You can also verify the sending unit resistance cold VS operating temp. The sending unit can also be removed and partially immersed in boiling water to test. I'm not 100% sure on yours but they do have bi-metal heat sensor that they make or break at certain temp (calibrated ) that could run a bulb or a buzzer. "Off" being when the connection is broken and "on" when the connection is made.
if Your temp sending unit is purely resistive you can use numbers below for reference. I got data below testing my Ford 1700.
JC,
I do like the combination of analog gauge and "idiot light" but never light alone. Just imagine what happens if you have a burned out bulb.
Cold: sender=690 ohms, T housing = 98 , Radiator= 98 , Head=98 , Exh Man=98
Operating temp: sender=140 ohms, T housing = 172 , Radiator=170 , Head=174 , Exh Man=192
1/8" , right of middle: sender=92 ohms, T housing = 196 , Radiator= 196 , Head= 197 , Exh Man=210
Hot
1/8" left of H: sender=66 ohms, T housing = 210 , Radiator= 210 , Head=212 , Exh Man=250