Humming AC

[barbil]

Front AC is rather loudly humming even though the thermostat is off. Am connected to shore line. Once I switched Cool on and off and it stopped. Later tried it again, no change. Opened the filter cover and something caused it to shut off. Vibration? Thought it might be because of cold weather (Texas-- 40 and going down) but---? Bill

[TXiceman]

Do you have a heat pump by chance?

Ken

[barbil]

TXiceman, Thanks for your reply. It is the front unit. Apparently there is no heat pump, never was aware of one at least, no setting on the t'stat for one, just heat, off and cool. Was checking the rig because it is supposed to be 21 tonight. Have the furnace on of course. The hum appears to be cycling, as twice I have heard the hum get louder just before it abruptly stops. Am puzzled.

[TXiceman]

Have you tried to trip the breaker to the A/C unit?

[Rex]

WillyD, it does sound like you may have a heat pump section on the front air unit. The hum could be a bad air conditioner compressor, frozen blower motor, or a motor trying to start but not receiving enough power. When it is warmer, go on the roof and remove the AC cover. Inside you should find a metal box with removable cover. It should be on one of the sides. Inside you will see some metal tubes which are capacitors. Look closely at those WITHOUT TOUCHING THE WIRING on top of these. If you see black splatter inside the box or tar like substance running out of one of these capacitors, it is bad. Use a screwdriver and short the wires on each capacitor together, and then do the same to each wire to the box case to discharge them. You may hear a pop or see a spark and that is OK. Make sure you are disconnected from shore power before you do this. You may be able to find the capacitors at a local air condition company or supply house or an RV Dealer.

While you have the big AC cover off, see if you can turn the motor by hand. There may or may not be a point to lube the motor on one or both ends just above the shaft. Only will need a few drops of 3 and 1 oil available at any hardware store.

If the capacitors are OK and the motor shaft moves OK, then you may need to replace the entire unit. My coach has two roof units and one furnace. I have one 5-button thermostat. My selections are Fan Only, Heat Pump, Cool, Furnace and at the bottom Zone 1 or 2. I set the temp I want to maintain and if the Heat Pump is selected and it quits providing heat (around 40 degrees) as the outside temp drops, the thermostat will start the Furnace. Yours may work this way also even if you only have the 4-button thermostat.

So what coach do you have? Is the thermostat a Duo-Therm 4 or 5 button unit?

[itdave]

I sometimes get a hum when the A/C has frozen up. In either Heat Pump or AC mode. Once the condenser unfreezes the hum goes away.

[barbil]

Hi Rex, Many thanks for your detailed instructions. I did get up on top of the rig today (At 89 I am learning that I will not be doing that much more!). Shore line unplugged, took the top off. The capacitors had no charge at all, fan motor turns OK. Down again, plugged shore line in. Vibration again, and drawing about 12 amps. Vibration is from the cylinder on the left, --- compressor? Thermostat has 3 switches, Heat, off, cool; and Fan: on,auto and hi, lo. What puzzles me is why the hum when all switches on the t'stat are off. Might that be the clue we're looking for? New unit??? (Just got 6 tires today!) Oh well. Bill

[Rex]

WyllyD you encourage me going on your roof at 89. Gives me hope that I will continue being able to do that for a while now.

If you want to go up there one more time, try tapping hard on the reversing valve while the power is on and the unit humming. It is the brass colored unit with all the copper lines going to it in the center of the compressor. Or at least it is close to it. Here are a couple of pictures that may help. Sometimes the reversing valve will stick and once you jar it loose it will work fine. If that does not do the trick, well a new AC is in your future.

[TXiceman]

The brass item in the center of the photo is the 4-way switching valve. You might try tapping it with a wood block to see if the problem clears up. The valves will sometimes hang up.

The tapping is called percussive maintenance.

ken

[pipedream2]

Willy
I tend to try the simple first....it may be that the 4 long bolts have come loose that hold down the AC against the sponge gasket...they do that from time to time and need to be re-tightened .... the vibration sound may go away.The temperatures you say you are having is nearing if not at the shutdown temps of the unit anyways.....your shut off may well be a coincident of this temperature

[Rex]

Willy if you tighten the 4 hold down bolts from inside the coach, they only should be tightened to 30-40 INCH pounds. If you do not have an Inch Pound torque bar, then tighten them hand tight and then one more turn.

First thing to do before tightening them is to loosen them all enough to let the gasket expand for about an hour, then tighten.