Wiring problem Have a Black$&Decker MM850 electric mower. While cutting one day the switch burned up. When I installed a new switch it worked for afew seconds then stopped. If I cross the wires the mower comes on when plugged in without depressing the switch. Is there a wiring diagram available for tis mower?

mower, then blade, then plastic part, then metal square washer, then nut

should have the switch or the brush connection checked

The "wedge" of the plastic lever wears, so you need to replace the lever or build up that section of the lever. Some cyanoacrylate (Super Glue / Krazy Glue) and a piece of a zip tie will work.

The rectifier is a flat plate with 4 posts that attach the electric wiring inside. You will need a flathead screwdriver to remove the four screws holding down the cover on your mower. on top where all the wires run (you will probably see one burned) There is a screw in the middle holding down the plate, you will need to replace the plate and resplice the wires that may be burned.

You can go to the black and decker parts page by going to this site:

http://www.blackanddecker.com/CustomerCenter/parts.aspx

I was having a similar problem. I tried starting it up with the switch case open. I saw a very tiny puff of smoke come out of the switch. I replaced the switch and it was fine.

can't just replace the switch with on-off.original switch has interlocking with brake, once u pull the trigger the brake circuit will open & close when triggger is release. it is also parts of safety.must common problem is inside the switch terminal eventually the contact will burnt.

maybe something shorted inside mower.
you need atleast 14 gauge wire

I have repaired mines. It was found in the trash dump, just before street curb side trash collection. It is a very sophisticated lawn mower. It taken me a month to restore it. The problem is the housing is poor design and does not hold the pin switch securely. This is not a simple on/off switch. It is switch module with two momentary switches built in to it to work simultaneously. The hot is a normally open momentary switch and the neutral is a normally closed momentary switch. Two things must occurred simultaneously for the lawn mower to work. The AC hot has to close and the neutral to open. This is a split phase AC motor. The switch has four connectors to it. The first two has black wires to it, for the hot, that is normally open. The second two has a white wire and red wire to it for the motor's start up winding. The orange connects with the plug's white, neutral. A new substitute switch will be two individual momentary switch, one for the two black wires will be normally open and the two for the red and white wires from the motor start coil will be normally close. It will take two fingers to operate the lawn mower. I recommend you to buy a new and each switch from Black and Decker.
The problem with the system is not the switch, but the poorly designed switch box housing. Often, there is nothing wrong with the switch. I tested mines and it works fine. The problem is the switch box housing does not keep the switch in the correct position for the higher tension spring lever to press against it to operate. I use small washers to stack between the rear of the switch inside and reassembled the switch box upside down by turning the handle upside down, so the washers and switch do not fall out. Once I am finished assembling the switch box and position it on the handle, then I position the handle back to the normal top side and attach the handle back to the lawn mower. I am finished.
http://en.wikipedia.org/wiki/AC_motor#Split-phase_induction_motor

"Split Phase Motors
Split phase ac motors are widely used for low power applications as for example in oil burners, furnace blowers, garage door openers and major appliances. Such motors employ separate run and start windings connected in parallel across the acpower source. When the motor is initially energized both windings are energized, and when the rotor speed reaches about eighty-five percent of the normal running speed the start winding is disconnected from the circuit. Various devices are utilized forthis latter purpose and include centrifugal switches and current and voltage relays.

A problem which is characteristic of these motors is that they have a relatively short operating life. The principal reason for this is the failure of the switching devices used to cut out the start windings when the rotors approach the normalrunning speeds.Failure of these switching devices in turn cause the start windings to burn out.

One of the reasons for the failure of these switching devices is the high currents which they are carrying at the time the contacts open. Attempts to increase contact life by use of improved designs and materials have not been altogethersuccessful. Another disadvantage associated with these switching devices is that they are necessarily quite large and occupy a substantial space in the motor housing." -from another web site.

the pic says the connection are not right.all left side connection are for brake which is the red & white wire. all right side connection are two black wire.