We would like to know:

1) What the fans are doing
2) What the engine temperature is

ABS sensors give the ECM rotational speed information for brake application, power retardation.
A similar sensor could give us the RPM of the aux fans. Two gages can be mounted on the A pillar to show the rpm for each fan. So, that solves the problem of what the fans are doing, whether they are rotating or not.

For the second problem, we could continue to use the temp gage.

This is how it would work, you are driving around and the temp needle goes to 107 C, you look at the rpm gages for the left and the right aux fans to see how fast they are rotating. If the rpm is low, you will know that they have not gone into high speed mode.

Another feature can be added. If the engine temperature goes above a certain temperature, an Led lights up above the aux fan rpm gages. The existing temp sensor resistor could be utilized.

For the coolant loss situation, the low coolant light will turn on and the AC will not provide any heat.

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Zafar
94 E320 58000 Miles