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Author
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Topic: How to check your alternator
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Mike W
Gearhead Posts: 135
From: CA
Registered: Jul 2003
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posted 07-19-2005 05:00 AM
 
I have seen a few posts about dim lights, etc so I thought I would throw this out. Get a voltmeter, even a cheap one. Check the battery voltage with the engine running. If it is 12.6 or lower the alternator is not charging.Now, measure the voltage at the F terminal of the alternator. If you have around 11 volts, the alternator is bad or there may be an output wiring problem. If there is no voltage at the F terminal, the voltage regulator is bad or there is a wiring problem not allowing the regulator to apply voltage to the F terminal. F is the field terminal. It must have current flowing thru it for the alternator to produce any output. There is more to it but this is a quick test that works.
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Scott H
Gearhead Posts: 1480
From: Chicago area
Registered: Mar 2005
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posted 07-19-2005 09:09 AM
Mike,
Well put, and thanks for the tip!
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68 Coop
Gearhead Posts: 5847
From: Mesquite, NV. 89027
Registered: Oct 2004
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posted 07-19-2005 09:40 AM
Thanks Mike, I'm having doubts about mine. I've replaced the regulator with the newer version, and came to find out I already had the new reg, so I got a back-up if necessary. Just gotta test the alt now. Thanks for the info.------------------
William
68 Coupe
289
Edelbrock 600
Performer Intake
Headers/X-pipe/Flowmaster 30's
C4
8"rear/2:79gears
"Restomod in Progress" Good friends are hard to find, even harder to leave, and impossible to forget!
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Toronado3800
Gearhead Posts: 1163
From: St. Louis, MO
Registered: Jul 2001
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posted 07-20-2005 12:11 AM
Good timing and thanks for the tip! Nothing I love more than replacing 40 year old ground wires I should have swapped out when the car was in pieces 
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Mike W
Gearhead Posts: 135
From: CA
Registered: Jul 2003
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posted 07-20-2005 04:44 AM
Thanks for the thanks. I drove a F-250 with a 60 amp alternator for years. When I finally installed a voltmeter, I noticed a strange jumping in the voltage. I took the alternator apart and found that 3 of the 6 diodes had open leads.I got a $13 repair kit from an auto store and replaced the diodes, brushes, and the front bearing. It is an easy job once you get the alternator cleaned up. I used my parts washer.
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