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Sent to Car Experts December 27 01:34 PM

I HAVE A 1973 MUSTANG WITH A 351C ENGINE WITH 32000 MILES AND ANTIQUE PLATES (NOT DRIVEN MUCH). THE PROBLEM IS AT IDLE THE ALTERNATOR GAGE FLUCTUATES AT IDLE BUT SMOOTHES OUT WHEN YOU REV THE ENGINE UP. WHEN I TURN THE LIGHTS ON THEY ALL FLUCTUATE, DIM TO BRIGHT, AND STAY ON STEADY WHEN YOU REV THE ENGINE UP (SAME AS THE GAGE). THE BATTERY IS 5 YEARS OLD BUT SEAMS OK BEACASE IT HOLDS A CHARGE AND HAS PLENTY OF POWER TO START THE ENGINE. I CAN DRIVE THE CAR AND DON'T NOTIIC THE FLUCTUATING AND IT IDLES FINE. IT HAS THE ORIGINAL ALTERNATOR (32000 MILES). PLEASE HELP ? THANKS

Optional Information:
Pre-1980 Ford MUSTANG 351C 4 BARREL

Already Tried:
CHARGED THE BATTERY AND IT HOLDS A CHARGE

Customer (name blocked for privacy)
Status:  Closed   Value: $7
Answer
December 27 2:59 PM (1 hour and 24 minutes and 54 seconds later)
         
ACCEPTEDCheck Mark
Make sure the idle speed is correct and then check the voltage across the battery terminals you should have between 13.5 and 14.8 if you do the charging system is ok.Some older cars and still some of the newer ones will let the lights dim at idle and it is considered normal.If your voltage is outside of that range you have a voltage regulator or alternator problem.Most auto parts stores can test the alternator on or off the car for you.If the battery starts the car strong it's probably ok but 5 years is starting to push it for a battery it may fail soon but since this is not a daily driver I would wait until it dies before buying a new one.

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