If there are 2 diodes, and a thermistor right at the brushes, most likely the field voltage to that point is A/C. The 2 diodes, are there to induce half wave D/C into the rotating field, which is most likely much greater than the 12 D/C you applied. If you can post the wiring diagram, I may be able to walk you thru it. Many small units use half wave, some use A/C into the fields, controlled via caps. You can test the diodes, with most any meter, but if your looking for the forward opening voltage you will need a meter that has a diode function test. The thermo can be bypassed for testing. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements. Are you sure it's a Thremo, or is it a MOV? Hope this helps.