Built for a car which never had one. The owner has just paid out for a new engine and he wants to protect his investment. Uses a spare light position found in the car's existing instrumentation.
When the water level is good an immersed probe completes a circuit to ground via the coolant in the pipes. When the level drops the probe is no longer in contact, the circuit is broken and the warning lights up. The probe is located in the highest part of the sealed and pressurized coolant, where the level is checked.

The conductivity probe is a brass rod inserted into a short length of silicone tube, pressed into a hole drilled into the expansion tank. Epoxy putty holds the silicone tube in place. The brass rod dip depth is adjustable by moving as required the rod up or down through the silicone tube. The build up of pressure in the cooling system does not push the rod out because the cross sectional area isn't enough force to overcome the friction keeping in place.