Alternator Light Stays on Until 3,000 RPM - Team Camaro Tech
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post #1 of 7 (permalink) Old Dec 30th, 11, 06:04 PM Thread Starter
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Alternator Light Stays on Until 3,000 RPM

On my brother's classic car he has an apparant problem with his charging system. The Alternator light stays on until the engine is revving about 3,000 RPM when driving. He took the alternator to a shop who confirmed that it is functioning correctly and changed the voltage regulator without resolving the issue. He also found that there is a 2v draw of power when the car is not running. If the car is idleing and he pulls the + terminal off the battery the car shuts off. Is that normal?

Now I have to confess that my brother is a member of the dark side and owns a '65 Mustang Fastback with the little 289 engine. Now don't flame me! In the interest of hotrodding please help resolve.

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post #2 of 7 (permalink) Old Dec 30th, 11, 06:10 PM
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Re: Alternator Light Stays on Until 3,000 RPM

LOL a FORD ? Found On Road Dead.

Okay had to dig just once....I would say if the Alternator is working by putting out a charge, and the regulator telling it to charge, and the battery is taking the charge the rpm's would suggest the belt is loose and is slipping? That's where I would look first.


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post #3 of 7 (permalink) Old Dec 30th, 11, 06:43 PM
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Re: Alternator Light Stays on Until 3,000 RPM

Check the voltage at the battery off and running at an idle. I've never seen a 60 or 70's model car that wouldn't run after pulling a cable off if it had a good charging system.

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post #4 of 7 (permalink) Old Dec 30th, 11, 07:14 PM
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Re: Alternator Light Stays on Until 3,000 RPM

If you lift the batt cable and and the engine doesn't continue to run then the alternator is not charging. You already know it's not charging until 3000 rpm so go from there. I don't know my Fords very well but if there is a voltage regulator then there is probably an "excitor wire" that signals the alternator to start charging when you turn the key (starter is running) and start the car. This is a hard one to trouble shoot unless you know what to look for. The excitor wire only has battery on it when the key is being held on to start the car. It should run from the voltage regulator to the alternator.

If the charging system has been modified and a self exciting alternator is in use it could have too large of a pully on it (a horsepower trick) and the alternator isn't turning fast enough to self excite and start charging until 3000 rpm.

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post #5 of 7 (permalink) Old Dec 31st, 11, 05:45 AM
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Re: Alternator Light Stays on Until 3,000 RPM

Could it have the wrong size pulley on it causing it to turn too slow since the motor needs 3000 before showing charge ?

I know when I went to an under-drive pulley that happened to me....but not to that extent.

And you must not like your brother much letteing him drive a ......stang

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post #6 of 7 (permalink) Old Dec 31st, 11, 07:20 AM
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Re: Alternator Light Stays on Until 3,000 RPM

Was it working before or a problem arrive overnight?
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post #7 of 7 (permalink) Old Jan 9th, 12, 02:19 PM
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Re: Alternator Light Stays on Until 3,000 RPM

Check the battery voltage with it turned off - should be around 12V.

Then check it while running- it should be 13.5-14.

You can also let Autozone test it.
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