poormans alternator test

ganderson · Post by **ganderson** » Mon Oct 24, 2005 5:25 pm

You may want to try the poorman's alternator test.....

Jump start your dead battery and runs it runs for a few minutes...Disconnect the negative or positive cable from the battery either is fine...

If the car dies once you've done this there is a HUGE chance that your alternator is no longer charging...and running solely on the battery which is already burdened

Or it could be the voltage regulator...especially the ground at the base of the regulator

It does not sound like a starter issue

goodluck

gene

shifty · Post by **shifty** » Mon Oct 24, 2005 9:14 pm

I've heard this is a bad idea.

From http://www.thecarconnection.com/Enthusi ... A7030.html

Every now and then a customer will come in with an electrical problem, either starting or charging, and tell me he knows the alternator is working because he unhooked the negative battery terminal while the car was running and it didn't shut off. Apparently the look on my face must be sufficiently horrified because after a few seconds they usually say, uh, did I do something wrong?

I suppose this alleged test dates back to the Model T Ford and probably wasn't valid then. It is rather the equivalent of setting a woman on fire to determine if she's a witch or not. The answer won't much matter. As the negative battery cable is pulled away, a tremendous voltage arc occurs. The alternator starts to both overcharge and burn with nowhere for the voltage to go, and every circuit in the car is desperately hunting for ground. Think of all the computer processors, diodes, solid state circuits, relays, and modules all getting an enormous voltage spike. You may as well fly a kite into a thunderstorm and hook it up to the back of your PC. Heck, BMW says you can blow the radio in their car with just a jump start.

So please, if you believe nothing else I've written, do not disconnect your negative (or positive) battery cable while the car is running. It is not a valid test, it tells you nothing, and is likely to cause future electrical problems.

ganderson · Post by **ganderson** » Mon Oct 24, 2005 10:06 pm

Hello Leigh,

It may be a bad idea to you..BUT it really works...Live alittle try it.It might save you some money and time..

I'm glad that you brought up history.You'd probably be in the group of "Nay sayers" that told Christopher Columbus that the "world is flat"..I'm sure glad that he did not listen...Otherwise we'd have no day off in November

I have done this several times through the years and I was fooled once by my "voltage regulator"ON MY 1970 Roadster.Turned out to be a corrided ground .....

And according to you..I NEVER experienced the "HUGE arc" that you spoke of..You embelished just a PINCH..Let the "star wars "analogy go

I would never suggest anything on a tech forum that I have not tried...successfully

MAYBE WE SHOULD SEND THIS TO "Myth Buster"then you'd have your science....

I don't agree with you Leigh

TR · Post by TR » Mon Oct 24, 2005 10:18 pm

On an older car you don't run the many risks associated with newer car electronics. But...If running a car with a weak battery is hard on the charging system, imagine how hard running it without a battery is. The poor regulator is not made to deal with such issues.

I would never do this, because the risk is just not worth it. If the alternator is charging, something has a high chance of getting fried.

Also, if you have ever seen a person who has blown up a battery in their face, you will never (and I really mean never) do anything that might cause a spark near a battery...

ganderson · Post by **ganderson** » Mon Oct 24, 2005 10:24 pm

I guess we'll agree to disagree..

No problem happens all the time

ambradley · Post by **ambradley** » Mon Oct 24, 2005 10:41 pm

Poor man's alternator test? Take it out, take it to your auto parts store for a free test. Make sure they hook it up right and watch the meter to see how it does.

Post by **spl310** » Mon Oct 24, 2005 10:59 pm

Gene,

Those aren't Leigh's words, they are from the link he posted. He pasted the text so that it would be more available (it is at the bottom of that page...)

ganderson · Post by **ganderson** » Mon Oct 24, 2005 11:06 pm

Thanks Sid....

I guess I should have added my disclaimer.."This is what I would do"

Maybe, I should have added that..

Sorry,Leigh

I would still do the "poorman's alternator test"everytime.

shifty · Post by **shifty** » Tue Oct 25, 2005 12:51 am

No problem, Gene.

I'd recently read about that test and was a bit leary. This is a place to offer ideas both pro and con and discuss them.

spyder · Post by **spyder** » Tue Oct 25, 2005 1:49 am

Your battery usually has around 12.5 volts with the car off. Running, I would expect between 14 to 15 volts depending on your charging system. I get 14.6 with a GM alternator. A stock charging system uses a relay regulator which goes from high charge to low charge based on the regulator relay's tension. When it goes to hi charge, the "GOOD" battery absorbs the extra energy the alternator develops until the regulator goes back to lo charge. The time the system spends in the hi charge transition is based on the electrical load. If you have a bad or no battery the charging system will not properly regulate the alternator voltage. Here is the rub, old system i.e. points and condenser perhaps no problem but if you have any voltage sensitive newer electronics they could be damaged by the voltage swings.

Moral of the story: buy at least a cheap digital voltmeter to "test" your charging system. Under 20 bucks @ Radio Shak or Harbor Freight.

Post by **dbrick** » Tue Oct 25, 2005 11:38 pm

I'm with Spyder, whether the test works or not the voltmeter ALWAYS works. Anyway, if the ammeter is hooked up, it tells you if you're charging, or rev the engine and see if the lights get brighter

:D

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poormans alternator test

poormans alternator test

???

alternator

no problem