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Old 09-09-2013, 11:39 AM   #1
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rear AC help please

I have a 1998 safari 40' dp with 50 amp. the coach is new to me and when i turned on the rear air it " hums" when i turn on the fan by its self it does nothing you feel no air movement at all with fan with the air on it only hums
any ideals? im up to try anything to fix it
thanks for all the help
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Old 09-09-2013, 11:44 AM   #2
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the unit is a Dometic DUO THERM
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Old 09-09-2013, 11:48 AM   #3
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get up on the roof, take the plastic cover off the AC unit and see if you can spin the fan. Many times mud daubers build nests on them, especially if the coach has been parked for a while.

If fan spins freely...go and retry the AC...the fan might just be stuck...and all your hearing is the AC compressor coming on
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Old 09-09-2013, 11:53 AM   #4
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thank you sc3283 im heading out there right now " in the rain " i will let you know what i come up with and thank you so much for the fast response.
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Old 09-09-2013, 01:11 PM   #5
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Originally Posted by sc3283 View Post
get up on the roof, take the plastic cover off the AC unit and see if you can spin the fan. Many times mud daubers build nests on them, especially if the coach has been parked for a while.

If fan spins freely...go and retry the AC...the fan might just be stuck...and all your hearing is the AC compressor coming on
ok went and took the cover off and the fan spins freely ( was not stuck at all) i turned the ac back on and just hear the hum again i didn't know if there is a compositor that runs the fan or maybe the motor in the fan is just bad.
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Old 09-09-2013, 01:30 PM   #6
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When that happened to me it was a bad capacitor. I could see it was melted on one side. Damage to a capacitor may or may not be visible.
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Old 09-09-2013, 04:34 PM   #7
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The motor itself may be bad, but it is rare. There will be 2 capacitors and the small PTCR mounted on the side of your Dometic Duo-Therm in their own holder compartment on the roof with a cover over them. The cover has a paper diagram of the cap(s) hookup. The START capacitor is for your compressor. It is the smaller capacitor (probably dark in color). The larger RUN capacitor has 2 sections. One section is from the COMM (Common)to the FAN connector, and you will notice the FAN connection wire goes to your fan motor. The other COMM to HMCR section goes to your Compressor also and is used after it starts to keep it running. I had the RUN Cap. HMCR section go out, and replaced the RUN cap in both roof units. the symtoms were: Fan=OK, compressor starting, but not running long enough to satisfy thermostat.
Sounds like your FAN section may be bad. The RUN capacitor is available thru Dometic parts. They will need your model number available by taking off your inside filter cover and looking up at the paper on the control box. It should have Model and serial numbers on it. Their web site has their phone number. I believe they were around $40 ea. When you go to "COOL" mode, the fan should come on first, then the compressor later. The compressor will get hot quick without the fan's 2nd squirrel cage to help cool it, so I wouldn't run it too long this way. By the way, if you take the nut off the rear of the compressor, you will see the compressor's over-temp sensor. It shuts down the compressor if it gets too hot, and then will enable it again after it cools down. Here is a hint--swap the good A/C's RUN cap or both of them into the Bad A/C. That will see if it is the caps---they are easy to remove--just label or make a diagram. There is a paper diagram of the hookup also on the Cap. compartment lid. Hope this helps some
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Old 09-12-2013, 12:44 PM   #8
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Ok I changed the compositors out with the front and still no fan so I took the compositors all of them from the back ac and moved them to the front ac and it works as it should so no bad compositors so I guess the fan motor is no good what do you fine ppl think fan motor junk or is there anything else I can try first
Thanks
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Old 09-13-2013, 02:31 PM   #9
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do you have a selector switch on ac for fan and cooling if so you could have bad selegood luckctor switch swap that out with other working ac.
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Old 09-13-2013, 03:21 PM   #10
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do you have a selector switch on ac for fan and cooling if so you could have bad selegood luckctor switch swap that out with other working ac.
yes there is a button that has fan on or auto and when i hit that to on i here a click at the ac so i think its working ( as far as sending a signal to the ac fan ) i do know the blower motor is like why rusty it spins freely but tons of rust on the motor it self. and you are talking about changing the thermostat control unit with the working one is that right??????
thanks for your help
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Old 09-13-2013, 04:59 PM   #11
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Is your compressor coming on? And just not the fan.

If you can start fan off spinning by hand it is a weak motor and needs to be replaced.

If compressor is not coming on check t-stat.

Check start capacitor for compressor .

Is contactor coil calling for cool and make sure it is energized .

Make sure both legs are hot to contactor .

Hope this helps,

James
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Old 09-13-2013, 06:01 PM   #12
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Originally Posted by Jimooo View Post
Is your compressor coming on? And just not the fan.

If you can start fan off spinning by hand it is a weak motor and needs to be replaced.

If compressor is not coming on check t-stat.

Check start capacitor for compressor .

Is contactor coil calling for cool and make sure it is energized .

Make sure both legs are hot to contactor .

Hope this helps,

James
i put it to cool the compressor comes on and i move the fan with my hand to kinda help it start and still no fan. all the capacitors are good ( took all from the back and moved to front and front to back ) the front ac worked and when i changed the capacitors out the front still worked with the back capacitors. so idk what to do from here to try to trouble shoot it.
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Old 09-13-2013, 07:13 PM   #13
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Sounds like your fan motor is shorted to ground, but it would have to be ohm out to be sure.

If you have a volt meter to make sure voltage is to the fan motor.

Good luck,

James
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Old 09-13-2013, 07:20 PM   #14
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Sounds like your fan motor is shorted to ground, but it would have to be ohm out to be sure.

If you have a volt meter to make sure voltage is to the fan motor.

Good luck,

James
i will check and see if i have voltage tomorrow morning and if i do then i guess the motor is shot. thanks for your help and will let you know what i come up with
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