[Flat Ernie]

The '40 BATT meter requires a precision 60 Ohm resistor wired in series with the gauge from 12V to get the correct reading.

A voltage drop will always read 6V or thereabouts. You want to read the relative voltage (12-14) and how it varies with the generator/alternator charging.

The absolute very best way to do this is to get a potentiometer (variable resistor) and wire it up with about 70 Ohms in series with the BATT meter and a 12V batter. Then adjust it down until the BATT meter reads roughly in the middle to lower end of the yellow band. Then just leave that in series with the gauge and 14V should read towards the very top of the green band and maybe slightly into the red band. 13.5V should be roughly the lower portion of the green band.

The value of 60 Ohms is derived from the Ford Service Bulletin outlining the overall resistance of the BATT circuit as stock. By doubling the resistance, you should keep the current through the BATT gauge equal with double the voltage resulting in equivalent readings (green = good charging)