Can anyone tell me how the "auto" setting on the  MK3 Supra works?

The fan will not switch on automatically anymore with the fan selector set to auto. It was working, then stopped abruptly while I was driving, briefly restarted, then quit again.

The fan work fine at all speeds manually.

All of the solder joints have been re-flowed on the controller
The VSV is working.
The heater is working.

The circuit diagrams are pretty confusing.
AJ Hunter
Graphic Designer

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027 201 9738