scott Carl

Re: How do you test an alternator? - 10/30/08 07:24 PM

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(note to self- dont buy used cars from scratchin or scott)

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Posted By: scratchnfotraction

Re: How do you test an alternator? - 10/30/08 07:39 PM

for sale
440 max wedge hemi with a stock 3/4 cam and no alt/charging system

gots a cut off switch in the trunk

missing some of the factory smoke that comes in the wires in the dash,they got a leak when I tested the alt

the lights are dim cause its low on the blinker fluid in the master cyl

bring your own batt,mine is blowed up,sorry

its on a hill,I will help push you down it to pop the clutch on the auto tranny

BR549

I have a car for you!

note to myself...undo the batt when running and drive it home tonight,I will post back if it leaks any smoke out

Posted By: CYACOP

Re: How do you test an alternator? - 10/30/08 08:24 PM

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for sale
440 max wedge hemi with a stock 3/4 cam and no alt/charging system

gots a cut off switch in the trunk

missing some of the factory smoke that comes in the wires in the dash,they got a leak when I tested the alt

the lights are dim cause its low on the blinker fluid in the master cyl

bring your own batt,mine is blowed up,sorry

its on a hill,I will help push you down it to pop the clutch on the auto tranny

BR549

I have a car for you!

note to myself...undo the batt when running and drive it home tonight,I will post back if it leaks any smoke out

How do you test an alternator?