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Millermatic 251 Fan Not Working

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  • Millermatic 251 Fan Not Working

    I have a Millermatic 251 Serial No. LE436902 the cooling fan won’t operate, was welding for quite a long time on a project and the fan never kicked on, then the thermal overload shut the machine off. Can anyone point me in the right direction as to how to check the cooling fan or thermostat?? That tells the machine to turn the fan on.

    Additional note I have owned this machine for around 10 years and the fan had always worked the way it’s supposed to up to this point.


  • #2
    How comfortable are you with checking voltages and electrical connections on your welder? There is a relay, CR1 (you should see it in your parts breakdown) that turns the fan on and off. It isn't turned on directly from the thermistor (temperature sensor), but is turned on by the board. Your fan sees 230V across it when the relay energizes. First thing is to make sure nothing is blocking the fan and it spins freely. If you're comfortable working around mains voltages, I could probably help guide you with what to check next.


    • #3
      Fan spins freely, took the whole cabinet off to expose all wiring and components and to clean it had a good layer of dust on everything, no obvious loose or exposed connections. I don’t have a owners manual for it, but I’m sure I can find one to download. I can navigate around with a volt meter fairly decent and have a decent quality one at that, not a $8.99 jobber from harbor freight


      • #4
        Ok, if you're looking at the main circuit board, the relay will be just to your right. Either the fan has an open coil, the wires from/to relay and board/motor are toast, the relay isn't working, or the board has an issue. I recommend you attack them in that order, first disconnecting the machine from power, then testing with power if you didn't find the issue.

        Check for continuity across the fan motor - easiest way is from the white lead on the input circuit breaker to the wire that connects to the relay ( a picture of the relay would help me help). If it's open loop, you either have an issue in the motor winding or in one of the leads to the motor.

        If the motor leads check ok, check for good continuity from the board to the relay. The two wires closest to the panel on the relay go to connector RC1 on the main board. Find pins 16 and 18 on that connector and measure resistance across them. I don't know the spec on that relay, but the resistance you measure should not be open. Any measurement other than a short or open should be good.

        If that checks out, your relay should be clear, so inspect the contacts through the case to see if there are any issues.

        If these don't reveal anything, it will likely require powering up to see where the problem could be.

        Hope this helps and please post a picture of the relay label and the bottom/connector area.



        • #5
          Pins 16 and 18 measure 2.156 on the left hand side facing. Pins 16 and 18 measure 06.09 on the right hand side. I get no reading when I test the motor leads. Unless I’m missing something on to check the motor??
          Last edited by Triple.tfarms; 10-30-2019, 02:56 PM.


          • #6
            I didn't quite understand what you meant by left hand side or right hand side. Are you talking about the board side of the connector and the cable side of the connector?

            Also, I saw you had some message in your photo, but couldn't figure out how to read that message.
            Last edited by jjohn76; 10-30-2019, 06:21 PM.


            • #7
              Those terminals with the voltmeter probe in the background is the power wires going to the fan, it’s those two wires that I zero reading on.

              the circuit board highlighted in red is the left side I’m referring to and the ones circled in black is what I’m calling the right side.
              Attached Files


              • #8
                Ok, you should not have an open circuit on your fan power leads. If you are certain those are the leads to the motor and there is no continuity across the fan windings, I would recommend a replacement fan, Miller part number 188706. Here is the cheapest one I found, from Miller service parts.

                The pins on the left side in your picture are 16 and 18. The ones on the right side are 6 and 8. That resistance value between 16 and 18 seems very low. If you have a diode tester, you should get about .5V when you put the positive lead on pin 16 and negative lead on pin 18. You should get open loop when you put the negative lead on pin 16 and positive lead on pin.


                • #9
                  Thanks I’ll check it out in the next couple of days.