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parvenu 09-08-2005 04:54 PM

alternator test - diesel engine
How do I test my alternator to test whether or not it''s charging my battery?

I have a diesel engine (Westerbeke 27).

Someone suggested that I disconnect the positive terminals of my batteries. If the engine keeps running, that means it''s not working off the batteries. Problem with that, in my mind, is that a diesel engine doesn''t depend upon sparkplugs as a gas engine does. So what would disconnecting the terminals tell you?

So any suggestions?



PaulBl 09-08-2005 05:41 PM

alternator test - diesel engine
<So what would disconnecting the terminals tell you?>

Nothing important.

dman 09-09-2005 03:48 AM

alternator test - diesel engine
Pulling the cable off your battery will blow your alternator if it is working.The current has to go somewhere,even jumping it and running on a dead battery can damage it.If you need to check the alternator, all you need is a voltmeter. Make sure you have a good battery. Start the engine and set the voltmeter on DC voltage. Take the red lead of the voltmeter and put it on the large red wire at the alternator. Put the black lead on the alternator housing. With the engine at high idle, you should have a reading of 14.2 - 14.6 volts. Take the voltmeter to the battery and place the red lead on positive and the black on negative. You should be reading 14.2 - 14.6 volts. If not, you have a wiring problem to the battery. If you do have 14.2 - 14.6 volts and you still have problems, test the battery.Also remember that the diodes in the alternator can flow backwards when they malfunction,draining it when it isn`t running.

sundown2 10-20-2005 08:40 AM

alternator test - diesel engine
a real simple test to check an alternator is to take a piece of metal (screwdriver etc) and hold close to the back of the alternator while it is running. If it is producing electricity it will be creating a magnetic field and you will be able to feel the pull on the metal. (be careful not to hit the output post).
The surest way to test output though is with a multimeter (Radio Shack about $25). with the alternator running check from the output (+) to ground (-). reading should be above 13.5.

GordMay 10-21-2005 03:02 PM

alternator test - diesel engine
and check out “BASIC Alternator Testing” lternator+test

roline 12-22-2009 08:40 AM

Disconnecting the positive terminal of the alternator while running will only tell you that you need to buy a new alternator.. Most alternators require a battery to prevent blowing up the diodes. There is an external diode that can be placed on an alternator, ZAP STOP, I believe that also will protect the diodes should the battery switch disconnect the battery from the alternator. The best way to know what is going on is a current shunt and current monitor/meter, or the cheapest way is an inexpensive voltage meter to monitor the battery voltage. The alternator should output around 14.2 volts to give the battery full charge. There are cheep digital voltage meters that plug into lighter sockets available from auto parts stores....

flatracker 12-22-2009 07:18 PM

You can simply
Take a small voltmeter and read the voltage. If it is around 14.2 volts with the engine running, the alternator is working. Or just take it off the boat, and take it to almost any auto supply for testing.

Omatako 12-22-2009 08:08 PM

Is this too simple?

Put a volt meter across the battery with the engine not running. Take a reading.

Start the engine and take another reading.

If the second reading is the same as or lower than the first, you can be 99% sure that the alternator is not charging.

The easist way to be sure that it isn't charging? Disconnect the positives while the engine is running. If it was charging before, it isn't any more.

TQA 12-22-2009 08:21 PM

If no voltmeter at hand switch on all incandescent lights and any other heavy consistent load, look at lights, start engine and rev it up. The lights will brighten just a little if the alternator is charging.

N.B. Best done at night.

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