Alternator & ampmeter are fine...NOW WHAT?

[mrtexas]

Originally Posted by Texas Tom

Yesterday I posted a question about why my ampmeter would not 'excite' when I started the engine and revved it up. Several folks suggested my alternator is likely the problem.

This morning I tested the alternator with a voltmeter and found that when I started the engine, the voltage dropped. This usually indicates a faulty alternator. So I pulled a spare (new) alternator from my parts pile and installed it. Same result!

So I took both alternators to my NAPA store and asked them to test them. Test result: both alternators work just fine. Yikes!

So I'm back to square-one. There is nothing wrong with my alternator, and apparently nothing wrong with my ampmeter (I tried three ampmeters, which yielded identical results: very slight discharge).

So why is my system (12 volts) discharging when the engine is running? I have no clue. Everything worked fine before I took the car out yesterday to give it some exercise.

HELP!

Did anyone suggest you charge the field on the alternator? If you let a one wire sit around too long it won't charge because the field loses it' magnetism.
Touch a hot wire to the field wire and it will resume charging. A three wire has juice to the field all the time I think. I bought a perfectly good $200 alternator for $25 because it wouldn't charge. Worked fine after exciting the field.

[Mike V. Florida]

Originally Posted by Texas Tom

In answer to questions:

-All electrical components (horn, lights) work just fine.
-System is wired per diagrams for 12V systems found in various Model A publications.
-Battery shows 12.6 volts on analog voltmeter prior to starting engine.
-When engine is started, voltage on voltmeter drops to 10.5V or so.
-NAPA store says both my alternators (1-wire) are fine.
-CarQuest store says my 12V battery (which is sealed and is 15 months old) is fine.

Tomorrow I'll try an alternate battery and will report results.

Thanks to all for your assistance.

1) With the battery fully charged from a battery charger take a voltage reading with nothing connected to the battery.
2) With the battery installed, take a reading. (should be the same as above)
3) Start the car and double check that the voltage changes as before. (12.6 down to 10.5)
4) Step on the gas and see if voltage changes. (the Alt may just need to speed up before it generates voltage.)
5) Shut down the car. Take a voltage reading.
6) Remove the wire from the Alt. Start the car and take a reading.

Does the voltage drop as before?

[Tom Wesenberg]

10.5 volts is OK while cranking, but if that's the reading after starting the engine, then the battery is way low on charge, or no good. Put a long slow charge on the battery and try that again.

[Mike V. Florida]

Originally Posted by Tom Wesenberg

10.5 volts is OK while cranking, but if that's the reading after starting the engine, then the battery is way low on charge, or no good. Put a long slow charge on the battery and try that again.

I hope he clears that up. I read, "-When engine is started, voltage on voltmeter drops to 10.5V or so." as after it is started and running not when the engine is starting.

Like I said, he will most likely clear it up for us later.

[Willie Krash]

Stop replacing parts....please.
Test the voltage at the alternator's battery terminal and at the battery. Repeat with the engine running at fast idle.
the alternator should go to 14.5V fast idle, the battery should read very close to the same. Report back.
While you're at it touch the pulley and make sure it isn't hot.

If the two readings are more than 3/4V apart you have an issue, 1 to 1 1/2V you have a problem.

No offense to NAPA but their alternators are nothing to write home about, they keep me in business..

Think plumbing, your alternator is a pump, if the pipe from the pump to the water-meter (amp meter), is broken you will see nothing on the water (amp) meter, it a pipe is broken or plugged you will never fill your pot (battery) with water. So we need to see it the water from the pump is the same at the water pot. If not with that voltmeter you will follow the water pressure from one point to the other, alternator to battery..
The simple view. It late, I'm tired, hope it reads right.

[Texas Tom]

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PROBLEM SOLVED!

Well, I charged up my 15-month-old CarQuest 'premium' 12V battery so it read 12.6V on my voltmeter. I reinstalled the alternator and battery. I had 12.6V at the alternator. I started the engine and bingo! The ampmeter needed moved to the positive side and the battery voltage increased when I revved the engine, just like it's supposed to.

The only thing I can think of that caused this entire issue to occur must be that my battery's charge was low after sitting idle for several weeks. There was plenty of cranking power to start the engine, but its low voltage apparently caused the alternator to discharge the battery rather than charge it. There may be another answer, but that's the only one I can think of.

Anyway, all's well that ends well.

Thanks again to all who provided me with your wise counsel.

Happy Motoring!

Texas Tom

[James Rogers]

Guess I was close since it was the battery after all.

[Willie Krash]

TT, if the battery is low but the car will start the amp gauge should reflect a heavy charge, it is what it is there for...
Keep an eye on things, maybe you are out of the woods.
Just an fyi, typically if the battery is bad it will not take current. If you put a line charger on it you would see the voltage rise but not the current. It one of many indicators but not a definitive test.
A shorted battery will take a lot of current but the voltage will not rise, typically.

I only mention this as withot tools it is a way to narrow it down.
As Bill W suggest batteries are a mystery. We have a $6,000 battery checker and charger. It is pretty damn good but not at 100%. That is to say dispite a positive good reading, if all other issuses are good too, out goes the battery.

[Quigley1930]

See how it does in a couple of days just sitting. I had the exact same problem. Charge the battery and the truck would start. Even overnight no go. Turned out the battery had a dead cell. Hope this isn't your case.

Sewall Tyler

[James Rogers]

Originally Posted by Quigley1930

See how it does in a couple of days just sitting. I had the exact same problem. Charge the battery and the truck would start. Even overnight no go. Turned out the battery had a dead cell. Hope this isn't your case.

Sewall Tyler

Exactly the way mine acted. I could jump it off or charge the battery and start it but the ammeter would show nothing nor would the alternator charge. Shortly the battery would go down till it would no longer start the car or light the lights. New battery and all has been good for the last 4 years. Bout time for a new one now.