# Trying to fix switch on B&D electric lawn mower



## ilyaz (May 17, 2014)

I picked up a broken Black & Decker electric lawn mower MM275. Previous owner said it just stopped running. Took off the switch cover and pressed the switch button a couple of times. The 1st time nothing, the 2nd time the motor started running but then one of the switch contacts started smoking. Bought a replacement (*681064-01 switch*) but forgot to mark which of the four wires connected to which connectors on the switch. So I know only one of them: the contact that was smoking that had one of the 2 black wires connected to it.

Got the new parts and then tried to connect it. Since I know very little about electricity, and also since I had 3 wires (2nd black, red, white) that had to be connected to 3 connectors, I decided to simply try the 6 different combinations. What I saw for at least 4 out of those 6 combinations was this: the blade would start spinning but then after about 1/2 second the circuit breaker would get tripped as if there was some sort of short circuit somewhere in the mower.

Does this mean that there is something wrong with the motor? When I tested it with the old switch, even though it was smoking, the motor continued running and the circuit breaker was not getting tripped. So I am hoping that the motor is actually OK. What else could be wrong? Or is this the motor?

Thanks!


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## Wuzzat? (May 17, 2014)

ilyaz said:


> but then one of the switch contacts started smoking
> *>high contact resistance, either internally or at the switch push-on connector.*
> but then after about 1/2 second the circuit breaker would get tripped



Not good.  
If you look at the trip curve for a residential breaker, 1/2 second means a pretty heavy overload, several times rated current.

This switch probably puts DC into the motor in the off position to brake it.  You need to take some ohmmeter measurements before you apply real power to this setup, there might be a diode inside the switch.

For a four terminal switch, checking with the switch on and off, two terminals at a time and reversing the ohmmeter leads to check for diodes, this is a lot of permutations.  
If you post all your zero or infinity readings the resulting truth table can be decoded.

Plus your switch may be clobbered in a way that an ohmmeter cannot detect.  If only the diode is clobbered you may be able to put one on externally for a buck or so. 

You can check the motor by running 120v directly into it without the switch.


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## ilyaz (May 17, 2014)

Wuzzat? said:


> Not good.
> If you post all your zero or infinity readings the resulting truth table can be decoded.



Sorry, you are dealing with a newbie here. Could you explain what exactly I should measure and how? I do have a multimeter so that's a start.  Thx



Wuzzat? said:


> Plus your switch may be clobbered in a way that an ohmmeter cannot detect.  If only the diode is clobbered you may be able to put one on externally for a buck or so.



But this is a brand new switch. Do you mean I might have damaged it while doing the connector permutation exercise?



Wuzzat? said:


> You can check the motor by running 120v directly into it without the switch.



Good sanity check, will do.


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## ilyaz (May 17, 2014)

ilyaz said:


> Good sanity check, will do.



I think something's wrong with the motor. I bypassed the switch and as soon as I turned the power on, the motor started worked and then the breaker got tripped. I also took apart the old switch and turns out it's extremely simple. No diodes or anything like that. Just a plastic "tray" with copper plates and springs above it.


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## Wuzzat? (May 17, 2014)

ilyaz said:


> I think something's wrong with the motor. I bypassed the switch and as soon as I turned the power on, the motor started worked and then the breaker got tripped.



Game over


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## nealtw (May 19, 2014)

I borrowed this from their help site
Hello nannie,

Hopefully you will be able to follow my explanation on where the wire leads go to the 681064-01 push button switch. From the top of the switch, where the push button is. The white wire from the motor cord is attach to it. The second terminal down is where the black wire from the motor cord is attached. The third terminal down is where the red wire from the motor cord is attached. The fourth terminal down is where the black wire from the power cord is attached. And then the orange wire from the motor cord and the white wire from the power cord are wired together with an appropriate wire nut. This is the diagram for a Black and Decker MM850 lawn mower. If the colors of the wires do not coincide with the diagram I have just described. Post your model number and we will make sure to get the correct diagram. If you do not feel comfortable wiring this switch. Take your mower to a Dewalt Service Center and they will be happy to wire this for you.

-WJA
http://www.ereplacementparts.com/switchpbutton-p-103549.html


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## WyrTwister (May 26, 2014)

With nothing plugged in & no power present , turn the blade by hand & see how freely the motor turns .

     If the motor does not turn freely , you may have dry bearings .  Try lubing the bearings , if you can .

God bless
Wyr


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## Wuzzat? (May 26, 2014)

WyrTwister said:


> With nothing plugged in & no power present , turn the blade by hand & see how freely the motor turns .
> 
> If the motor does not turn freely , you may have dry bearings .  Try lubing the bearings , if you can .
> 
> ...


That may be why the current draw is excessive.  I'd give it 50-50, frozen bearings vs. shorted windings.


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