Okay, I have addressed the problem. The fans are driving by 24V PWM signals named FAN PWM, in which is similar to LED PWM. The PWM signal works fine. However, one fan is dead, since two fans are connected in series, therefore both fans are not spin.
I'll replace the bad fan with a new one. These fans are rated at 12Vdc, 30mm x 30mm x 10mm.
Thanks.
Best Regards,
Kevin LO