Roof Top AC Noise

[2 Sinks]

My main Dometic roof top ac unit is emitting a high pitch noise /squeal.The noise does not appear to be a bearing noise,but an electronic capacitor charging type noise instead.Could the capacitor be failing?

Has anyone else experienced this type noise and what did you do to repair it.The ac cools and cycles on and off with no problems.

Is OZ still on the forum?

Thanks
2 sinks

 

[Ozz]

Not the capacitor, could just be the refrigerant going through the expansion device. I don't really know. Is it real loud?

 

[1retired06]

I would not rule out bearing noise.

 

[2 Sinks]

Thanks for the replies,Ozz it is pretty loud...we could hear it aprox 200 feet from the camper the other day and don't want it to disturb other campers.I was running the bedroom ac during the day and the main ac at night when everyone was in for the evening.It sounds like a electronic load tester I used years ago to load test large industrial size dc diodes.The high pitch sound is quite annoying.

I'll pull the cover off tomorrow and troubleshoot a little more.I'm thinking of swapping parts from the bedroom ac to the main ac to try and isolate the problem.

Thanks,
2 Sinks
2007 Mountaineer 336
2006 Dodge 3500 DRW 4x4 QC
2000 Lance 1130

 

[DQDick]

You might also check to see if the fan blades are clear. If a mud dauber or something else attached something to a fan blade it could be out of balance enough to be affecting the fans bearing.

 

[Ozz]

Mike probably hit the nail on the head.

 

[2 Sinks]

Pulled the cover,fan assy is free of any wasp/mud dobber nest.No debris of any kind,been through the dobber nest problem before,the fan shook like an out of balance tire when a dobber nest was on the blade assy.Pulled the fan/motor as far forward as possible,the shaft play is normal...no rough feel when rotating by hand.Put some silicon lube on the shaft in the front and rear motor bearing area and no change,the shaft will pull the lube in and quiet even a sealed bearing temporairly,done this many times on dc/ac motors for noise troubleshooting.I've taken apart sealed bearings before and repacked to end motor noise in industrial applications,this appears to be something different.

Listened with a hose all around the units pressure lines/low/high and no determination of the noise.Put the hose on the rotating motor shaft and motor housing and nothing.The noise sounds identical to microphone feedback just not as loud and can be heard easily from the ground.The fan speed hi/low setting doesn't change the noise.

The hot Florida sun is hitting the roof now so I'm done till early evening.Open to any and all suggestions.The ac cools down quickly and cycles on/off as it should.

 

[Ozz]

Very good troubleshooting!
Turn the A/C off, (Compressor) run just the fan. If the noise is still there, you know it is the fan.

 

[Irlpguy]

It sounds like you have done a thorough job of troubleshooting and as Ozz suggests if you run the fan only it will eliminate the AC part, but not necessarily point to the fan. When I bought my new truck and installed a ham radio antenna and mount on it that had been on my old truck for years, I developed what I thought was a squeal coming from my dash, thought it might be the speedo or something else that was turning at high speed. It turns out the antenna and mount were creating a harmonic vibration at a frequency that sounded like a squeal. This was very bad as my dog would not get in the truck after having ridden in it once.
Changed the antenna to a different one and the noise was gone.

If you are convinced it is not a bearing, and you have checked both fans, then possibly a harmonic may be the cause, how or where I have no idea, I am just giving you something more to consider. Good luck on finding the problem

 

[2 Sinks]

Thanks for the replies/suggestions,ran the fan only for over an hour switching between lo/hi speed and no noise also less/almost no vibration than when the compressor is running.

Something else came to mind...this is the original slide type/non digital thermostat in the 5ver.The noise occurs immidiately when the ac is switched on and the compressor comes on at that point,no time lapse at all.When the temp cools down inside I think the compressor used to cycle off and the fan would run a few minutes and then the system would turn off and not come on again until the temp went up.Could there be a sequence problem created by a failing thermostat?

Earlier when I said the unit cycled on/off I meant it came on/compressor included at the same time and all shut off together,no compressor cycle off,it just all cuts off at once.

This is the identical thermostat we had in our 1998 Coachmem TT and it failed after about seven years but had different symptoms,it came on/off at will.I still have that old t-stat and might put it in to see if the noise goes away,I remember reading somewhwere that the compressor pressure needs to drop before coming back on ?

If that is correct then this part of the sequence could be gone and maybe the high pressure is creating the noise?

Harmonic noise is a possibility... I have heard that type of noise from industrail refrigeration applications.I'm going back in the 5ver and look inside the ceiling part of the unit and t/s further.The noise cannot be heard inside the 5ver when the ac is on,only outside and there is no wave to it.It sounds like constant microphone feedback only at a lower level and is very annoying.

I post back later...again thanks to everyone for the help.

2007 Mountaineer 336 5ver
2006 Dodge 3500 DRW 4x4 QC LWB
2000 Lance 1130
2 Camping Collies 8 & 12 Yrs Old

 

[2 Sinks]

Found the noise Now to silence it.The noise is coming off the dryer which mounts to the compressor in an area 1/2 inch wide x 6.5 inch long.

The noise is right in the center of the dryer which is clamped to the compressor with a stainless steel clamp.The compressor housing may be enhancing it.I isolated the area with cardboard and used Goodyear ac high pressure hose to listen to it.

The squeal is continuous,I'll post back on the fix

 

[2 Sinks]

The noise continues,swapped out thermostats with a spare I had,tried insulating the dryer and nothing.The noise is definitely coming from the compressor.When the fan speed is switched to high the noise is somewhat masked by the increased fan rpm noise.I'm going to try and insulate the underside of the ac cowling to see if it will absorb the noise,but not to where it will restrict the air flow paths.

Looking into some sound barrier material now.

 

[Ozz]

Compressors are electric motors with a...compressor hooked up to it, valves can rattle, crankshafts can be troublesome when loose to the connecting rods, they are also sprung on springs, they can be broke making noise but it could be a Scroll compressor, a whole different bag of tricks. Long story short, if it is internal, you can't do anything about it.
Might be time for another unit.

 

[2 Sinks]

Ozz- Thanks for the reply.I researched scroll type compressors and thats the type compressor I have.It seemed to be a little quieter yesterday during the intense heat part of the afternoon but started the noise/squeal again this morning.I worked on it awhile today and nothing.

As you said a replacement may be the only fix.

Thanks to Ozz and everyone for the help...If I find a way to silence it I'll post the results.