Dometic A/C not working

[Teoma]

Hi I have a Duo-Therm Penquin Roof Top A/C unit made by Dometic on my travel trailer. The on/off switches and fan speed control are located on the unit's inside roof panel not on a wall type thermostat. When I turn the unit on there is no air cooling or even any air flow. If I just switch the fan only on (no a/c) there is no air flow either. All I am getting when I turn the a/c or just the fan is some humming noise but no air flow. Before I drag the trailer to a repair place and have them invariably tell me I need a new unit does anyone have any ideas for some preliminary troubleshooting. My manual is not very helpful. The unit came new on my 2010 trailer.
Thanks
Ambrose

[j-d]

The diagram on Page 19 of this manual indicates Dometic used a two-way capacitor. One side for Compressor (may be marked "HERM" for Hermetically Sealed part of the A/C, compressor, coils, etc.) and the other side for the Fan. Coleman tends to use use two separate capacitors with their common terminals wired together. Same thing. If you find one side of a two-way capacitor is bad, you can get a single of the correct value and wire the Fan or Compressor to that side alone. From the diagram, it seems the Brown and White wires from the Fan Motor lead to the Fan Run Capacitor or part of the two-way capacitor.

Please understand, it's not the Capacitor(s) every time one of these things won't run. But it Often is. If an RV A/C has a leak, a damaged coil (condenser or evaporator), a clogged expansion device, or a bad compressor, it's junk. What's left, where repair could be economical, is Fan Motor, Fan Blade/Blower Wheel, Capacitors, Switches/Thermostats etc. Anything that's not HERM.

[_40Fan]

Have you checked the breakers?

[enblethen]

There is two capacitors, one for the fan and one for the compressor. trace out the wires is the easiest. There should be a drawing inside the housing to show them and what their rating is.

[Teoma]

Thanks for everyone's ideas- hopefully it is something related to the fan. How do I identify the start capacitor? Is it tied into the fan or the compressor?

[j-d]

If the fan stops running (bearings, windings, capacitor, switch) the compressor will soon overheat. A built-in safety will cut the compressor out to protect it till it cools. If fan is now working, all's OK. But if it's still not running then compressor will again run till it overheats.
Do you have adequate voltage (110 or more is best, but it should run down around 105)? And adequate amperage (not using a small or excessively long power cord)?
A 5-year-old A/C should be good for many more years.

[bdpreece]

While you have the shroud off look at the top of the start capacitors. If the top is bulging the start cap is bad. A very common problem but fairly cheap to fix.

[Chris_Bryant]

Go up on the roof, remove the shroud and try to spin the blower wheel. It could be (from easiest/cheapest up):
A bug nest blocking the blower.
A bad fan capacitor.
A bad fan motor.

[enblethen]

Go onto the roof, remove the AC shroud. Try spinning the fan by hand. Power to AC needs to be off. This is to check for a seized motor.