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TOPIC: Checking Alternator Output

Checking Alternator Output 26 Aug 2017 15:18 #1

  • Old Man With A Boat
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What is the proper way to check the output of my alternator with a voltmeter?

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Just love being on my 3870............Bill
1985 3870
Twin 130 Mits. not turbo charged
Name of boat is "Plenty Of Fish"
Live on board full time.
North Myrtle Beach, SC

Checking Alternator Output 26 Aug 2017 17:56 #2

  • makonnen
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To properly check alternator output, you will need a clamp style volt meter that can read DC amperage, something like this:
www.amazon.ca/Tacklife-Auto-Ranging-Cont...ive+clamp+volt+meter

1. Clamp the device around the single thick red wire on your alternator
2. Put your positive lead of the volt meter to the + terminal of the battery or selector swith
3. Put the negative lead of the voltmeter to ground somewhere
4. Switch the volt meter to measure DC voltage 0-20V
5. Turn on your engine
6. Volt meter should read 13-14V give or take
7. Turn your volt meter to measure DC amerage
8. It should read anywhere from say 5-20 DC Amps depending on the state of charge of your batteries
9. Turn on all your interior lights, radio, bilge pumps, water pumps, windlass etc...
10. Look at your volt meter set to measure DC amps, it should increase
11. Increase your RPM, DC amps should increase too
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1998 Bayliner Ciera 2655, 5.7 BIII

Checking Alternator Output 26 Aug 2017 21:23 #3

  • makonnen
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Another thing is that those DC clamp meters that measure DC amperage are DIRECTIONAL. So if you are not getting the amp output you expect, try to reverse the clamp. I just tried it on my trickle charger. Clamp it one way and I get .7A, clamp it the other direction and I get the expected 2.2A.

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1998 Bayliner Ciera 2655, 5.7 BIII
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