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Old 11-07-2004, 07:08 PM
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generator lite stays lit while engine is running

have "69" Chevelle with the idiot lights and am having a problem with the generator lite staying on while engine is running. Have checked alternator out put to the battery with a voltmeter and it is putting out 14.3 volts at idle. Any suggestions on what to check or where to look ? Thanks.

Ted

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Old 11-07-2004, 07:59 PM
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Generator Light

More than likley the Diode in your Alternator is out. It will still show it's charging even when the diode is out. About $15.00 to fix it, but if you have any age on the Alternator as cheap as they have become provided your not running some high amp model you might just replace it.
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Old 11-08-2004, 03:34 PM
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To restore 60's ;
Is there any test that I could do with the alternator still on the car to confirm that this is the problem ? Thanks for the response.

Ted
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Old 11-08-2004, 06:10 PM
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Gen. Light

The only test I know is with the Alt. taken apart. I've checked the diode trio with a meter. If the diode is burned out you are getting a feed back thru the indicator light. The diode keeps current flowing in one direction. The only other problem I have ever seen was blown fuses with melted cavitys that wires like the gauge fuse wiring would touch another hot wire but that would cause a problem with all gauges.
If Doc's listening he might shed some light if you can test a diode without going into the alt.
I just got so used to seeing that problem you are having that years ago I would replace the diode but now since alternators have gotten into the $30-40 range I just quit building them.
The charge gauge or light should be a direct feed from the Alternator.
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Old 11-09-2004, 01:06 PM
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Test

Doc here

There are 2 tests you can perform on the Diodes, without tearing the Alternator Down, one is simple and uses tools you have at home...The other is also simple but requires an oscilloscope (I have one, spectrum analyzer in fact) but most folks don't have one at home.

The first is a DMM set for "Diode Check" in the forward bias direction it will read .5 to .7....reverse the leads and it will read "OL" .. If your getting a reading in both directions or the readings are higher than above, The Diodes are bad...

The second is simple too, just start the vehicle and place your scope probe on the output line...It should place a flat line across the screen at 12 Volts IRE(the screen is gridded in 0.5 volts IRE units depending on your settings)...on the screen.Looks like this:-------- If you get a Waveform +6 volts IRE that passes through the 0-volts line down to the lower 6 volt line...or a waveform that looks like this ~ ...The Diodes are bad..

The waveform indicates AC in the presents of DC (or non filtered DC )

The 12 volt flat-line indicates good clean DC.

If you do this kind of thing a lot...(work on Alternators AND Computer controlled Vehicles) You CAN get an OLD used Oscilloscope on EBAY (Electronic Test Equipment) for about $50...Just be sure it has all the plug in modules (Vert and Horiz) and it should be above 400 megahertz to be useful in many electronic projects.

Handy to have, but for Cars I use mine about 3 times a year...mine is Primarily used for Electronic testing of TV Broadcast equipment...and is wayyyy more expensive than a standard Oscope...about $5000.00

Doc
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