adding low coolant warning

[92EcoDiesel Jetta]

I deconstructed the MK2 circuitry and it's an op amp with the engine temp on one input and the level sensor on the other.

Have you posted the circuitry before? The coolant level sensor is a probe with 2 electrodes about 1/2" apart (not sure of exact dimension) that measures conductivity of the coolant, very much like the water in fuel sensor. Makes sense an op amp is needed for such a high impedance sensor. If the system is filled with 100% distilled water, the  level sensor will not work since distilled water is  non conductive.

[92EcoDiesel Jetta]

The float switch idea is not bad, if you can find one rated for ~100^oC.

Would be nice to know what the OEM sensor actually does and hack a circuit for it.

Even if it's rated to 100C, it may not physically be able to be installed into the expansion tank.

[fatmobile]

 No I don't think I posted the circuit and would have to look for the box of stuff that's dedicated to that project.