The three pin connector (with two wires) is approximately 12 volts AC. Set your meter to AC volts, one probe to one wire, the other probe to the other wire. It doesn't matter which probe (red or black) you use for either wire. This is AC. AC doesn't care about ground and positive (in this application).
According to my schematics....all black/white wires and red/white wires are grounds. Electronically speaking, they are all the same point....and are all connected under the assembly. So....set meter to DC volts, black probe goes to any of the above mentioned wires, red probe goes to red wire...you should have 18VDC (give or take). Keep black probe where it's at, put red probe on one of the black wires (the schematics show two, they are both the same).....you should be 10VDC (give or take). Done.
Edward