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$30 Alternator for a John Deere 1020

posted Apr 21, 2010, 8:35 AM by Matt Francis   [ updated Apr 21, 2010, 11:10 AM ]
So, I got to put my second spring electrical fix to use last night, while mowing one of our pastures by the now insanely bright lights of my John Deere 1020.  Since I got the tractor several years ago, the alternator has never worked, so I was always running extension cords to keep the battery topped off so I could start 'er again.  After mowing through and repairing yet another extension cord, I decided it was time to let the tractor provide its own juice for once.

The JD 1020, as it rolled off the assembly line in 1966, came with a little 30amp Motorola alternator and a separate voltage regulator, located under the dash.  A regulator limits the output voltage of the alternator, so that you don't, say, overcharge your battery and cause it to maybe explode?  So, it's pretty important ;)  These early regulators couldn't handle the heat and vibration of the motor, so were located separately.  Though the innovations of power electronics, they can now be directly integrated into the alternator.  So, I decided it was time to upgrade to a more modern and, conveniently, much cheaper alternator setup.

After doing some research, I learned that an AC Delco 10SI or 12SI alternator would fit this tractor with no modifications to the mounting brackets, and is just such an integrated model.  And, since such alternators have been used in about a billion GM products since the 70's, they are the cheapest alternator on the planet; $30 bucks (plus core) for a re-manufactured unit from your local autoparts store.  Read here for A LOT more great info on these alternators.

So, here's how it went down; using the above reference, I went to my local parts place and asked for an alternator for a "1978 camero with 350 V8."  While this should have been a 10SI, they gave me a 12SI for $30 plus core.  Since the 10/12 are pin compatible, I went with it.  I also grabbed a pigtail for the "pin plug" on the side of the alternator for about $2 to simplify the connections.

The 10/12SI you get from the autoparts store is a "3-wire" model.  It has three pins to connect for the device to work properly.  The main "output" lug of course supplies the current.  Pin 1 provides what's called the "field" or "excite" connection.  When wired from the accessory line of your ignition through a light bulb, it gives you the "idiot light" on the dash (If the light is on, you're not charging!) and also "turns on" or "excites" the alternator.   Finally, a "sense" pin (pin 2) senses the output voltage at the terminal block of your vehicle, to allow the alternator to adjust the output current accordingly.  Since this is a tractor with a really short wiring harness, that pin just gets connected back to the output; the output really is the terminal block on a 1020.  Read here for a great, detailed discussion on what the sense pin was really designed for.

Since the JD 1020 already has an alternator light on the dash, this is a really easy wiring job;(1) just connect RED to the output (just like original), (2) jump the "sense" pin to the output, and (3) connect the "field" wire to the ORANGE wire of the original wiring harness.  The remaining original wires (green and black) aren't needed; they ran to the old regulator.  I soldered all of my connections and heat-shrinked them.  Then, just reach under the dash and (4) disconnect the regulator.  The only other part is of course getting the it mounted, and getting some mechanical energy into the alternator!

Here is my mounted alternator.  Since the JD1020 uses a rather wide fan belt, I had to (0) swap the pulley for the one off of the original Motorola alternator.  This only proved slightly tricky in that the Motorola alternator had a smaller shaft; I had to bore out the inside of the pulley just a little with a drill to get it to fit.  The original pulley is smaller, so the alternator will never spin as fast as it could; however, since we're upgrading from a 30amp model to essentially a 60+ amp model, it is lost in the noise.  If everything is wired correctly, the alternator light should come on when you turn the iginition key to "on" and then go out after you start the tractor.  If it stays on after you start, the alternator is not charging.  On my 1020, this usually means I need to wiggle they key ;)  The whole thing took me an evening from start to finish; I spent much longer tracking down the bad ground on the headlights.  I now have a 100% functional electrical system that is more reliable and powerful than the original, for less than $50 in parts.

Note that a lot of folks are selling "one wire" alternator kits for tractors, for rarely less than $100.  These are typically just a 10/12si with a "hacked" regulator designed to self-excite.  On a tractor, this only saves you from having to run that "idiot light" wire.  On a JD 1020, a one-wire alternator is a particularity poor choice, since you already have the orange field wire at your disposal!  Save your self some $$$ and just get a standard 10/12si from your local store.