Kitchenaid KSM150 Mixer's speed Off has become High and speeds 1-10 have become Off

SOURCE: I have a 350 watt

replace the stir speed switch

SOURCE: mixer speed settings

It could be the phase control, or it could be that the governor assembly or associated spring is missing or malfunctioning.

SOURCE: won't shift speeds

ok so the set-up looks sortof like this (lets hope the formatting of this website/your computer doesn't mess it up...)
       C           D
        /  /    \  \   \/                                    |    | ___   /\ |    |   <B
  A
A is the rivet holding the arm to the plastic body B is the contact with a little tab. C is the copper arm D is the arm's contact
there is a copper arm (C) that has a contact on one end (D).  If this contact is ALWAYS open (should only be open when mixer is off) it is either bent (carefully reform) or has come loose at the rivet marked A.  On mine, the rivet at postn. A had come loose, i simply rotated it so that the contacts touched and super-glued the copper to the plastic (which should be rivited).
Since the copper was not discolored, I knew that the temps should not be too high for the super glue to handle (not in my opinion anyway). 

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.

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.