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  • How do I troubleshoot electric lawn mowers?

    Hello,
    I've recently picked up a dead Craftsman 19" electric lawnmower at a garage sale. It's a popular model that shares the platform and parts with many other B&D mowers. It uses the very popular 681064-01 switch (push button with four contacts). At first, I thought the switch may have been the culprit, but after some testing, I'm not so sure anymore. Let me explain my findings:

    The switch has four connections. #1 black wire from power plug, #2 black wire to motor, #3 red wire to brush, #4 white wire to brush. There is also an orange wire #5 direct from power plug bypassing switch altogether. Wires #2, #3, #4, #5 connects at the rectifier above motor. On operations: if I press the power button, the #1 & #2 black wires would touch; if I release power button, #3 & #4 wires would touch. This is the basic function of the switch and it's doing it. So based on this alone, could I confirm the switch is not the problem?

    Now when I actually plug in power and do the testings, it gets more confusing. If I press the power button, all that happens is a little smoke rises from the rectifier and then the circuit breaker trips. It appears the switch is delivering the power down, just that something is happening at the rectifier and/or motor.

    I would appreciate if anyone could help me with the troubleshooting steps. Could you explain what the relationship is between the rectifier, shunt brushes, and motor? Is there a way for me to deliver power to this motor (bypass the switch) to test out to see if the rectifier/motor is even functional? Thank you!

  • #2
    Re: How do I troubleshoot electric lawn mowers?

    Smoking rectifier is not good, it will need to be replaced.
    The rectifier is required to switch AC (wall power) to unregulated DC.
    The brushes are the power input to the motor, with the switch off the brushes are shorted to provide a dynamic break which quickly stops the blades. You will need to test the motor to see if that caused the rectifier to go. Not sure what the specs would be for resistance but I would guess in the 10 ohm range for a 12 amp motor (120 volts/12 amps=10 ohms)

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    • #3
      Re: How do I troubleshoot electric lawn mowers?

      Once you let the smoke out you might as well toss it. Will probably cost more to repair than its worth.
      SSG, U.S. Army
      Retired
      K.I.S.S., R.T.F.M.

      Comment


      • #4
        Re: How do I troubleshoot electric lawn mowers?

        Wayne & TOD,
        Thanks for the reply...I was so busy that I didn't get a chance to update. Basically, I can pretty much say for sure the switch is still good. I bought it hoping the switch may be bad (ya know if happens very often with these things). I suspected the rectifier is bad, so I went ahead a bought a similar one off ebay (about $3.00). I'd just received it yesterday. I plugged it in and tested it and I'm left fearing the worst may be true! Now, if I press the power button, it would spark & run (though a little unstable) for about 30 seconds and then trip the circuit breaker. ALL my wirings are supposedly in stock positions. So I switched the + and - output on the rectifier to be sure...and the same result. The spinning motor leaves a burning smell. My worst fear is the motor may be culprit. What is the proper way to test this motor? I appreciate your help!

        -Dennis

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        • #5
          Re: How do I troubleshoot electric lawn mowers?

          This is a good read
          Google Test DC Motor for videos as well
          You may just need new brushes and clean out the slots between the commutators

          Comment


          • #6
            Re: How do I troubleshoot electric lawn mowers?

            Originally posted by wbrooks View Post
            Smoking rectifier is not good, it will need to be replaced.
            The rectifier is required to switch AC (wall power) to unregulated DC.
            The brushes are the power input to the motor, with the switch off the brushes are shorted to provide a dynamic break which quickly stops the blades. You will need to test the motor to see if that caused the rectifier to go. Not sure what the specs would be for resistance but I would guess in the 10 ohm range for a 12 amp motor (120 volts/12 amps=10 ohms)
            Hi, wbrooks,
            I have a similar lawn mower problem. I have made quite progress. But I still have some questions to be answered. Would you please give me some advice?
            What I have done:
            * I have identified that the rectifier MP254 is broken. The points between two AC are shorted. I followed the test on youtube.
            * I have tested that the motor is good. I tested using a multi. meter. Again, the test can be found on youtube.
            * I found out the MP254 is 25 AMP, 400 V rectifier.
            * I opened the switch. The switch is burned out. The two points connecting the black wires were melted down from the sparks. It was burned off bit by bit. In the last few years, gradually it needs pushing swtich handle harder and harder to get the mower turned on. Later, I put the switch in the fixed ON position all the time by holding in the push button. Mower will start when the 110 V AC wire is plugged in. This way it worked for another year at least. I wonder if this is the reason that rectifier broke.
            What problem I still have:
            * Rectifier question.
            1) I found rectifier in nearby store (Surplus Gazimo). It is GBPC3506 which is 35 AMP, 600V. My concern is that will it work? Are those values the maximum only or it may hurt other parts if installed ?
            I found that GBPC3506 cross reference to MP356. Both are 35AMP, 600V.
            2) Are all MP254 interchangeable? My rectifier has label MP254 A90. Others online is MP254 with some other texts behind.
            * Switch question.
            1) May I skip the red & white wire connection when switch is off? That is, I will remove those two wires. I will only use the other two wires from 110V AC. (I plan to skip the switch also. I will use 110V wire plug to control mower since the switch will burn down soon again. i understand the original switch design has safety feature.)
            2) Will this two-wire only connection harms the motor or the rectifier?
            I know that the MP254 can be ordered online. However, the curiosity of knowing how it works is greater than making it work now. I wish that I had learned some knowledge of electricity and electronics.
            Your answer is greatly appreciated.
            Thank you very much.

            Lee

            Comment


            • #7
              Re: How do I troubleshoot electric lawn mowers?

              The GBP3506 will work fine, it is just overkill for the mower.
              I personally would not own a mower with all the safety equipment bypassed but it will run without it.

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