DC Fan Malfunction Indicator
As I understand it, the yellow tacho wire is just a pulse stream proportional to the frequency. How about using a 555 timer configured as a 'missing pulse detector'? Choose a fan speed to trigger the alarm, and calculate the pulse timing (I believe the pulses are 2/rev) below which the circuit should trigger, and set the timer RC parameters for a pulse train of that timing. Any speed lower will then result in a 'missed pulse' output. This of course would be an intermittent output from the 555 (as another pulse does eventually arrive), so use another 555 or suitable circuit to 'latch' the output, until manually reset.

You could of course make R variable in the missing pulse detector circuit, allowing the alarm speed threshold to be adjusted. 

Both the missing pulse monostable circuit and the latch can be done using a 556 dual timer rather than 2x 555's.