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  1. #1
    New Member
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    Nov 2012
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    2
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    Pilesgrove
    Tractor
    John Deere 790

    Default JD790 Battery/Alternator Problem

    Tractor is 8 years old with 900 hours on it and the battery died. Replaced with new battery and was OK for a few weeks and then dead again. I cleaned up battery cables and put on new battery terminal clamps which seemed to help, but after a week....dead battery. Frame is rusty where negative lead is welded so I installed a second negative lead (tapped framed and bolted) and I now have two negative leads attached to battery. That seemed to help for about a week as well....but now, dead battery! The problem has seemed to worsen over time to the point where I need more power to jump start than before; used to jumpstart with just jumper cables on truck, now I need jumper cables and a portable jumpstarter battery. I started to suspect the alternator and took some measurements:
    Key off - 13 volts
    Key on - 13 volts
    Key start - 13 volts until you here a click (starter?) and then zero volts

    Jump start and measure with jump cables still attached - 14 volts
    Remove cables - 7 volts

    If I turn the lights on the tractor dies. If I leave the tractor running it only runs for a short while and then dies.

    It sure seems like an alternator problem to me, so I removed the alternator (belt was good and tight) to get it tested. Had my wife take it to the local guy that most local farmers use and he took one look at it and told her these never break! He said I should check all wires and the regulator. I already checked the wires and everything seemed OK. I'm not sure how to check the regulator, I think I read somewhere that you can disconnect and jumper around it to see if the voltage is higher than battery which would indicate the alternator is working, but I'm not sure how to bypass. At first I thought that the regulator not working would just not clamp the voltage from going over 14-15 volts and how could it keep it to 7 volts, but then I thought the regulator could have multiple diodes in series and if one of them was shorted it could be holding the voltage down to 7 volts.

    Any thoughts on my next best step? Are these alternators truly that good?

  2. #2
    Veteran Member zzvyb6's Avatar
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    Dec 2006
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    2,493
    Location
    michigan
    Tractor
    jd 1070

    Default Re: JD790 Battery/Alternator Problem

    My 1070 is a close relative of your tractor (I believe, maybe so). I had identical problem with it TWICE. Check the voltage regulator connectors (wire clip that goes onto the regulator. On my tractor, one of the wires must have frayed, became hot, melted the plastic covering and insulator and broke the connection. A new regulator did not fix the problem because it was the wiring that connected to it. Once I fixed that, things have been fine. It would takes two weeks for the battery to get low enough to require a charger. You could tell because the crank speed would gradually slow down. It takes very little crank time to start my tractor, usually less than two seconds.

    Good Luck to you.
    There is no "I" in team, but there is a "Me" if you want to jumble it up a bit...

  3. #3
    New Member
    Join Date
    Nov 2012
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    Location
    Pilesgrove
    Tractor
    John Deere 790

    Default Re: JD790 Battery/Alternator Problem

    Quote Originally Posted by zzvyb6 View Post
    My 1070 is a close relative of your tractor (I believe, maybe so). I had identical problem with it TWICE. Check the voltage regulator connectors (wire clip that goes onto the regulator. On my tractor, one of the wires must have frayed, became hot, melted the plastic covering and insulator and broke the connection. A new regulator did not fix the problem because it was the wiring that connected to it. Once I fixed that, things have been fine. It would takes two weeks for the battery to get low enough to require a charger. You could tell because the crank speed would gradually slow down. It takes very little crank time to start my tractor, usually less than two seconds.

    Good Luck to you.
    That was quick! Thanks, that is exactly the kind of feedback I was hoping to get. I will definitely look closely at those wires now. I will keep you posted.

  4. #4
    Gold Member
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    May 2011
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    332
    Location
    NKY
    Tractor
    Yanmar YM186d,John Deere 1050, Case 211B

    Default Re: JD790 Battery/Alternator Problem

    You have a Dynamo (alternator) that puts out AC,not DC. Should be able to unplug it and rev it up,and see if it is putting out over 20v AC. The regulator is usually the problem-it contains the diodes to change it back to DC. 7 volts sounds like a junk battery! Try charging it up and see if it recovers. A dead battery will burn out the regulator, so if it ever goes dead,don't just jump it,charge the battery first.
    JD1050,YM186D,Case211B,Homemade- sawmill & log splitter & log hauling arch trailer
    JD14T,Ford 532 balers,
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  5. #5
    Veteran Member Mickey_Fx's Avatar
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    Aug 2005
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    1,131
    Location
    Vancouver Wa.
    Tractor
    Yanmar Fx24D, Cub 3204

    Default Re: JD790 Battery/Alternator Problem

    I was wondering if JD kept the same charging system as Yanmar uses. From Barry's comments it sounds like they did. These charging systems are very sim to what is typ found on lawn and garden tractors. Permanent magnet alternator feeding power to the "voltage Reg". The regulator rectifies the AC to DC and controls the voltage, it does this by shunting some of the power to ground, just enough to pull the voltage down to the 14V range. Over the yrs the reg has been a trouble spot. With this kind of system the alt is always putting out max power. A portion goes to ground and the rest goes to batt and acc.

