get out your trusty Ohm meter and go do the following:
View attachment 1715490783
I know it seems like you are doing the same thing by doing each test twice but with the leads reversed, but I'm looking for diode like devices in the system
MY guess
Red = power +12V
Black = neg terminal on the coil
White = light
Housing = ground for light.
The more I dig the more I think you are missing a wire (green) or the unit does not have a light and the wiring would be something like this:
Red = power +12V
Black = ground
White = neg terminal on the coil
OR
Red = power +12V
Black = neg terminal on the coil
White = ground
ALSO if you can open it up and look where each wire goes that would help too and PHOTOS
this is for Autometer
View attachment 1715490793
Now the plot thickens ( there are current driven tachs... who knew)
from:
https://shop.e3sparkplugs.com/images/pdf/5927253042154.pdf
"...There are two styles of tachometers, Voltage Triggered and Current Triggered. The wiring for both of these types of tachometers is different. First, you must determine what type of tachometer you have in your vehicle. Once you have done that follow the wiring instructions for your tach type.
Voltage Triggered: If the tachometer trigger wire was originally connected to the coil negative terminal, then the tachometer is Voltage Triggered.
Current Triggered: If the tachometer trigger wire was originally connected to the coil positive terminal, then the tachometer is Current Triggered.
Wiring the Tach Adapter harness for a Voltage Trigger tachometer; When wiring any DiamondFIRE component we recommend Weatherpack connectors. If you are not using Weatherpack connectors and prefer making wire to wire connections then we recommend soldering the wires together and applying an appropriate heat shrink. Step 1: Connect the Red wire to a switched 12 volt positive source Step 2: Connect the Black wire to ground Step 3: Connect the White wire to the Gray wire coming from the DiamondFIRE 6CDI Ignition Control Box (p/n E3.1440) Step 4: Connect the Violet wire to tachometer input trigger
Wiring the Tach Adapter harness for a Current Trigger tachometer; Step 1: Connect the Red wire to the tachometer input trigger Step 2: Connect the Black wire to ground Step 3: Connect the White wire to the Gray wire coming from the DiamondFIRE 6CDI Ignition Control Box (p/n E3.1440) Note: in this application the Violet wire is not used!
Note: Never install any tachometer input wire to either of the positive or negative coil terminals when using a CD Ignition Control box...."