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Farmall & IHC Tractors Discussion Board

Re: Farmall M 12 volt neg gnd loses charge


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Posted by Janicholson on June 15, 2009 at 07:33:46 from (66.173.50.88):

In Reply to: Farmall M 12 volt neg gnd loses charge posted by David Boo on June 14, 2009 at 23:58:37:

With a on off switch to the alternator "excite" terminal, (assuming you shut that switch off when the tractor is not running) the tractor seems to be wired OK. (I prefer either a parking light bulb (filament type not LED), or a diode in the excite wire because it is convenient. The most likely problem is the internal diodes in the alternator. If one of these has gone bad, it will pass current in the wrong direction letting the battery go down.
The remaining diodes will work well enough to charge the battery when used.
Use a multi meter with a built in amp meter as follows:
Disconnect the alternator big wire. Set the amp meter on its highest amp setting (you may need to move a lead to a different spot on the meter) connect the amp meter leads with the black on the post of alternator and the red on the wire you disconnected. The reading should be less than 150 milliamps. You may need to adjust the meter settings to get an "in range reading". 1000ma equals one amp, so 150 milliamps equals .15 amps.
If it is more like one amp, the the battery will drain in approximately 50 hrs (this depends on the battery and is not a hard number)
If it is less than the 150ma, the diodes are OK.
A battery with a cell going bad can also self discharge. This mimics the situation you have. Even new batteries can be bad. JimN


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