Sparking under the speed control switch on kitchenaid mixer

SOURCE: Kitchenaid Mixer K45SS

I just did this last week!
1 Unplug Mixer 2 Remove screw on TOP of mixer body, at back. 3 Remove rear cover and paper-gasket thing (its not the end of  the world if you end up tearing it a little). 4 the phase control (little circuit board w/wires) is at the top (you  should see it very easily) held on by one screw, remove it. 5 when the circuit board is unscrewed, disconnect all the wires  that are connected FROM THE CIRCUIT BOARD to the  black control plate (NOT all wires-there should be 4 left on  the control plate at this point).  you don't need to  remember where all the colors go as the control plate is  labeled->but looking at them and making sure you  understand the abbreviations would be beneficial  (BK=black, BL=Blue RW=Red&White W=white R=red etc.   6 install new phase control (screw), either buy new thermal  grease (used to keep the board cool->it transfers heat to  the mixer's housing) or try to salvage what you can from  the old one. 7 connect wires, paying attention to labels. 8 reinstall paper gasket, then cover, then screw to hold on back- cover.

SOURCE: Blakeslee (Kenwood) A717 mixer speed control

I had the same problem. Had to strip the machine down to get to the motor. On the top is a moving plate with various capacitors etc mounted. On the front side there was a Capacitor (At least I think it was) with the wire broken.
I resoldered it and the machine now works fine.

I hope this helps

SOURCE: speed switch doesn't work, Hi speed only

CAREFUL, do this unplugged. Behind the back plate (one screw on top to remove) there is an adjustment single security screw, lower center. Adjust this screw out or in as needed. Hit or miss but very easy. Repeat till all speeds work as designed.

BobbyT

SOURCE: My Kitchenaid K5SS 300 watt standing mixer launches into the highest speed regardless of the speed ....

I would recommend that you first check the Hall effect sensor which determines the speed.  I had this exact problem (but mine occurred with a 600 professional model) and I actually went out and bought another Hall sensor for about $13.  I replaced the original sensor and made sure that the connection between the sensor and control board was nice and snug. My blender worked as good as new! But I was curious to know if my original sensor was defective so I put it back in mixer and again made sure all of the connections were tight.  Guess what?  My original sensor also made my mixer work!  There was nothing wrong with my original Hall effect sensor except the connection between the control board and sensor was not adequate--unfortunately I didn't check the connections before I decided to buy another one. My guess is that the connection between the control board and the sensor probably loosens or comes undone over time from use and vibrations. 
1.  Download the mixer parts and diagrams from the Kitchenaid website to see what the Hall effect sensor looks like. 2.  You'll need a phillips head screwdriver to open up the mixer. 3.  Make sure that the connection between the control board and sensor is absolutely tight and snug. 4.  Also, make sure that the orientation of the sensor connection between the control board and sensor is correct. it's hard to know which way the sensor should be connected because the control board has three prongs and the sensor can attach facing up or facing down. If, after you've made the connections, the mixer still doesn't work.  Disconnect the sensor and reconnect it with the prongs facing the opposite direction and try it.  I actually found the right orientation through trial and error.  By going through this trial and error, I also had my Eureka moment when I realized that the original sensor probably would also work it were reconnected with correct orientation.   5.  This whole process probably takes less than 10-15 minutes to perform.

SOURCE: kitchenaid mixer speed

I would recommend that you first check the Hall Effect sensor which determines the mixer’s speed.  I had this exact problem and I actually went out and bought another Hall sensor for about $13.  I replaced the original sensor and made sure that the connection between the sensor and control board was nice and snug. My mixer worked as good as new! But I was curious to know if my original sensor was defective, so I put it back in mixer and again made sure all of the connections were snug and tight. Guess what?  My original sensor also made my mixer work!  There was nothing wrong with the original Hall Effect sensor except that the connection between the control board and sensor was not tight--unfortunately I didn't check the connection before I decided to buy another sensor. My guess is that the connection between the control board and the sensor probably loosens or comes undone over time from use and vibrations. To fix your problem:
1.  Download the mixer parts and diagrams from the Kitchenaid website to see what the Hall Effect sensor looks like. BTW the part number is 9703312.
2.  You'll need a phillips head screwdriver to open up the mixer gear case cover.
3.  Make sure that the connection between the control board and sensor is absolutely tight and snug.
4.  Also, make sure that the orientation of the sensor is correct.  It's difficult to tell which way the sensor should be connected because the control board has three prongs and the sensor can attach facing up or down. After you've made the connection, run the mixer. If it still doesn't work, disconnect the sensor, reconnect it with the prongs facing the opposite direction and try it again.  I actually found the right sensor orientation through a little trial and error.  But by going through this trial and error, I also had my Eureka moment when I realized that the original sensor probably would also work if it were reconnected with correct orientation.  
5.  This whole process probably takes about 10-15 minutes to perform.
6.  Finally, use your common sense and make sure to unplug the mixer before checking, connecting or disconnecting anything.