JD 820 - 6 volt alternator on a 12 volt tractor?

   / JD 820 - 6 volt alternator on a 12 volt tractor? #1  

glenhenshaw

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Sep 5, 2011
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Tractor
1973 John Deere 820 3 cylinder
Hi all,

I have a John Deere 820, serial number 065272L. I've owned it since 2005 and have used it sparingly. For several years now I've known it had a problem charging the battery. I'd put a new battery in it and it would start 5 or 6 times, and then I'd have to start jumping it; after ten or twelve more starts that wouldn't work either, since the car doesn't really provide enough current to turn the JD starter, so I'd put a new battery in it.. Since I only use it five or six times a year, this system worked okay.

But this week I couldn't start it for anything and so got serious about fixing it. At the suggestion of my local JD dealer (really small place in southern Maryland) I pulled the alternator and took it in to be tested. And... lo and behold, it looks for all the world like it's a six volt alternator; it even says "6V" on the service plate. Which explains why it won't charge a twelve volt battery, but leaves us with a couple of bigger mysteries. First, the alternator looks nothing like the alternators shown on the various OEM parts supplier websites for this tractor. Second, how and why did anyone get a six volt alternator that actually fits - there are no apparent modifications to the tractor to make the alternator fit - and install it in what is clearly a twelve volt electrical system?

The JD mechanic, who in my experience is very good, is scratching his head. He's afraid to order a replacement OEM alternator since they all look different from the one that came off the tractor, and can't figure out how you could even find a 6V alternator that would randomly fit the wrong tractor.

Any ideas?
 
   / JD 820 - 6 volt alternator on a 12 volt tractor? #2  
Is it a 6volt alternator or generator. I have never heard of 6 v alternators but most old tractors had generators.:confused:
 
   / JD 820 - 6 volt alternator on a 12 volt tractor?
  • Thread Starter
#3  
kuboman said:
Is it a 6volt alternator or generator. I have never heard of 6 v alternators but most old tractors had generators.:confused:

Errmm... dunno. How do you tell the difference? I'm pretty good with my hands but have no mechanics experience.

In looking at pictures the external appearance of the part I removed looks like a generator (it's fairly long with respect to its diameter, and it has a front mounting plate with two offset bolt holes and a rear mounting plate with one hole). However, the parts websites all list alternators, not generators, for this tractor.

When did JD switch from generators to alternators? I think my tractor was built in 1971. And did the older 6v generators have the same mounting points as the 12v generators did?
 
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   / JD 820 - 6 volt alternator on a 12 volt tractor? #4  
The 820 shows a generator for serial numbers up to 075344 and an alternator for above that number. They are both 12 volt and both are made buy Robert Bosch. I attached a parts diagram for both versions. Can you post a photo of your generator/alternator?
 

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   / JD 820 - 6 volt alternator on a 12 volt tractor?
  • Thread Starter
#5  
What I have is clearly the generator...
 

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   / JD 820 - 6 volt alternator on a 12 volt tractor?
  • Thread Starter
#6  
So would a six volt generator fit in the same spot? And do I need a new voltage regulator as well as a new generator?
 
   / JD 820 - 6 volt alternator on a 12 volt tractor? #7  
glenhenshaw,
Looks like somebody stuck that 6v generator on there before they sold you the tractor.:(

I would get the proper generator and check the voltage regulator to see what it is. I would be inclined to change both to eliminate damage to the new generator.

I would heed the advice you get from jd110 an experienced Deere mechanic.
 
   / JD 820 - 6 volt alternator on a 12 volt tractor? #9  
unless you need the correct part i would put a 10-SI alternator in it looks like the one in the right in the pictures some one else posted

I'll second installing a Delco alt. as long as there's enough clearance under the hood.
 
   / JD 820 - 6 volt alternator on a 12 volt tractor?
  • Thread Starter
#10  
Tx Jim said:
I'll second installing a Delco alt. as long as there's enough clearance under the hood.

This is great, guys, thanks. That sounds like the best idea. I have a fair amount of electrical experience so I'm pretty sure I could wire it in. But I'm worried about physically mounting it. Does anybody know whether a Delco alternator will fit on the same mounting holes as the original generator? Otherwise, does anybody know how involved the required modifications are?
 
 
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