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Alternator low output

hukre's Avatar

Join Date: Nov 2002
Posts: 121

11-05-03, 10:22 AM   #1  
Alternator low output

1968 Ford Mustang.
Just installed new alternator and voltage regulator, with the alternator rated at 61 amps.
When the car is idling and all accessories and lights are on (around 700 rpm), the alternator can not keep up with the load and the battery discharges. I measured the alternator output and it is 31 amps. However, at 1100 rpm it puts out about 45 amps. I measure roughly the same outputs with the alternator fully fielded. Shouldn't the alternator put out more than 31 amps when the engine is idling, or is this normal? Are there curves available from alternator manufacturers which show output in relation to alternator rpm? Could I put a smaller pulley on the alternator to increase its rpm when the engine is idling and thereby increasing its output?

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Joe_F's Avatar
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11-05-03, 10:30 AM   #2  
Sounds like you got a poorly rebuilt alternator. Most of them should put out about 10% difference against what they are rated when full-fielded (at least that's what I've read with most Delco units).

Bear in mind an alternator's output is diminished at idle (it's turning slower) and putting a different pulley on there won't accomplish anything.

I don't trust any off the shelf ones but OEM rebuilts. To this end, I rebuild my own.

Desi501's Avatar

Join Date: Oct 2003
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11-05-03, 04:41 PM   #3  
That's not the way to test an alternator at all. First of all, you are consuming some of that amperage before you get a chance to measure it. It's perfectly normal for an alternator not to keep up with that much load at idle. To test it properly, you have to bring the RPM's up over 1500 with all accessories off and load it from an external source, like a Vat 40 or similar equipment to get an accurate output of that alternator. A 61 amp alternator is not very large at all. All your going to see out of that is in the area of 50-55 amps at best. It doesn't appear anything is wrong with your system.

carguyinva's Avatar

Join Date: Nov 2003
Posts: 1,147

11-06-03, 06:02 AM   #4  
charging system test

here is a nifty way to check a charging system...it's not foolproof but it catches the vast majority of problems.

step one...measure the battery voltage with the engine off and write it down

step two...start the engine and bring it up to 2000 rpm's and measure the voltage at the battery and write it down

step three...turn on your electrical accesories while keeping the engine at 2000...lights on high beam, wipers, blower on high...that represents a decent load (altho a carbon pile load tester is really good for this too) measure the voltage and write it down.

now...your second reading should be no more than 2 volts above the first reading and the third reading should be AT LEAST half a volt higher than the first reading

the last thing you should do is connect your voltmeter to the positive post of the battery and the big terminal on the back of the generator (output terminal) and with the engine running and a load on the charging system...note this reading

do the same thing from the negative terminal of the battery to the case of the generator and note that reading (engine running and the charging system loaded)

both of these readings should be less than 100 millivolts or.1 volt

do all of these tests with a digital voltmeter

PS...i agree with the previous posters...within 10% of rated capacity (is your belt tight enough?), i prefer OEM rebuilts, and NO...the charging system will not keep up with high electrical load at idle

hope this helps...oh...and post back if you come up with something!!

Joe_F's Avatar
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11-06-03, 06:30 AM   #5  
Good advice and procedures.

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