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Posted: Sat Mar 19, 2011 9:42am

Post Subject: Alternator output

Hello. I want to test the output of my alternators. How can I do it? Do I need anything other than a multimeter? And where would I connect the probes? Thank you.


Posted: Sat Mar 19, 2011 7:25pm

Post Subject: Alternator output

First of all you need two pieces of data for your particular alternator. The regulated voltage and the output in amps. To measure the regulated voltage give the batteries a very good charge or run the boat for several hours at a reasonable charging speed. Connect your multimeter set to 20 (200) volts DC (brackets for 24 V system) across the batteries and rev the engine to at least 1500 rpm. Take a direct reading of the voltage. It could be anything from 13.8 to 14.8 depending upon your alternator but most modern ones will show between 14.2 and 14.4 volts. Check against the manufacturer's data. Current (amps) is all but impossible unless you have an accurate onboard ammeter connected to measure the alternator output. You could also spend a fair bit of money on a clamp type DC (thats important - cheap ones tend to be only AC) ammeter that has a large enough scale to measure the output expected. Do not even think about trying it with an ordinary multimeter - its highest amps range will be 20 if you are likely and more likely 10! You then need to ensure your batteries are well discharged so the alternator will run to maximum output. Turn off all other loads, put the clamp around one of the battery supply leads (pos or neg) start & rev the engine. The meter should then read close to the manufacturer's data. If the battery is not discharged enough you will get a false low reading. In that case try to pull the main alternator output wire (the big thick one) away from the rest and put the clamp around that trying to keep the clamp away from all other cables and magnetic fields. Repeat the above but with some heavy loads applied. Again you should get close to the manufacturer's data. Do you not think that if you described what lead you to want to test the alternator you may have got more helpful advice. Tony Brooks

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