While the drawing above is very nicely done, and probably the best description of how it works that I've seen, but  it is not quite accurate. It says that on high the circuit would have 225mA. The IVR (Instrument Voltage Regulator) supplies an average of 5VDC, So using Ohms Law, R= 5V /.225A= 22.22 ohms. Since the sensor is 10 ohms of that total, 22.22 - 10 = 12.22 ohms for the gauge heater. That's a reasonable number, as I have measured 12.4 and 12.8 ohms resistance for the gauge heater.
Here is where the first problem arises- The Ford Shop Manual says that the sensors are variable resistors that range from 10 ohms on high to 73 ohms on low, not 75 ohms. So lets add 12.22 ohms for the heater to 73 ohms for a total of 12.22+73=85.22 ohms when on high. The amperage would then be 5V / 85.22 Ohms = .059 amps, not .006 amps. It looks like they missed it by a decimal point. Its closer to 60mA (.060A) instead of 6mA (.006A).
It may not seem like much, but in electronics it makes all the difference in the world .

Last edited by Mach1Driver (7/17/2021 12:42 PM)

Bearing Bob...ok, gotta remember that- I like your RPM avatar