    One way to make sure this is the charging system, take a look. There should be 2 wires coming from the alt that is the source of the AC power. The reg should have 3 wire connections. The 2 from the alt and 1 DC output that supplies power for the battery and acc. As Barry said, disconnect the 2 wires from the reg and with engine running measure voltage with engine at speed. Again as Barry said it should be >20V and somewhere in the back of my mind I think it should be closer to 25-28 volts but don't quote me on that. If the alt is putting out proper voltage, I think that settles it as problem is the VR. Good luck.
    Yanmar Fx24D,
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  6. #6
    Member Soots's Avatar
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    Oct 2012
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    Inverness, Florida
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    mt372

    Default Re: JD790 Battery/Alternator Problem

    Mickey, I think you said that very well. even the standard idiot like me understood that. I'll remember that for future problems. Rick.
    Green Earth. If you're not part of the solution, then you're part of the problem.

  7. #7
    Silver Member
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    Jan 2006
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    249
    Location
    Stone Mountain, GA
    Tractor
    Case 1210, Mitsubishi D2050

    Default Re: JD790 Battery/Alternator Problem

    Well, not to argue about what has been posted, but some of it is not how generators (dynamo) and a voltage regular work. The points about checking the wiring at the voltage regulator, and the regulator itself are valid. In addition, if the tractor has a number of hours on it, the generator may need servicing, such as replacing brushes, refinishing the commutator, new windings, or bearings. Typically, if the regulator is sticking open, you can tap on it with the handle of a screwdriver to get it to work for at least a while.

    With that said, the dynamo does *NOT* put out AC voltage, it is DC. With a non-functioning regulator, the charging voltage can be high or low. Here is a quick discussion of how the dynamo and voltage regulator function:
    The generator is like an electric motor in reverse. Instead of applying electricity to it to make it spin, when you spin it, it makes electricity. It does this by spinning a series of windings of fine wire (called the armature) inside of a fixed magnetic field by connecting them to a belt and pulley arrangement on the engine. As the armature is spun by the rotation of the belt and pulley, it gets a current and voltage generated in those windings of wire. That current and voltage will be directly proportional to the speed that the armature spins and to the strength of the magnetic field. If you spin it faster, it makes more and if you make the magnetic field stronger it makes more current. The speed of the spinning is controlled by the speed of the engine - that's why you need to rev the engine up to help charge the battery faster. The magnetic field is controlled by an electro-magnet, so by changing the amount of current supplied to the electro-magnets that make up the field you control the strength of the magnetic field. This current is referred to as the "field" current and it is controlled by the regulator in response to the electrical needs of the automobile at any given time.

    The voltage of the generator is controlled by the number of windings in the armature. The current output varies widely from zero if the battery is perfectly charged and nothing is using any power up to the maximum rated output of the generator. The current output is controlled by the field current, but also by the speed at which the armature is spinning. This is important because a generator can only put out it's maximum rated current at or above some speed - at lower speeds the output drops off very quickly. This is why a generator-equipped car will not charge (or even maintain!) the battery at idle and this is one of the main reasons for the development of the alternator.

    The current generated in the armature is AC - not DC. To get it converted to DC so it can charge your batter and run your headlights, a device called a commutator is used to "rectify" this situation. It is inside the generator on the armature and has a series of contacts along it's outer surface. Two spring-loaded brushes slide on the commutator - one brush is connected to ground and the other is connected to the main output of the generator. As the armature and commutator assembly rotates, the brushes come touch the different contacts on the commutator such that the polarity of the current moving in the armature is always connected to the correct brushes. The net effect of this is that the generator output is always DC even though the current inside the armature windings is always AC.

    Hope this helps!

    Tom

  8. #8
    Member Soots's Avatar
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    Inverness, Florida
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    mt372

    Default Re: JD790 Battery/Alternator Problem

    Ok, so the two wires coming off the alternator going to the VR. is DC, not AC.
    and the wires coming off the Generator is also DC. not AC. correct?
    Green Earth. If you're not part of the solution, then you're part of the problem.

  9. #9
    Silver Member
    Join Date
    Jan 2006
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    Location
    Stone Mountain, GA
    Tractor
    Case 1210, Mitsubishi D2050

    Default Re: JD790 Battery/Alternator Problem

    Yes, all voltages on the wires at the generator/dynamo are DC.

    The voltage regulator should have one of them marked FL or Field or something like that (and probably the generator). This is the lead that the regulator controls, in order to control the output of the generator. So when the relay sticks in the voltage regulator, or there is a broken wire, intermittent connection, or corroded connection on this wire or the ground wire from the regulator, the generator will not function as it should.

    The other lead is typically the output of the alternator, which energizes a second relay. If this relay is not energized, it lights the charging warning light on the dash. This lead will be heavier, and is typically the lead that gets hot and may burn up a connection or something like that. It is imperative that this wire and lead be in good condition and tight on the regulator.

    In general, assuming that the generator brushes are in reasonable shape (they are a regular maintenance item), then problems typically are broken, intermittent, or corroded wiring or connections; or a sticky regulator.

    You can take the covers off of most regulators and look at the condition of the coils and contacts. If any of the contacts are pitted or discolored, it's time for a new regulator. Many regulators have solid state replacements, which make them more reliable.

    Hope this helps.

  10. #10
    Elite Member
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    Northwest, WA

    Default Re: JD790 Battery/Alternator Problem

    Wow.

    The JD 790 uses a alternator, not a generator.

    And it's brushless.

    ((EDIT; got the wrong "pg4", give me a minute))
    ((OK, fixed it.))
    Attached Thumbnails Attached Thumbnails -pg3-jpg   -pg2-jpg   -pg1-jpg   -pg4a-jpg  
    Last edited by Willl; 11-06-2012 at 09:24 PM.
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