We have several of these blowers with same PN# that have different resistance from the PWM input to the Common (Between yellow and black wire). They are 1.6 kOhm, 659 Ohm, 593 Ohm and 235 Ohm. The fans with lower resistance cannot be controlled because we are not able to transmit a PWM signal with a high enough voltage amplitude for the fan to recognize. Is this normal to have such a large variation in resistance? Is the fan damaged? If it is damaged, what could cause this? We are operating at 12 VDC.