19 replies to this topic

[LarryH50]

It was a warm and sunny afternoon. The Dometic 15,000BTU AC started and in moments was blowing cold . . . after 30 minutes, the fan kicked off, compressor continued to run, and there was a buzzing sound.

On the serious side, I had an RV tech look at this a week ago. He replaced the control board. Before he had his paperwork written up, the fan kicked off after 30 minutes.

He came back today to replace the capacitor (not the one for the compressor); ran 30 minutes, fan kicked off.

Of course, he hasn't been paid, and he said he can't charge for parts and labor on the earlier work.

I spoke with someone today that mentioned there is 12 volt power going to the controls, and sometimes a bad battery or converter can cause problems.

Quite frankly, I don't know, and it appears the RV tech doesn't either. I have very limited choices of available techs.

We're in Texas, and it's starting to get HOT!

HELP!

Thanks in advance to any and all who can help.

[Kirk]

If you have one thermostat that controls both the furnace and the air conditioner, then it will have a 12V supply that is used for that. If I understand the problem, the blower has stopped turning but the compressor did not stop? If this is the case, I see little reason to suspect a lack of 12V. It would be very easy to check that with a meter if you have one, but if not, first try your lights as they use 12V-dc power, as does your water heater, furnace, refrigerator(RV type), and water pump. If all of those are working, then it is highly unlikely that the problem is a lack of 12V.

One thing that you may try is to use the thermostat to turn off the a/c if you do have one that controls other things as well as it should have a switch on it to turn the a/c off. It it does turn off, then the 12V control circuit is clearly working. If you can turn on the furnace, even without the a/c having stopped, that also tells us that the 12V control system is working but some relay has stuck closed and kept the compressor running. It is quite possible that the control circuit board has failed again That would be very possible if something else caused that to happen and the board was replaced without finding what caused it to happen.

I suggest that if you can't stop the compressor in any other way, go to your 120V power panel and turn off the circuit breaker to the a/c as it will do more damage if you allow it to sit with the compressor running and no blower, and probably whatever is buzzing will also burn up. Do not just let it keep on doing those things!

[Ozz]

It is either a bad fan capacitor...I know he changed it... or a bad fan motor, about a 50% chance of either one.
The buzzing is just..probably.. the contactor, or relay to the motor. Could also be a wiring issue, bad wire on the 120 volt circuit to the fan, anywhere along the circuit to the motor. Less of a chance since it runs.
If you can feel the fan motor after it quits, it will probably be 'burn your hand hot'. That motor/fan is a condenser fan and evap fan, blows air through the coil on roof and downstairs through the cooling coil.
He will eventually probably change the fan motor out.
good luck!
Ozz

We will be heading for home base the 11th, K.C. Mo. and I will be woring on A/C's for the rest of the summer. Now we are in beautiful Tonopah Az. West of Phoenix...no humidity

[LarryH50]

Thanks Kirk and Ozz for your replies.

The thermostat is the 5 button Duo Therm comfort control. When the fan stops running and the buzzing starts, I immediately turn the AC off with the thermostat, and the AC stops.

As for the fan motor, the tech has taken readings, and they are well within normal, and according to him the motor is not hot, and turns freely and sounds normal.

I should have thought about the other 12 volt things; they all work normal.

The mystery continues.

[Ozz]

I stand by my post, hope he gets it fixed for you.
Without being there, it is impossible to be 100 correct on my diagnosis, but I have been doing this HVAC work all my life, and that is my opinion on it.
I have been wrong a time or two...
Good luck, I know it is frustrating..and hot!
Ozz

[LarryH50]

I wonder if I'm on to something? I read the Dometic AC Bulletin A29/2A, January 2002 - Troubleshooting for the 5 Button Comfort Control Center System. on page 4, I read point 8.a Roof Top Air Conditioners, the following comments:

The operation of the AC control box can be checked at the 6-pin plug connection. Disconnect the unit and use a 115 volt light bulb to check from terminal 5 (the white or common wire) to:

Terminal 1 (blue) is the compressor
Terminal 2 (black) is high speed fan
Terminal 3 (yellow) is medium speed fan
Terminal 4 (red) is low speed fan
Terminal 6 (green/yellow) chassis ground

This got me to thinking. I didn't know about the different terminals for different fan speeds. We always run our fan on high. So, 1 hour and 5 minutes ago I started the AC with the fan on medium. It has run without fan shutting down, and the temperature has dropped.

