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Old 08-03-2011, 08:34 PM   #1
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Default What is factory amperage for alternators?

I think my alternator is starting to kick the bucket. Had the car on a load tester last week. Car running 1100RPM, no load on anything and the alternator was pushing under 20 AMPs. Turned the dial on the tester and the car almost stalled.

I am still getting good voltage at the battery though (14-14.3v).

I'm told this could also be the cause of a small lean condition I'm having right now if the charging system can't keep up.

Last edited by falcon; 08-03-2011 at 08:45 PM.
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Old 08-03-2011, 09:07 PM   #2
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It's not 20A for sure. I've heard anything from 60A to 100A.
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Old 08-03-2011, 09:25 PM   #3
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Quote:
Originally Posted by Savington View Post
It's not 20A for sure. I've heard anything from 60A to 100A.
+1 At 20 amps I'd shitcan it and get a new one.

EDIT: 1100 rpm? Alternators are usually load tested at 2000+ rpm. I remember forgetting to take a 2005 Corolla off idle and getting similar numbers, something like 30 amps max, and that alternator is rated for something like 90+ amps.
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Old 08-03-2011, 10:17 PM   #4
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He tested it while revving and got a number not much higher. Don't remember exactly what though.
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Old 08-03-2011, 11:45 PM   #5
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60 amps. I had a 100 amp core put into my old alternator and it worked great
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Old 08-03-2011, 11:45 PM   #6
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Quote:
Originally Posted by falcon View Post
I think my alternator is starting to kick the bucket. Had the car on a load tester last week. Car running 1100RPM, no load on anything and the alternator was pushing under 20 AMPs. Turned the dial on the tester and the car almost stalled.

I am still getting good voltage at the battery though (14-14.3v).
This does not make sense.

I mean, the stalling thing makes sense. You need to open the throttle and get the revs up. But if you apply a load to the system and cannot get the alternator to put out more than 20A, and yet the system voltage is not dropping, then something is wrong with the test.

If you want a definitive answer, pull the alternator out and take it to the auto parts store to be tested on the load bench.
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Old 08-03-2011, 11:48 PM   #7
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Ok. Will do.
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Old 08-04-2011, 02:34 AM   #8
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Quote:
Originally Posted by falcon View Post
I think my alternator is starting to kick the bucket. Had the car on a load tester last week. Car running 1100RPM, no load on anything and the alternator was pushing under 20 AMPs. Turned the dial on the tester and the car almost stalled.

I am still getting good voltage at the battery though (14-14.3v).

I'm told this could also be the cause of a small lean condition I'm having right now if the charging system can't keep up.
If the voltage is above 12.8V the alternator is putting out whatever load current is on the electrical system. If you can load it at >45A at 1100 RPM the alternator is probably fine.
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Old 08-04-2011, 08:39 AM   #9
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1.6L = 60A manual 65A auto
1.8L = 65A manual 70A auto

Last night I logged 11v with the headlights on and a/c running, suffice to say, I'll be getting load tested tonight, I doubt I'm anywhere near 60A.
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Old 08-04-2011, 12:42 PM   #10
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NB alternator?

BTW a common failure of NB alternators is they get flaccid at *high* RPM.
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Old 08-04-2011, 12:44 PM   #11
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If my 3 month old "refurbed" alt is truely bad, im going to see if I cant get an FC alternator as a replacement unit from them. Should be a direct bolt in (just need a new lower bolt) and produces 80A.

I've been having major electrical gremlins since I replaced my truely bad oem alt, shich allowed voltage spikes-the regulator failed. I wonder if it's the cause...
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Old 08-04-2011, 12:49 PM   #12
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You don't always have to stay with a Miata alternator. Often there are numerous options that can be adopted. I used to run 200+ amp Chevy tow truck alternators on an old BMW with a booming sub. They ran around 100 bucks and were bulletproof compared to the considerably more expensive and weaker Bosch alts. All that was needed was new wire terminations and an 8 dollar muscle car L bracket. It had a separate, adjustable VR and could be set to cut back at WOT. There are likely some nice uberlight alts you could load onto a track only Miata.
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