Is 12.9-13.3 an acceptable alternator voltage? - Forums at Modded Mustangs
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post #1 of 102 Old October 6th, 2009, 11:16 PM Thread Starter
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Is 12.9-13.3 an acceptable alternator voltage?

I have a high output alternator but the voltage is only 12.9-13.3, is this normal?

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post #2 of 102 Old October 6th, 2009, 11:19 PM
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a little too low, but since it a high output alt try checking it at about 1500 rpm & see if it make's a diffrence.

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post #3 of 102 Old October 6th, 2009, 11:22 PM Thread Starter
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Quote:
Originally Posted by casper gt View Post
a little too low, but since it a high output alt try checking it at about 1500 rpm & see if it make's a diffrence.
This was at all rpm ranges. At WOT it's about 13V and 2000 rpm is about 13.1-13.2. It's a 220 amp alternator.

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post #4 of 102 Old October 6th, 2009, 11:23 PM
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post #5 of 102 Old October 6th, 2009, 11:25 PM
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eh.. idk.. 12.9v is kinda low... check output at 2k use a VAT 45 if ya can.. check the amp output as well.. my .02

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post #6 of 102 Old October 6th, 2009, 11:26 PM
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Quote:
Originally Posted by hardhitter View Post
This was at all rpm ranges. At WOT it's about 13V and 2000 rpm is about 13.1-13.2.
thats not good then, is pulley dia. right (ie. stock or underdrive,etc)? other than that sounds like you might have a problem with the alt./wiring/etc.

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post #7 of 102 Old October 6th, 2009, 11:27 PM
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it should be around 14.2-14.4 across the batt. Check all the wires to make sure they are getting good contect and that nothing is loose.

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post #8 of 102 Old October 6th, 2009, 11:27 PM
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Quote:
Originally Posted by hardhitter View Post
This was at all rpm ranges. At WOT it's about 13V and 2000 rpm is about 13.1-13.2. It's a 220 amp alternator.
Regardless of what amperage the alternator is rated for voltage should still be around factory/close to 14.4 while running.
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post #9 of 102 Old October 6th, 2009, 11:32 PM Thread Starter
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Can someone recommend an alternator that doesn't suck then? I went through 3 alternators on my old car because none of them could stay at 14V and now I'm having the same problem again. They were all "high output" alternators.

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post #10 of 102 Old October 6th, 2009, 11:34 PM
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post #11 of 102 Old October 6th, 2009, 11:36 PM
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+1^ try getting one from the dealer.

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post #12 of 102 Old October 6th, 2009, 11:37 PM
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Or if youre stuck on a high output one for some reason look into ea/excessive amperage.
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post #13 of 102 Old October 7th, 2009, 12:49 AM
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Yeah that voltage is low. As mentioned it should be in 14 range at 2k + rpm. Even at idle it should be `13.9's if a performance alt that is geared up top.

My alt is awesome, got it from a guy that custom makes em with high quality parts. He specializes in other cars but saw that stangs had trouble with aftermarket alternators and the cobra guys were over spinning them.

Its about 300 or so + shipping, i don't remember how much for sure. Its a 160 amp alt. Here is thread i found him on, Brand New Usa Made Hi-output Alternators - ModularFords.com.

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post #14 of 102 Old October 7th, 2009, 10:30 PM Thread Starter
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Today I tested it again and it's reading .3 volts higher with a voltmeter than the datalog I was looking at so this makes it about 13.75 volts at idle. At WOT it goes down to 13 volts and then when I let off it drops to 12 volts like the alternator quits working. I took 30 seconds before it decided to suddenly return to 13.6 volts while cruising. Still sound like an alternator problem?

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post #15 of 102 Old October 7th, 2009, 10:37 PM
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I believe my old one put out a constant 14 (at idle). its low in my opinion. i know the alt was replaced before i got the car so it could have been a high output but either way yours sounds low.

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post #16 of 102 Old October 7th, 2009, 10:37 PM
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Quote:
Originally Posted by ajm View Post
Regardless of what amperage the alternator is rated for voltage should still be around factory/close to 14.4 while running.
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post #17 of 102 Old October 7th, 2009, 10:45 PM
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WOT will drain your power so you will get a drop in voltage. It sounds like your alternator is junk. GET A NEW ALTERNATOR. You should be getting 14.2-14.4V across the batt when it's running. WOT is not going to gave you a reading of it output back to the batt because it is giveing motor voltage to the motor to run. If you can, return it for a new one, or go to AutoZone and have them check to see if it's the Alternator or not.

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post #18 of 102 Old October 7th, 2009, 10:50 PM
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post #19 of 102 Old October 7th, 2009, 11:38 PM
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yes, 12.9 -13.3 is an acceptable ET
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post #20 of 102 Old October 8th, 2009, 12:08 AM
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Quote:
Originally Posted by hardhitter View Post
Today I tested it again and it's reading .3 volts higher with a voltmeter than the datalog I was looking at so this makes it about 13.75 volts at idle. At WOT it goes down to 13 volts and then when I let off it drops to 12 volts like the alternator quits working. I took 30 seconds before it decided to suddenly return to 13.6 volts while cruising. Still sound like an alternator problem?

First with car off, what is voltage, at battery? Start car, what is voltage at battery? Then check voltage at the charge post on back alt.

Also a marginal battery will cause bad alt readings. Have battery load tested! alt output of less then 13.6 volts is bad in my book.
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