Testing alternator output

Joined
Jan 14, 2013
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Hello, I have a 1995 Wellcraft Excel 21 GL with a Volvo Penta 4.3GS PHUB and am trying to figure out if my alternator is putting out the proper voltage. I read the voltage across the two large posts protected by boots and during idle it was like 8 volts. I just bought this boat and it has a new battery but the dash gauge reads about 11 volts almost all the time so I'm pretty sure it's just stuck there. I would like to make sure my output voltage is ok before it dies on the water when I need it. I am not sure I tested the correct way.

Thanks!
 

dennis461

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Aug 11, 2011
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Measure voltage at the battery. Its the most important part of the system. Expect to see around 13.5 volts
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Take alternator off motor and take to alternator repair shop. They will test it for you, and if there is a problem, they can fix it
 
Joined
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Measure voltage at the battery. Its the most important part of the system. Expect to see around 13.5 volts

So lets say when the boat is off it reads about 12, and when running it should be about 13ish AT THE BATTERY? That sounds like a first easy step, then perhaps taking it to the alternator shop like the post above states.
 

bruceb58

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You can measure the voltage at the alternator output. Measure it from the post to the case. Should be 13.5V. If it isn't, make sure the excite terminal has 12V on it. That voltage is applied when you turn on your ignition so you can test that terminal with the engine not running..
 

Money Taker

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Aug 3, 2018
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Wiring can be weird on boats...... Agree -- check at the battery while the boat is running -- unless the battery is bad and overloading the alternator -- should read at least 13V. If voltage is less than about 12.7 -- try disconnecting the battery while running -- if it keeps running -- likely battery.
 

Bt Doctur

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Wiring can be weird on boats...... Agree -- check at the battery while the boat is running -- unless the battery is bad and overloading the alternator -- should read at least 13V. If voltage is less than about 12.7 -- try disconnecting the battery while running -- if it keeps running -- likely battery.

NO,NO.NO its the quickest way to have the alt run wild and overload the electronics and anything else connected
 

Money Taker

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That is a myth, the alternator will only provide the current required by the load -- its ohms law.
 

Lou C

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Then why do the makers of all battery switches say NOT to switch off the batteries while the engine is running?
 

Money Taker

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I apologize -- I missed the part you had a new battery..... Since your volts are low even at the battery -- something is loading down the system -- it may or may not be the alternator. Since folks are concerned about sparks and transients -- you can try disconnecting loads while the battery switch is off and the boat is off. Then turn back on and using your helm gauge, see if the volts go back up to 12.7 plus( normally ~13.5v for a fully charged battery)
 

Bt Doctur

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That is a myth, the alternator will only provide the current required by the load -- its ohms law.

Any the owner who follows that advice and wipes out the electronics and ecm you will pay for the repairs?.
Why don`t you go pull the battery cable off yours or your car and report what gets destroyed?
That battery disconnect test was only done with generators
 

Bt Doctur

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dash gauges are never that accurate so put your meter on the battery with the engine off, turn key to on" voltmeter should be a close match to your meter Start engine and go to 1500 rpm . Reading on the meter should increase and come close to the gauge. If the meter reading increases you are charging , no increase means no charging
 

DouglasW

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Apr 20, 2018
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That is a myth, the alternator will only provide the current required by the load -- its ohms law.

First, it's called Ohm's law, named for a guy named Ohm ;-) Second, Ohm's law involves Volts also and it's the Volts that kills stuff. In order for the regulator in the generator to supply the needed Amps, it needs to adjust the alternator's Volts to comply. With little load, the Volts can soar. By the way, Volta (Italian inventor of the battery cell), and Ampere (French physicist) were guys too ;-)
 

Money Taker

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First, it's called Ohm's law, named for a guy named Ohm ;-) Second, Ohm's law involves Volts also and it's the Volts that kills stuff. In order for the regulator in the generator to supply the needed Amps, it needs to adjust the alternator's Volts to comply. With little load, the Volts can soar. By the way, Volta (Italian inventor of the battery cell), and Ampere (French physicist) were guys too ;-)

Exactly == and those guys Farad and Henry get added to parts to save the day. But I'll yield to all of you -- troubleshooting a boat can be risky, and made even riskier by poor or unknown manufacturing quality. If you don't want to assume any risk, take it to the dealer as the troubleshooting tools available are limited
 
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