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Old 07-16-2019, 09:46 PM   #1
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RV A/C

my 1990 class a has 2 roof A/C units. One is completely dead, it has 120 volts to it but only hums.
The other one works for about 5 minutes then quits. This is during a warm day (70 degrees). Puts out good cold air when it works.


Thank for any ideas



Bob C.
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Old 07-17-2019, 08:38 AM   #2
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Go up on the roof, remove the covers, take pictures, then get the values of the motor capacitors. Those will be round or oval shaped and will be marked with things like this: 20 MFD, 270 VAC.

Note that you can get the model number off the units and look up a service guide online...that might tell you the capacitors valves and show where they are too.

Replace all of those, and that will help the units run better...might fix your 'buzzing' unit. While in there, clean out the debris, vacuum the coils. Also note that the motors have tiny holes over the bushings on either end of the motor cases. Most stores will carry 3-in-1 oil with a pull out snout. Squirt that oil in your motors bushings that those holes lead to to make them run quieter and smoother, it also helps make starting easier.

Those units are so old though, you might consider new.
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Old 07-18-2019, 05:45 PM   #3
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A/C problem

Thanks Jim.


i thought that the front ones capacitor may be bad. Do you think a bad cap would cause it to run then shut down?


Any thoughts on soft start capacitors.


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Old 07-18-2019, 05:51 PM   #4
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Not Jim, but most units I have worked with have a run and a start capacitor, so yes.

Does your coach have a 30 amp service? If so you might have a load controller, or just a switch to run the units.
If you can't read the numbers for the a/c unit take the cover off inside the coach and look up in the hole and see if there is a tag in there, or a sticker.
Some older 30 amp coaches could run only one unit at a time.

I would not think soft start yet, If the caps don't fix it and you have good voltage on a 30 amp service I would replace the unit, or units, with Coleman Mach POWER SAVER units. These draw way less power than a standard unit and is what my 30 amp coach came with from the factory.
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Old 07-18-2019, 06:00 PM   #5
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Yes, a bad start capacitor on the compressor can cause a shutdown after running for a while. It'll also have hard starting, and stuttering while starting.

A soft start circuit for a motor reduces strain so it's a good thing. Not sure it's all that necessary in most cases.
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Old 07-20-2019, 01:26 AM   #6
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Thanks for the info guys.
I'll check the capacitors and clean the unit coils also check the motors.
I only have 30 amp coach plug.
I'll check lock-rotor and run amps.
Hopfully I can run both at once.


I also have a 1977 class A that I can run both A/C at same time...run amp are about 24.
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Old 07-20-2019, 08:47 AM   #7
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What I did with my 30 amp coach was to open the inside cover of the bedroom A/C and attached a heavy duty cord as the supply. Ran that out the window and plugged it into an unused 20 amp outlet on the shore power post...or a neighboring post. I'd often park in the back were it wasn't crowded. Over here in the west, RV parks start to empty as it gets hot so there are often many empty spaces.

BTW, when you're working on the A/Cs, if you see some little yellow plastic caps on the motors, remove those and squirt some 3-in-1 oil in there to lube the bushings. Really helps operation and helps quiet them.
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