[AllenV]

This is fascinating. I had no idea how the engineers of the time solved these challenges. My EE education dates to about 1980, when transistors were fully matured and electromechanics was old-school. Anyway, I found some further info over at FordTruckEnthusiasts. The attached image is for a 60’s ford. It pulls together much of what dmsfrr has been offering and adds detail of the senders and the voltage regulator. What I am learning is that in the 60s and earlier, before transistors were economical and the all-electronic alternative would have been vacuum tubes, the 12V supply from the alternator-regulator combo was not well controlled. Without further voltage stabilization the instruments would have variable readings caused by voltage supply fluctuations. The clever bi-metallic switch instrument voltage regulator used the thermal time constant of the heating caused by the 12V input to build a switching 5V power supply. The effect was to isolate the instrument panel from the 12V supply variation. No doubt the inductance of the gauges and resistance of the sensors would have smoothed out the switch pulses.