Coleman roof AC, low fan not working

[Chae]

Hi guys, I have a Coleman AC unit and it's been working really well. I don't know if it's the original unit, but it's blowing nice and cold while I'm rehabbing the interior.

All of sudden, the low fan stopped working. The compressor is working. High fan is working, but whether as Low Cool or just Low Fan only, it's not spinning. I know the compressor is working on Low Cool because I can hear the hum, and when I switch to High Cool, the initial blast of air is super cold.

I don't know if it's related or not, but the AC unit was making a rattling sound. What I figured out is that while the completely gutted AS was being transported back and forth from storage to my home, the AC was bouncing up and down (yup, axles are toast) and tore a 4" rip on the exterior skin, causing the AC to tilt and sag. I braced that and the AC is upright and no more rattling noise.

It's a connection problem, I think. The compressor and the fan start up no problem (fan only on high, of course) so it's not a capacitor issue or a bearing issue. Air gets cold so no issues with refrigerant, either. I rechecked the connections on the lower unit and everything is connected to where they're supposed to be. So my guess is the connections to the upper unit.

Before I jump in there, is there anything I should know about how the low fan is wired or is there any additional electronics that govern the low fan speed? As I said, both low and high were working before, and the low fan issue's occurrence coincided with the rattling issue. The only other I can think of is thermostat issue but inside is usually super hot and the compressor shouldn't be running even then. Much thanks.

[HowieE]

There are 2 ways to control motor speed. A resistor to reduce voltage and a second winding in the motor. On smaller motors the first is the common approach.

The other possibility is a bad switch contact on the low fan speed. If you open the unit disconnect the compressor power, you don't want the compressor running without cooling over the condenser coils for a long time, and while the low fan is turned on test for voltage along the connections from the switch and up to the fan motor