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Old 08-09-2012, 12:22 AM   #1
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Dometic A/C cools nicely but cycles even with the thermostat turned all the way down

The Dometic A/C in my '92 Class B blows ice cubes just fine but the compressor will cycle off even when the thermostat is turned all the down to its lowest setting. Then after a few minutes it wIll cycle back on. I haven't bothered to really time it but it might be close to 50% cycle time. I know the thermostat works because when the compressor is running I can turn the thermostat up to purposely shut the compressor off and then back down to bring the compressor back on. When i do this I hear the distinctive clicking of the thermostat. I don't hear this noise when the problem occurs. Shouldn't the A/C compressor be able to run indefinitely? I've only had one other RV with air before but it seemed like the compressor would run all day if i wanted it to.

Is there some other thermal switch or protection circuit causing the compressor to turn off?

Thanks,
Steve
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Old 08-09-2012, 03:31 AM   #2
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Steve, more than likely your air conditioner is cycling off due to the freeze sensor located on the evaporator coil. I would suggest you inspect the evaporator coil and clean if necessary. Also make certain that the divider between the discharged air and the return are not leaking by. There is the chance the unit is low on refrigerant but that is the least likely. Please give us more detail as to the type of ceiling assembly you have. I would think it is not ducted. If you can measure the compressor amperage and measure the return and discharge temperatures we can ascertain if the is a low refrigerant issue causing any freeze up. Good luck and keep us posted.
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Old 08-10-2012, 11:46 AM   #3
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This is a Duo-Therm unit, model 55908.209. By ducted I assume you mean ducting throughout the RV. No this A/C unit pulls air directly and exits cool air directly. The cool exit is nicely isolated from intake with aluminum tape sealing all seams. I see no opportunity for mixing air. Unfortunately I don't have a watt meter but I'll borrow one from a friend. And I don't even have a thermometer to check temperatures but I'll get one pronto. When it cycles on it does blow cold though.

I't didn't even occur to me to check for a dirty evaporator coil. So this morning I took the shroud off and inspected it. It's actually fairly clean, maybe a very fine layer of dust. I'll go ahead and clean it. The freeze sensor you commented on travels down to the bottom of the coil on one side. It's not really accessible so I can't see if anything is amiss there.

I live in the NW so it's not used that often. But, I turn it on monthly when I run my generator for half an hour. This is when i noticed the compressor cycles even thought the thermostat doesn't trip. Can freezing occur in such a short amount of time?
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Old 08-10-2012, 02:56 PM   #4
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Ok, I have some more information.

1) I bought foam cleaner for good measure even though coils didn't appear that dirty. Didn't make any difference as far as I can tell.

2) Measured between 1600 watts and 1700 watts with wattmeter while plugged into thin extension cord. My inverter was beeping at me so I knew current was being limited by extension cord and went back to generator operation. BTW this is not a problem when parked at a site and plugged in properly.

3) With a dual thermometer I can now give real temperature readings. With the coach reading 78 with probed tucked into a/c intaked I turned on the a/c and saw output go down to 61 deg. Not quite the 20 difference I read about but I figure this is good for a 20 year old unit. The coach then goes down to 75 as read at the intake. Then the compressor cycles off until the output reaches ~75. At this point the compressor comes back on until the lower output temp is reached. But here is the strange part. The minimum temperature at the output gradually rises at each cycle by a degree or two till about 70 is reached. I didn't measure the cycle time but we're just talking a few minutes here and I would think plenty of time for the compressor to get the temp down.

Thanks for the help,
Steve
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Old 08-11-2012, 03:31 AM   #5
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If I understand you correctly, I would suggest you might want to remove power and inspect the wiring connections on the compressor. If any are burnt or loose, this could cause the over current/temperature overload to trip out; shutting down the compressor until it cools. If this area is in good condition then I would come to the conclusion you have a sealed system problem and without further testing, suspect the valves in the compressor may be going out. I could be completely wrong, since I do not know how long the unit has run during each of the periods you mentioned.
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Old 08-11-2012, 02:14 PM   #6
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The compressor will cycle off and on when temperature you wish is reached and come back on when it has to bring temp back down what you have temp set for.
Your AC COMPRESSOR may have a over heat devise to cut it off if over heating.
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Old 08-11-2012, 08:56 PM   #7
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The following items can cause it to cycle

Low temp (in the room where the Thermostat is located
Low temp (Freeze sensor.
High temp (Compressor over-temp device/circuit breaker

And on some air conditioners LOW VOLTAGE (I do not know if yours is like that)


And an EMS in teh coach if you are on 30 amps.

IF you think it might be the frost sensor shoot it with an I/R temp gun or measure the outflow temp with a thermometer.. If it is below 40 it might be the frost sensor.

