Thursday, December 8, 2022


Think you need to replace your AC fan motor? Maybe not


By Steve Savage, Mobility RV Service

It’s warm out and air conditioners are humming — or should be. Then comes the request for service. A common one concerns air delivery, or more specifically, “My air conditioner is not blowing as hard as it should,” or “My air conditioner fan will not start.” Before getting in line for service, let’s talk about possible problems.

Let’s take the “fan motor won’t start at all” first.Air conditioners sit for months without running and one of the things that often happens during that time is the motor shaft rusts and sticks in the bushing. Before ordering a new motor, try freeing the motor shaft by turning it by hand. In severe cases you may need pliers. Do this while the unit is off (OK, sometimes I do it with the fan switched on, but that is me). What you will often discover is once the shaft is freed up and the motor starts to run, the problem goes away.

I do this dozens of times each year on dealer lots. If you call me on the phone (customers only, not Internet readers), I will discuss this possibility with you before I fire up the truck and charge a bushel of money for a very simple and brief fix. I have run into this so often, I have made a bent hook out of a piece of aluminum trim and don’t even have to unscrew the cover in order to get it spinning. In my experience, the problem is more common with Dometic Duo Therms rather than Colemans, but they all do it some.

Next up is “air delivery at the vents is too weak.” Again, this is not a motor replacement issue, at least in my experience. More likely than not, your coil is “iced.” When humidity is high, running the fan on high is often the cure to put more heat across the evaporator coil. That’s the coil you can see from inside the camper, and if you drop the filter cover and look up, you often can see the ice. If running the fan on high does not cure the problem, your freeze control that is sticking in the coil and is designed to shut off the compressor when the coil starts to freeze may have been left out or has fallen out of the coil.

Sometimes the freeze control does fail. The good news is, it is not an expensive part and takes only a few minutes to take out the old one and plug in the new one. Duo Therms are a normally open circuit and Coleman’s are normally closed, so they are not interchangeable and can be tested for continuity to ensure one is not replaced unnecessarily. I replace less than one a year, so failure is uncommon.

Do fan motors fail? Sure, but not often. If you know how to use a multimeter to test an electric motor, you already know how to check them. In my opinion, Coleman’s are the easiest to replace, Duo Therms are not bad, and I hate replacing Penguin motors. As always, be sure to diagnose before ordering a motor!

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Brian Anderson
1 year ago

I just started using the AC in my travel trailer. When I turned on the thermostat, the condenser motor apparently came on, but with no blower fan. I found that if I spun the fan by hand, it worked fine with plenty of cold air blowing, but usually the fan would stop after about 15 mins. I bought a fan capacitor & switched it out and now the fan kicks right on with the thermostat, but I’m not felling any cold air now. I don’t think the condenser is working now, but I fail to see why that would happen after fixing the fan issue with a new capacitor, , because it worked nice when I had to “jump start” the fan by hand. Also, there are no markings on either of the fan motor capacitor’s 2 terminals or any color codes on the 2 wires connected to it, so its possible the wires got reversed, but could that affect the condenser operation? I was going to try reversing the 2 wires, but I don’t want to create another problem. Brian

Last edited 1 year ago by Brian Anderson
Tamara M. Geiser
1 year ago
Reply to  Brian Anderson

Did you ever resolve the problem? Was it a run capacitor problem?

2 years ago

My rooftop mounted AC unit will not work. The breaker kept tripping every once in a while on the inverter. Then it would trip immediately when the fan was turn on. Then the breaker stopped tripping. When I would turn on ONLY fan there was no airflow. Then if if turned the switch to AC the compressor would start, but the fan would not turn. I replaced the fan with the same issues. I did not have a chance to check fuses yet because I had to leave for back for home. I noticed some forms talked about the fan capacitor would be suspect. Any input would be greatly appreciated. Also should I replace all capacitors or just the fan capacitor?

2 years ago

Fan having a hard time starting then just stopped. I got it turned on but can’t set the air to Auto. Compressor is working and kicking on and off. As long as I don’t turn the fan off it all works. Do I need to replace or repair the fan? Will it hurt to have it running full time?

7 months ago
Reply to  Samantha

I have this problem now. What was the issue please.

2 years ago

When our air kicks on the fan sounds awful, and took a min for the cold air to come through, like it kicked on but then it took a min for the fan to start going then the colder air came through jist trying to figure out if i need to get a fan before it stops or if its something else.. Thanks

2 years ago
Reply to  Ashley

Its a 2006 camper I dont know the name just got it a few months ago…. Sorry

Chris Brown
2 years ago

I have a duotherm on my v-cross. I thought a wire had come loose from condenser but my fan was stuck. It came right on. However now its vibrating like its out of balance. Please help?

2 years ago

Thanks for the knowledge!
Fan wouldn’t spin and your advice worked perfect.

Nick Rima
2 years ago

Thank you for the honest information. I have not looked at it yet but fan just hums, never spins. Compressor does kick on. Hope the breaking lose fixes it. Any recommendations on spray line like Liquid Wrench that does not cause shorts like WD-40 can?

3 years ago

My dometic ac motor only starts on the high setting, check the switch is good. Amps out good once it’s running. If I try starting it in low it hums and amps out high

John Houston
2 years ago
Reply to  Aaron

My ac fan wants to run on high..if I start on auto It runs awhile then slowly slows to a stop then it just hums..what’s the problem?