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  1. #1

    Default Millermatic 35 Wire Feed SLOW


    I found a MM35 and hooked it up.

    The wire feeds super slow out of the Tweco #4 gun. The wire feed knob works, but even the 100% speed is too slow. It does not appear to be a problem with the gun liner. Feed motor brushes look good.

    I poked around the inside and 2 of the large diodes are shot, one open, one shorted.

    I'm replacing them tomorrow, do you think that the diodes somehow will also fix the wire speed problem?


  2. #2


    Replacing the two 150A 300v reverse polarity diodes fixed the problem.

  3. #3
    Join Date
    Jan 2009


    Hey just read your post.

    Just for clarification the large diodes do not affect the wire speed control circuitry. By replacing those 2 diodes you fixed the bridge rectifier that controls the DC voltage output that creates your welding arc. With those 2 diodes not functioning, your welder does not create the necessary voltage to make a hot enough arc to weld properly. You may have misinterpreted this as bad wire speed.

    I was just troubleshooting a MM35 that wasn't creating a proper arc, and I also felt that wire speed felt funky. After reading your post I decided to test the diodes for proper function. I too ran into 2 bad diodes. . . one shorted, one open. Thanks for posting your issue, it gave me a new avenue to test. That ended up being my power issue.

  4. #4
    Join Date
    Mar 2009
    DFW, Texas


    From what others have posted, the MM 35 powers the drive roller motor off of the DC welding output through a rheostat. So bad DC welding output will impact feed speeds.

  5. #5


    Yes, the bad diodes really did slow the wire feed down to a crawl. Just looking at the drive gear spinning, I could tell it was way too slow.

    My local Miller dealer did not stock the diodes, so I got them from Galco for $18 each and they arrived 2 days later. The reverse diodes are part # 037-306.

    My dealer said they often fail in pairs. The first shorts out, then the other burns open when it can't handle the current by itself.

    The welder is flying now, built 1/2 a trailer today with it.

  6. #6
    Join Date
    Feb 2013

    Default slow wire speed on a mm 35

    For 25 years I used .035 and .030 wire and this Miller matic 35 only welds at the 100 speed by lowering the speed---I just put a new liner---for .030 and it remains the same ----it only burns the wire back into the tip. Can you tell me how to diagnose and test the diodes -----and if you have a phone number ________of a source to buy them after testing is convincingly definitely the problem . Feel free to call me Thanks, Don 585-352-1165

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