I'll continue to monitor the AC for fan shutdown. If it does, I'll shut the unit off. If it runs for the next 4-5 hours, I guess i can tell the tech to look at terminal 1 black wire and connections?

If it runs for the next 4-5 hours on medium fan speed, I wonder if it will be safe to let it run when we're out tomorrow. Our cats would appreciate it, but I don't want to burn the compressor up if the fan were to shut down while we aren't here. I haven't trained the cats how to shut the AC off, yet.

Thanks again Kirk and Ozz, and if you have any comments about it running on medium vs. high, please send them along. And again, thank you for taking the time to respond on Easter weekend.

[LarryH50]

Hey OZZ,

On another subject, I happened to notice in your signature about repairing nuke boats; I was a navy nuke. Class 64-2 Gold crew at Bainbridge, MD for training. Was sent to USS Truxtun DLG(N)35 at the New York Shipbuilding Co., Camden, NJ. I'm a "plank owner."

[Ozz]

It's tempting to run it on medium, but there is a chance the coil will freeze without full airflow, these ducts are minimal anyway. You can try it, but keep an eye on it. Most systems have a freeze-stat, with a bulb, or cap. tube inserted in the coil, it will cycle off the compressor when it detects a frozen evap.
You probably have a bad 'tap' in the high speed section of the fan motor. (That portion of the winding could be bad, or the connection inside the motor.

[Ozz]

Hey OZZ,

On another subject, I happened to notice in your signature about repairing nuke boats; I was a navy nuke. Class 64-2 Gold crew at Bainbridge, MD for training. Was sent to USS Truxtun DLG(N)35 at the New York Shipbuilding Co., Camden, NJ. I'm a "plank owner."

I was up in New London, lived off base in Mystic, what a neat little town. I was a Machine Repairman at the machine shop there on the base. After that, off to 'Nam I went, U.S.S. Colleton converted flat-bottom L.S.T. I fixed all the stuff they broke on the ship. Brown water Navy.
The cooks on the subs treated me real well, hams, all kinds of foods, I would sharpen their cutlery for them.
Always envied you guys...except that claustrophobia deal I have going for me...

[LarryH50]

If it is the "bad tap" or high speed section of motor windings, does that mean a new fan motor is needed?

[Ozz]

If it is the "bad tap" or high speed section of motor windings, does that mean a new fan motor is needed?

Yes, I am afraid so. That may not be it, yours is not an easy one to diagnose, could be many things, but my money is on a bad motor, or bad cap. Usually, the capacitors are a dual, or split capacitors, one capacitor for both the compressor and the fan motor, he could install a slightly larger cap on the motor side, but IF he put in the correct one, and IF he put in a NEW one, it is probably the motor. Many times a tech puts in a used cap, because he doesn't have the proper size in a new one.
I carry around 30 different capacitors when I run service.
I do commercial and some residential, HVAC electrical and refrigeration work. Not looking foreword to going back to the heat and high humidity of Missouri.
I hope you get your unit fixed, if he installs a new motor and that doesn't fix it, you would have needed one in the future anyway if your system has a few years on it, and that will be one thing less it could be.

[LarryH50]

Thanks OZZ. I hope to know something by Monday. I did run the unit on Medium fan for 2-3 hours; cooled good with no coil freeze.

I'll post the solution when it's fixed. Sounds like it will be the fan motor.

