Ammeter/Voltmeter Discharge

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Jun 15, 2011 | 01:22 PM

captjohnws

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From: Tucson AZ

Ammeter/Voltmeter Discharge

I am a new owner of a mostly stock 73 w/350. I have noticed that my Ammeter usually shows a -15 to -10 discharge, but sometimes it kicks in and goes back to zero. I went out last night and it was showing -40 (completely pegged) for a couple of minutes. I decided just to go home and avoid problems. The car started fine and seemed to be running well. Any thoughts and/or diagnostic procedures would be helpful. Thank you for the help.

John

Jun 15, 2011 | 01:31 PM

...Roger...

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From: Dayton, Ohio

Its possible its just a bad gauge,what were your lights doing ? Watch your courtesy lights as you turn things on and off,also rev the motor a bit while watching the courtesy lights to see if there is recovery from turn things on.
If you have a voltmeter get some readings from the battery and back of alt.

Jun 15, 2011 | 01:34 PM

MelWff

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Normally on a fully charged battery after running for a few minutes the gauge should read 0. So you either have inaccurate gauge or depending upon how long you have the car your battery is slowly going dead. When you first start the car do you get any readings on the positive side of the gauge? How long has it been doing this? If you have a volt meter you can check the output of the alternator which should be around 13 volts when running.

Jun 15, 2011 | 08:38 PM

7T1vette

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From: Crossville TN

The only reason an ammeter should show a discharge...with the engine running...is that the car is at idle condition and lots of electrical stuff is running---the alternator can't meet the energy demand at engine idle speed. If it shows a discharge when cruising, you have an alternator problem.

Jun 17, 2011 | 05:44 PM

captjohnws

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From: Tucson AZ

Thanks for the help. To answer the questions below:

- I never get a postive reading on the ammeter.

- It has been doing this for the past month.

I have a few questions:

(1) are the courtesy lights the gauge lights?

(2) Where do you put the voltmeter on the alternator to get readings? Do you do this when car is at idle?

(3) What should readings from the battery be? Does it matter if the car is running?

(4) Somebody told me to start car and then disconnect the negative battery cable (using quick disconnect) and see if the car stays running. If it dies, then alternator needs replacement. Is this sound advice?

Thanks.

Jun 17, 2011 | 06:09 PM

7T1vette

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From: Crossville TN

At the alternator, put + probe on the "output" terminal on the back (red wire with the rubber boot on it); put the - probe on the case or on the ground lug on the back of the unit. You can check it running (which would give you alternator output voltage (if you take a reading at idle and one at elevated rpm, you can determine if the alt. is 'lugging' at idle (less than 12.3 volts). If you hook up the voltmeter to the alt. and start the car, the alt output should show around 14 volts, indicating that it is working properly. After a minute or two, that voltage should drop to around 13 volts or less (indicating that the battery has been recharged).

And, "No".... if you remove the ground wire from the battery when the car is running, it will die because it has an alternator (instead of a generator). Ignore anything that "buddy" tells you in the future.

Jun 17, 2011 | 06:53 PM

...Roger...

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From: Dayton, Ohio

Quote:

Originally Posted by 7T1vette

Quote:

Originally Posted by 7T1vette

And, "No".... if you remove the ground wire from the battery when the car is running, it will die because it has an alternator (instead of a generator). Ignore anything that "buddy" tells you in the future.

I disagree,the engine should stay running.

Jun 23, 2011 | 01:14 AM

captjohnws

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From: Tucson AZ

Figured out

Thanks for all of the information. I learned a lot about the electrical system.

I fugured out my multimeter and tested the battery before and after cranking. My alternator seems to be working fine. I have driven the car a couple of times and the voltmeter is back to working normal. I have no idea what happened that night.

Thanks for all of the help.