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Square pulses that are 2.5 ms wide and fall from 8V to 0V. Frequency duplicates that of speedo input signal. Frequency is of course proportional to car speed. But pulse width for this signal remains steady at 2.5 ms whatever the frequency, therefore duty cycle varies with frequency, while signal from speedo input is roughly always 50% duty cycle.
Not sure this is clear.
By the way, the function of this is not to hold off the fan until car is moving, as you said, but rather to hold on the fan when car engine is overheating. In this situation, heater fan is helping to cool engine down when car is not moving or is moving at very slow speed, like in traffic.
But once car is moving past a certain speed, via this signal and some logic gates, relay module cuts off heater fan even if oil temp is still reported too high.
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