[DaveM]

It is either a bad fan capacitor...I know he changed it... or a bad fan motor, about a 50% chance of either one.
The buzzing is just..probably.. the contactor, or relay to the motor. Could also be a wiring issue, bad wire on the 120 volt circuit to the fan, anywhere along the circuit to the motor. Less of a chance since it runs.
If you can feel the fan motor after it quits, it will probably be 'burn your hand hot'. That motor/fan is a condenser fan and evap fan, blows air through the coil on roof and downstairs through the cooling coil.
He will eventually probably change the fan motor out.
good luck!
Ozz

We will be heading for home base the 11th, K.C. Mo. and I will be woring on A/C's for the rest of the summer. Now we are in beautiful Tonopah Az. West of Phoenix...no humidity

Oh oh, back to work buddy? Must be the guy that took over needs some help huh? Anyway, keep in touch ok!

Dave

[LarryH50]

The saga continues: Today, we ran the AC on high fan speed. After 30 minutes, the unit shut down, including the compressor, with no buzzing sound. The shut down sounded like a normal shut down when you turn the unit off. This was after a reset was done on the 5 button Duo Therm Comfort Control: turn switch off, hold zone and mode buttons down, release them when FF shows on screen.

So, could the thermostat be the problem?

[Ozz]

It could be. You are like me, I always wonder 'Why me?' It couldn't be a simple problem...
Are you saying this happened after a reset?

The thermostat couldn't have been satisfied and shut off normally?

[LarryH50]

Yes, it happened after the reset. I don't think it was normal according to the temperature being satisfied, because the fan and compressor shut off. I think with the fan set on high, the fan would have stayed on when the compressor shut down. Also, it happened at the 30 minute mark again.

[Ozz]

Unless you set to fan 'on', only, both the compressor and fan shut off when the temperature is reached. Even when the fan is on 'high' setting.
On mine and any I have seen, yours shouldn't be any different.

[Kirk]

Yes, it happened after the reset. I don't think it was normal according to the temperature being satisfied, because the fan and compressor shut off. I think with the fan set on high, the fan would have stayed on when the compressor shut down. Also, it happened at the 30 minute mark again.

I have been "out of pocket" for a couple of days, but thought that I'd jump back in on this one. I have experienced failure of the Dometic thermostats before. I have twice replaced one of ours in our coach that we had for 14 years, both times the front one as the bedroom thermostat was original when we sold it. When mine failed, I don't remember exactly what the symptom was other than that there was some issue of coordination between the blower and the compressor.

I got two from Northwest Supply because they were pretty cheap parts and eventually used both of them.

On another subject, I happened to notice in your signature about repairing nuke boats; I was a navy nuke. Class 64-2 Gold crew at Bainbridge, MD for training. Was sent to USS Truxtun DLG(N)35 at the New York Shipbuilding Co., Camden, NJ. I'm a "plank owner."

I too am a former "Navy Nuke" but a little before your time, I suspect. I went to basic Nuke school at Mare Island, CA from June through Dec. of 62 and then prototype training on the old A1W plant up in Idaho, Jan./July of 63. That was after qualifying submarines on the USS Seadargon out of Pearl Harbor in 61/62. After Nuke school I went through "emergency nuke welding school" in New London for six months and then to the USS Theodore Roosevelt(SSBN 600) for three patrols and then off to the USS Woodrow Wilson (SSBN 624) for six more patrols, leaving the Navy in 1968,

[LarryH50]

I didn't think about the fan and compressor shutting off together. The AC seems to shut down, when on medium fan, before the inside temperature is reached. Since doing the T-stat reset, the unit seems to cycle the compressor off and on more often. It hasn't been real hot out the last few days.

[LarryH50]

Kirk,

Thanks for your reply. I'm not very happy with the RV tech working on this; seems to be troubleshooting by replacing parts. His standard answer to everything is, "I'll call Dometic." He's already said he won't charge for his misdiagnosis of good parts. At this point, I'm desperate enough to suggest replacing the T-stat.

I went to Nuke school in 1964 (Class 64-2, Gold Crew) at Bainbridge, MD. Then to D1G prototype in upstate NY, near Saratoga Springs. Thought originally I'd be going to subs, but got selected for nucleus crew of USS Truxtun DLG(N)35. Spent 1965-67 in New York Shipbuilding, Camden, NJ. First cruise was around South America and Cape Horn, and on to Long Beach.

Yeah, I know I was "surface scum."

Again, thanks for your thoughts and suggestions.