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No power on miller bobcat 250

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  • No power on miller bobcat 250

    I have an older miller bobcat 250, prolly around the 2005 model. Was working good before it sat for the winter now I’m not getting any ac or dc voltage and nothing off the brushes. Check resistance between brushes and continuity with the shaft everything checks out alright. Any help would be appreciated.

  • #2
    Serial Number?

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    • #3
      Lb193859

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      • #4
        It's a 2001 model Bobcat 250NT. A little more information would be great. What was the ohm measure between rings? Should be 3.2. Did you measure the OCV at the studs. Completely missing or just low? Check the fuses, Check the diode and look at the cap. Is the red dot still intact or is there a hole in the top. All power starts at the brushes. If all that checks the fuse holder may not be making good contact.
        But i can help you more if I know the exact voltages at the studs and the 100v recpt.
        Last edited by 1997CST; 07-09-2020, 07:49 AM.

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        • #5
          The resistance was 3.5, I’m not getting any power at all. My hour meter won’t move either, I’m guessing because there is no power. All the diodes and fuses check out okay. I’m checking the power right of the brushes and I’m not getting anything

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          • #6
            Even the blocking diode is good? How about R2? your problem is in SR2 or 3, or the capacitor. You need that exciter circuit for everything. Your problem is in there somewhere and if everything checks it sends me to the cap.
            Last edited by 1997CST; 07-09-2020, 10:40 AM.

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            • #7
              So if turn the key on your hour meter doesn't click? Not trying to start it? Check if you have 12vdc on one side of that meter. If you have 12 vdc I wonder if that ground from the meter is bad. That grounds that whole circuit.

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              • #8
                I you want quicker advice send me your phone # by email. I'll try to walk you through it in real time if you like
                [email protected]

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                • #9
                  What do you mean by the cap? And I have not taken the capacitor off yet and checked it. Not seeming to get any voltage on the meter.

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                  • #10
                    If the unit starts with the front panel switch and stays running, but has no hour meter and no power at all to the field, I'd look for a loose, corroded, or burnt plug4, loose or corroded connections near it, bad chassis grounds near it, etc. I'd also check the diode between the ignition power and the field winding, in case the hour meter was a simple unrelated failure.

                    If the fuse or its associated wiring, or the ignition switch and its plug and wiring, were bad, you wouldn't have power to fire the starter motor and power the ignition module. If it's starting and running normally (and you're not sticking a screwdriver across the back of the starter and forgetting to mention that), they should be good. However, power from the ignition module goes to the hour meter and the diode to the field winding, so if you have ignition but no field, it would have been to be something between those points.

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                    • #11
                      Found this by plug 4. Might be on the right track, is that some kind of diode that is also burnt out?

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                      • #12
                        This also does not look healthy

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                        • #13
                          I'm not sure what the part in the first picture is, but it sure doesn't look healthy, and the red wire probably should be connected to something, possibly itself...

                          The second picture is your voltage regulator for the charging system. It's not directly connected to the parts that aren't working, To test it, once you have the unit working, check to see if it's charging the battery.

                          According to the manual and part number I found, the diode is mounted on a little plastic bracket with .250 disconnects, so it's not your diode if that's correct. I haven't worked on one of those exact units, though.

                          You might have to see where the wires go, and match it up to the wiring diagram.

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                          • #14
                            Besides the highly suspect red wire that looks like it corroded internally and then broke in half, the silver thing above the wires looks like one end of a 3AG/AGC fuse that has been broken off. Circled in red below. Wonder where that is supposed to go?

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                            • #15
                              Yea that’s where I’m stuck at. I was thinking it was a diode of some sort.

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