I should have noted that I reversed the leads on each combination and the only difference was the meter showed negative "-" polarity on some combinations and no difference on others. It's a decent meter(Fluke 115). If you think about it someone at some point has rebuilt one of these tach amps that didn't need to be rebuilt for a few more years. I have to wonder if it really is impossible to cull out a useful reading(s) with resistors involved. Maybe using a range of temperatures.