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12V electric issue....


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  • 12V electric issue....

    Not sure where to have posted this, as it has to do with a garden tractor. I want to check the voltage with it running and with it not running. I have an electronic vom that goes crazy when it gets near the machine when running. I separated the battery by using cables and merely attached the positive lead of the vom to the positive of the battery (loacted maybe six feet away!!) and the meter was jumping to between 14 and 19+ volts. Very radical and very fast. So I tried an analog meter and even after trying two different ones, I can't get a good read. I disconnected the charging system and got the same result. The primary (and separate) issue is the machine reads a 6 amp discharge just in the on position. When running it reads a 12 amp discharge. Back to the meter jump issue: the charging system is a circle of coils attached to the block and are surrounded by flywheel magnets that spin past them, supplying ac voltage. The two leads from those coils go to the regulator and get converted to dc volts. With the ac leads NOT CONNECTED to anything, the meter still jumps erratically. Is that because of the magnetic field?? I checked the meter by the truck and the car electrical and it's steady as a rock, even when placed touching the alternator or plug wires. No interference.

  • #2
    hmmm,back to the garden tractor

    I thought we did this already.
    there is one magneto with multiple windings on/off it. one goes to chassis ground, one is the high voltage lead that goes to the spark plug.
    two of the other leads goes to a bridge rectifier [which I believe you already changed] then goes to a regulator. the normal output of the magneto is about 15 volts AC and out of the bridge rectifier is 13-14 volts DC.
    the action between the magnets on the flywheel and the pole pieces of the magneto produces the voltage necessary to run your engine and charge the battery which is used "only" to start the engine and operate a fuel solenoid on the carb, if you have that type. my yardman has one of those solenoids.
    I have never needed to measure the voltage on my lawn tractor, but only to check it when the damn thing wouldn't start.
    a lawn tractor is a very crude sort of charging system where you really don't need a controlled output as you would for a truck or car where it is computer controlled and other electronic circuits are used.


    • #3
      tractor magneto

      think of the magneto as a generator, which in actuality it is.
      to produce a voltage a coil of wire is passed through a permanent magnet's field. this field is called "gauss" and is a measure of magnetic strength.
      as the wire goes into the field a voltage is induced in it in the positive direction, goes to maximum at the height of magnetic concentration, the reverses in the negative direction as the wire leaves the field. it does this over and over again. starts at zero, goes to max, then goes back to zero, then goes below zero [producing the negative] and goes back up to the other zero reference.
      this is alternating current.
      in a transformer is an iron core that gets magnetized because of this rising and falling of magnetic flux and produces a voltage in the coil. on the secondary side [another winding] the rise and fall of flux induces another voltage in the secondary winding but of opposite polarity that the incoming. it is 180 degrees out of phase with the input. now, depending on how many turns are in the secondary winding the voltage can be increased or dropped to whatever is needed.
      also there are many different connections available for the transformer to do what is needed for whatever load.
      step-up, step-down, buck and boost etc.


      • #4
        12V issue....

        Good memory, HayZee. We did go through a charging issue on a tractor a while back. Although this is the same tractor, it's a different issue. There's a constant discharge that is impossible for me to find. There is no magneto, as the plugs are points fired and it's a battery ignition. The field magnets are glued inside of the rim of the flywheel and spin past about 16 or so coils. And they are the charging system and are separate from the operation of the tractor electric. There are but two wires coming from the charging coils both producing ac current, and go directly to the regulator. Then, from that regulator is but one tab that outputs 12V dc. It's a Kohler regulator, by the way. That portion of the tractor electrical is only for the battery charging. Without the battery, and without the regulator, the plugs will not fire. The situation is such that there is an intense draw of 12 amps when running, far too much to allow charging of the battery. I wanted to verify the the voltage with the engine off and with it running. The problem is that I can't get the VOM near that engine when it's running. That's why my suspicion ran to that charging system being the culprit. Which I believe you answered with your explanation of the gauss field. I just didn't think it would be that strong of a field to affect a meter six feet away. I will be running an experiment in the next couple weeks. The engine must come out for other reasons anyway, so that will be my opportunity to yank the flywheel, remove the coil ring, reinstall flywheel and see if I can THEN check it with that same VOM and see if it jumps all over. At this point, the problem is becoming personal! Besides that, I'm still baffled by the over-the-top discharge. I just wish I knew what the voltage drop was, too.


        • #5
          tractor loads

          ok, what you might do is series your meter inline with each device on the DC circuit. in a series circuit the total amps is the sum of the individual loads, the potential or voltage is constant. ok. you say the output of the coils goes directly to the regulator. now, there has to be a rectifier somewhere in the system. the regulator can't regulate alternate energy because the polarity is constantly changing.


          • #6
            Kohler tractor engine

            Disconnect all the leads from the Rectifier/Regulator. That’s the B+ lead and the AC leads. With the engine off, use the Ohm feature and measure across the AC leads. You should show .064 to.2 Ohms. If it shows open your stator is no good. Measure each lead to ground. Neither should show continuity. With the Rectifier/Regulator disconnected start the engine and rev to about 3600 RPM’s. You should see 28 volts or more. If so the stator is OK and the regulator is no good. Less than 28 volts the stator is probably faulty.

            Merry Christmas



            • #7
              Jumping VOM....

              Thanks, Hawkins. I can check the resistance of those AC wires tomorrow. Will not be able to check anything with engine running, however.