+28 VDC on J1-A turns the blower on at 1500±100 RPM. When the air temperature sensor reaches 76.7±3° C (170±5° F), the speed switches to 3000±200 RPM. The blower switches back to 1500±100 RPM when the temperature drops to 68.3±3° C (155±5° F). If the air temperature sensor fails, the blower defaults to the 3000 RPM speed.
If J1-A optocoupler is off, the blower speed is controlled by PWM (Pulse Width Modulation) signal on J2-A as specified in PWM IN.