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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 07-10-2010
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Exciting the alternator

I have a Yanmar 4JH2E. I've just replaced the alternator only to find another possible issue. My new alternator will produce power but only after running at cruising RPM for 10 minutes or so. It's a Hitachi N-type alternator with a "T" connector in the back for the "sense" wire and the excitation / warning lamp wire. The sense wire works properly - the charge is correctly regulated (by the internal regulator). The engine panel has no charge warning lamp, which is typically part of the excitation circuit. After much searching I found that the excitation wire simply terminated about half way back to the engine panel. The alternator does start charging immediately by applying 12 volts to the excitation wire.

My questions are -

1) With no excitation current how does the thing ever start charging?

2) Should I attached the excitation wire so it starts charging as soon as I fire up the motor, or is it typical to not connect this wire and wait until the motor is run at cruising speed for a while before charging?

3) If I do connect the excitation wire to the ignition circuit, I believe I would need to add a resistor since I have no warning lamp. What is the proper type / ohm value resistor?

While it's not a huge problem as the alternator does start charging eventually, it seems to me that it would be better to excite the alternator at start up. I don't like using the windless to pull up the anchor with the batteries down a bit after using power all night. I'd rather have them closer to 14 volts as I believe this is better for the windless motor. Does this make sense?
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Old 07-10-2010
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Steve, your instincts are right on this. Having wires tied and teed and not used is simply quick & dirty, and setting it up with a few more feet of wire and five bucks worth or parts to get it producing power right away is a better way to go.

I wouldn't try to guess what voltage it wants to see on the exciter, but would try to find a schematic for that particular model number. From Hitachi or the folks who sold you the alternator, you should be able to dig something up. Heck, you might be able to find out which bulb they usually use, and simply add a bulb+holder into your panel so you have a charge warning light too!
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