Being a stickler for details...it has bothered me that the temp gauge on Shop Truck wasnt reading accurately - way too low. The reason...temp sending units for 70 yr old 6 volt electrical systems and gauges are non existent...or have a completely wrong impedance. So the fix...get the engine to known operating temperature of 170 degrees. In parallel, run 1 wire to gauge and a second wire shorted to ground. In the second wire, stack resistors until the gauge reads 170 (accounting for voltage drop in the 6 volt charging system at idle), then soulder the resistors together. From there, if the engine gets hotter, the sending unit will continue to function normally and taaadaaa....we now have a perfectly celebrated temp gauge at 94ohms worth of 1/4 watt resistors. Pics are just showing my testing with different resistance to narrow down which to use. 2 47 ohms did the trick.
