Wanting to know if the old way of testing the alternator is good or bad. eg. Start the motor (diesel) and disconnect the battery, if the motor dies, the alt. is NOT charging. Does this work on diesels??? Does it hurt the motor doing such a test??
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Easier Way
Get a volt meter and hook it to the battery. With everything turned off the voltage should read somewhere between 12 and 12.5 volts. Then start the engine. Within a couple of minutes the voltage should jump us to somewhere between 13 and 14.5 volts. If this happens, you know the alternator is charging. You can also buy meters that test amperage, but they have to be hooked in-line so the hookup is more complicated.
Get a volt meter and hook it to the battery. With everything turned off the voltage should read somewhere between 12 and 12.5 volts. Then start the engine. Within a couple of minutes the voltage should jump us to somewhere between 13 and 14.5 volts. If this happens, you know the alternator is charging. You can also buy meters that test amperage, but they have to be hooked in-line so the hookup is more complicated.
Alternator
A diesel works on compression not an electrical ignition. If you disconnect the battery while the diesel is running there is a good chance that you will destroy the alternator as there is no place for the generated electricity to go. Measure the voltage with everything off and then with the engine running. You should see about 1 to 1 1/2 volt increase when the engine is running.
A diesel works on compression not an electrical ignition. If you disconnect the battery while the diesel is running there is a good chance that you will destroy the alternator as there is no place for the generated electricity to go. Measure the voltage with everything off and then with the engine running. You should see about 1 to 1 1/2 volt increase when the engine is running.
at any RPM?
Does the RPM have to reach a point before the alternator starts sending a charge out?
Does the RPM have to reach a point before the alternator starts sending a charge out?
Why do you ask?
Is it because you replaced the alternator? Or are you just wanting a way to check your old one? I ask because if you replaced with a newer model you may need an exciter voltage before it will begin to charge. A recent post suggested that his will start to charge(without the exciter) if he runs the RPMs up to 2500 for a few seconds. But do NOT run the test you propose. As previously mentioned the engine will keep running and the diodes will fry in the alternator. A voltmeter or one of the Link instruments is the way to go.
Is it because you replaced the alternator? Or are you just wanting a way to check your old one? I ask because if you replaced with a newer model you may need an exciter voltage before it will begin to charge. A recent post suggested that his will start to charge(without the exciter) if he runs the RPMs up to 2500 for a few seconds. But do NOT run the test you propose. As previously mentioned the engine will keep running and the diodes will fry in the alternator. A voltmeter or one of the Link instruments is the way to go.
minimum rpm
Depending on the alternator, you might not see any increase in voltage (as previously described) at idle. Check at 1500 rpm, or more, to verify the alternator is producing. don
Depending on the alternator, you might not see any increase in voltage (as previously described) at idle. Check at 1500 rpm, or more, to verify the alternator is producing. don
amps
Please don't try to test amps "IN LINE" you will not like the result. Use a 360 degree split coil clamp meter(Sears) for best results. To successfully test the alternator you do need to check the amps and not just the voltage. An alternator can put out the voltage at reduced amperage output.
Please don't try to test amps "IN LINE" you will not like the result. Use a 360 degree split coil clamp meter(Sears) for best results. To successfully test the alternator you do need to check the amps and not just the voltage. An alternator can put out the voltage at reduced amperage output.
Test ran & results found
Hello all. I wanted to thank you for the ideas for testing the batteries as well as the alternator. I went to walmart and for $15.00 or so bought a digital volt meter. I was getting 12.77 volts on the batteries at rest but when I tried to crank over the motor they went down to to 7.68 volts (guess that means toasted) while trying to crank motor. With new battery we got 12.86 standing still and 14.6 on the battery as well as at the alternator while running at idle right after start up. Don't know how low the new batery will go upon trying to crank, the motor starts up to quickly to find out. What a nice problem to have, huh. Once again, thanks to all here.
Hello all. I wanted to thank you for the ideas for testing the batteries as well as the alternator. I went to walmart and for $15.00 or so bought a digital volt meter. I was getting 12.77 volts on the batteries at rest but when I tried to crank over the motor they went down to to 7.68 volts (guess that means toasted) while trying to crank motor. With new battery we got 12.86 standing still and 14.6 on the battery as well as at the alternator while running at idle right after start up. Don't know how low the new batery will go upon trying to crank, the motor starts up to quickly to find out. What a nice problem to have, huh. Once again, thanks to all here.
Measure DC amps easy
I have a Clamp on meter that measures DC and anything else electrical you would need on a boat Just clip over alternator wire to battery and measure amps. cost Aus$159 Details below AC/DC Current Clamp Meter http://www1.jaycar.com.au This small clamp meter will measure an amazing 200 Amps DC. It is ideal for car stereo installations and electrical trades people. It has a one touch zero adjustment for DC current measurement. Jaw opening is 23mm. There is a 10mA high resolution on 40A DC/AC range. Also measures DVC, ACV, Ohms, diodes, capacitors and continuity. It is supplied in a handy vinyl pouch, along with leads.
I have a Clamp on meter that measures DC and anything else electrical you would need on a boat Just clip over alternator wire to battery and measure amps. cost Aus$159 Details below AC/DC Current Clamp Meter http://www1.jaycar.com.au This small clamp meter will measure an amazing 200 Amps DC. It is ideal for car stereo installations and electrical trades people. It has a one touch zero adjustment for DC current measurement. Jaw opening is 23mm. There is a 10mA high resolution on 40A DC/AC range. Also measures DVC, ACV, Ohms, diodes, capacitors and continuity. It is supplied in a handy vinyl pouch, along with leads.
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