Alternator causing battery drain? Tractor


I know this isn't a tractor forum but I'm not about to sign up on one for a general electrical question.

I know nothing about tractors, i don't even know how to start this old thing. Trying to help the father in law that knows less than me.

Tractor is a '70ish John Deere. Battery goes dead after a few days of sitting.
I measured a .07 amp drain on the battery with everything shut off. I went around the tractor and unhooked electrical components until i saw a change in battery drain. I unhooked the single wire that screws on the back of the alternator and battery drain goes from .07 to zero. Is the alternator bad?


Engineer In Residence
If you still have an alternator repair shop in the area, they may be able to repair the unit for a reasonable cost. Replacing a single diode is usually not difficult. But it depends on the model.

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I found an alternator pretty easy but the plug is different. There’s A little note with the alternator explain how they have updated the style. Nobody locally seems to have any idea how to get that new plug though, it’s just a Chevy alternator apparently. I have to order it off Amazon.


Also I don’t know which wire goes to what. One side of the plug on the new alt is labeled R and the other is F.

Also the new alternator has a ground wire, but the old one doesn't. Tractor doesn't have wiring for it.


If you need more time to figure it out just pull the negative cable on the battery, that will stop the drain.


I replaced the alternator. Battery drain is gone now but it's not charging. I'm not sure it was ever charging. The wire that screws onto the alternator splits and feeds the R terminal at the plugin. Both of these are getting battery voltage at all times. The F terminal is only getting about three volts and only with the key on. Should the F terminal get full battery voltage with the key on?


Fixed. Parts store gave me an externally regulated alternator instead of internally regulated.

An alternator for a '78 chevy truck seems to be working now.