1939 / 1940 Ford Battery Gauge And Upgrading To 12-Volts ...What You Need To Know

Starting with model year 1939, Ford installed a "Battery" gauge in the dash of their Deluxe model Ford, Mercury and Lincoln Zephyr model cars, instead of an Ammeter. No one seems to know why this happened, some speculate that there was a shortage of Ammeter gauges or that production could not keep up with demand. It doesn't really matter as long as you know how to identify what you have in the dash. The use of a "Battery" gauge in the Deluxe models continued into the 1940 model year.  The Battery gauges used in the Ford, Lincoln and Mercury models are marked with the colors of Red, Orange, Green and Red again at the top of the gauge markings.

Here is a 1940 Ford truck Ammeter

If your dash gauge doesn't look anything like the one described above, but instead, the dial says DIS and CHG  with the needle in the middle, you have an Ammeter instead. When you upgrade to 12-volts, you do not have to do anything to the Ammeter, because an Ammeter measures the volume of current...not the voltage. Again all standard model cars, and the trucks got Ammeters instead of Battery gauges.

If you have a Battery gauge in the dash...which in reality is a volt meter, (although not calibrated as such) the Battery gauge is designed to display the amount of voltage present in the electrical system when the ignition switch is in the "on" position.

1939 Mercury Battery Dash Gauge

The markings on a Ford Battery Gauge are as follows...Green was normal operating voltage, and represented a voltage range from 7.1 to 8.25 volts. The Red on the high side identified voltages of 8.25 to 9.0 volts. The Red voltages on the high side were unsafe for light bulbs, and often resulted in the water being boiled out of the battery from overcharging. Excessive generator voltage output (from stuck points in the regulator) can cause permanent damage to the generator. (All Ford Lincoln and Mercury Battery Gauge markings represent the same thing.)

The Orange sector represented voltages from 6.2 to 7.1. When the needle dropped below the green on the dial but stayed in the orange that meant the headlights and electrical load was equal to the generator output, and no current was being replaced into the battery. If headlights were not on such as daytime driving it meant the generator was not recharging the battery and the charging system may need attention.

If the needle dropped into the Red on the bottom of the scale, it represented 6.2 volts or below a warning to the driver the charging system was not keeping up with the electrical load and the current in the battery was fast being used up.

This is a backside view of a Ford Battery Gauge.  Battery current flows thru heater wire which in turn heats the bi-metal which moves the needle on the front side of the gauge.

The battery gauge on the left is the 1939 design, while the battery gauge on the right is the 1940 design. Remember it was just the deluxe models that got the battery gauges, the standard models along with the trucks got the ammeter gauges in the dash.

Part of this information on the Ford Battery gauges came from a Ford service bulletin dated October 1939. I wanted you to be able to read and understand a Ford Battery dash gauge, and also know the difference between a Ford Battery gauge and a Ford Ammeter. 

So now the burning question becomes...what do you do with the Ford Battery gauge that is calibrated for 6-volts, when you upgrade your Ford electrical system to 12-volts? 

The answer is simple! Install a "Runtz" voltage drop onto the back of the battery gauge just like you will do for the gas gauge and the rest of the electrical dash gauges, (water temp, oil pressure) and you will live happily ever after. The "Runtz" will reduce the 12-volts down to 7.75 volts which will be in the "Green" markings in the original Battery dash gauge. The Battery gauge will then function just like it always did.

I know that often times the solution from the experts is to just swap out the "battery" gauge for an "ammeter" gauge and everything will be fine. That will work, but the gauge face of the Ammeter gauge will not match the rest of your gauge faces.

Now that you know the simple solution of installing the Runtz onto the original Battery gauge which allows to keep and use all of your stock gauges, the upgrade to 12-volts just became that much easier.