Exact cutoff temp I do not know, varies from manufacturer to model.
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Old 08-12-2012, 11:59 AM   #8
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I took a minute by minute temperature reading of input vs output starting at 99 degrees at both ends of the unit. I'll give results below in separate columns for easy reading. First column is time of day followed by input temp followed by output temp followed compressor on or off for number of minutes. Min temp was reached right before comp cycled off.

4:20 99 99 on 6 min
4:26 97 80 off 1 min
4:27 93 94 on 3 min
4:30 93 81 off 5 min
4:35 95 96 on 3 min
4:38 95 88 off 8 min

Big event here at 4:38 not only did comp cycle off but so did the circulation fan! No coach circuit breakers were tripped. I waited about 30 minutes and was able to restart. I assume it would have repeated the pattern above but I didn't bother running another test.

I assume that, as some of you have mentioned that some thermal protection is happening that cycles off the compressor. Seems a second thermal tripping event is occurring at the unit that also shuts off the fan.

There is a very small tubing that connects the two coils together. I have assumed this is the freeze sensor all along but I now have doubts. I do not see an additional sensor that could be a freeze sensor.

I need to get back to the unit to see if any electrical connections are compromised that would lead to over heating. There is a large easy accessible metal box that would be easy to open and inspect but the access door on the compressor is blocked by the frame that holds the coil. I would like to slap the designer's hands that placed it;^)

As you can see it does not take long for the problem to show itself. And when the compressor is allowed to run an approximate 20 degree delta is reached. And I feel the thermostat is working fine since I can manually turn is down and here a click to force the comp off when I want to. When the comp unexpectedly turns off the thermostat is at the max setting and no audible clicking sound is heard. Seems my best bet is to look at those connections.

Thanks for the continued help!
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Old 08-12-2012, 12:08 PM   #9
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Need to check power to compressor terminals when it cycles off. Could be the internal overload inside the compressor. Is this a 1992 unit? Is the condenser coils clean(the ones that get hot) ? Put an Amprobe on the 120VT line a see if it isn't running high amps.
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Old 08-12-2012, 07:33 PM   #10
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I just spliced in a Kill-A-Watt meter right at the junction box in the A/C unit where coach wiring meets the A/C wiring and measured 115 volts, 17 amps, 1950 watts. There was no appreciable increase right before the compressor cycled off.

It's 96 here in Oregon this afternoon so I think I'll try inspecting wiring on top in the morning.
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Old 08-12-2012, 07:49 PM   #11
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Isn't your unit only supposed to be fused at 15 Amps? Biggest unit I've seen is on a 20 Amp service. Is the condenser coil(outside) clean?

See if you can find the tag on the unit and see what the running amps are supposed to be.
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Old 08-12-2012, 10:10 PM   #12
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It's rated at 7100 BTU and I see "min circuit ampacity 18" and "maximum current protection 20". So my reading of 1950 appears dangerously close. This looks like bad news but I'm still hoping it's not the compressor!

I also see:

Compressor 7.2 RLA and 34.4 LRA
Fan motor. 1.5 RLA and 3.7 LRA

Seems like you need to be a trained HVAC person to really know what the average amp draw would be for this unit.

What do you think? Still a chance it could be something other than my compressor or is it still worthwhile to check topside for bad connections?

Thanks,
Steve
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Old 08-13-2012, 04:19 AM   #13
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The compressor should only draw 7.2 amps at the voltage rated on the data tag. Also the fan motor on high speed should not draw more than 1.5 amps at the same voltage. It sounds like you may have a run capacitor issue or even a possible compressor motor winding shorting out. The unit should not be drawing 17 amps under normal running conditions. Sorry but it does not look good if it is not the capacitor(s) possibility.
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Old 08-13-2012, 05:25 AM   #14
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No way a 7600 BTUH unit should be drawing 17 amps when running..

7.2 amps is rated load amps when running at 120 vac. At 115 VAC you will see a bit over 7.2 amps.

If you Kilo-Watt meter is tapped in on the whole unit, you will see 7.2 + 1.5 amps for a total of 8.7 amps.

Since you have the voltage in the proper range, you need to check the starting and run capacitors on the compressor. If the run capacitor is bad, the amps on the motor will run high and cause the motor to over heat. That old of a unit will have a Klix-on thermal switch clipped to the compressor case which is probably what is tripping off the compressor.

Basically you will have three things that will shut off the compressor...
The Thermostat.
The freezestat
The Klixon.

Newer compressor may use a solid state thermistor that is embedded in the windings to protect the compressor against over heat.

If it is not the capacitor, I think the unit is on i't last leg and not worth trying to repair.

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