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AC compressor comes on and off by itself, even when cool is set to off


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Hello folks, I have a perplexing problem and would appreciate some help. I have a RV with a Dometic Brisk Air rooftop air conditioner model 57915.531. This unit is controlled by a wall thermostat just like a home ac. The compressor has a mind of its own and likes to randomly come on and on for brief periods, even if the wall thermostat is not set to cool.

 

The unit cools just fine when you set it to cool, and the compressor pretty much stays on, although sometimes when it gets down to about 72 or less, the compressor will cycle on and off every 15 seconds or so.

 

Right now as I type, the wall thermostat is set to a higher temperature than it is now, and cool is set to off, and the compressor comes on about every 5 minutes and runs for about 45 seconds.

 

The only sure way to make sure that the compressor does not come on is to flip the breaker switch. I have tried disconnecting the wall thermostat and even replacing it, but this has not affected the compressors behavior. Also, there is a metal box inside the AC that contains the main circuit board with all the relays. I put in a brand new circuit board, but nothing changed. Any ideas what could be causing this?

 

I added a couple pictures. One of of the control box that contains the circuit board in replaced with a schematic on it. The other is a the wiring harness. There is a thin orange wire that is not connected to anything. I assume it's just an unused wire. There's no orange wire coming out of the wall to connect to the thermostat.

 

http://imgur.com/MA8ktyT

 

http://imgur.com/A7ZCwzG

 

Inside of thermostat wiring

 

http://imgur.com/CwV4FtT

 

Another label with electrical info and model numbers

 

http://imgur.com/vHC0WI0

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The compressor is activated by the yellow wire. Do you keep the fan running continously ? I have seen where the subbase has a short in it and does things like this. What I suggest you try is to remove the thermostat and take the yellow wire off of the subbase and see what happens. If that takes care of the problem then you need a new thermostat and subbase. If it doesnt take care of the problem then you most likely have a problem with that relay box.

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Do you have any fluorescent lights on in the proximity of the thermostat?

The lights sometime

Interfer with the thermostat. You might also try removing the 12VDC from the thermostat to "reset" it.

If the thermostat is set to "off" then any amount of heat radiating onto the thermostat should not bring on the cooling....if it does then you have a faulty thermostat.

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I got this response on an HVAC forum and though I would share in case anyone has another idea or is having the same problem.

 

"Sounds like a pump down. You have a leaking solenoid which is not a big deal because the leak is internal, not to atmosphere.

 

Under normal operation, your t stat opens a liquid line solinoid (among other things) and causes the pressure to rise in the compressor. The compressor is wired to a differential pressure controller that turns the compressor on when the pressure rises. The compressor runs as long as the LL solenoid is open.

Once the t stat satisfies the LL solinoid closes but the compressor continues to run until it pumps down the pressure to its cut off point.

 

This is a nice design because it protects your compressor from running and burning up if you ever lose your charge.

 

The down side of this design is exactly what you describe, which is only an annoyance and nothing to worry about, unless this occasional pump down kills your battery and leaves you stranded. (I doubt it)

 

So it is not a big deal and you should just get used to it.

 

If you can't live with it, have a qualified service station change out your liquid line solinoid, the whole thing, valve body and power head, not just the power head.

 

I have seen systems (on a much larger scale) run for years with this nuisance issue, and no ill effects came from it, but I must admit that I am not an expert on RV systems.

 

Good luck"

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I got this response on an HVAC forum and though I would share in case anyone has another idea or is having the same problem.

Very interesting response. Thank you for sharing it with us. Air conditioning is a pretty specialized field and most RV techs are not very strong in this area.

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No. This Dometic unit does not use a pumpdown system. I have worked on many RV rooftop systems and have never seen a pumpdown system on an RV. Pumpdown systems are used on some home systems and larger commercial and industrial systems and I am very familiar with them.

I had a quick look online at a wiring diagram for the above Dometic B597 and there is no pumpdown system. It would require a more sophisticated system at more cost....aint gonna happen on an RV.

The OP needs to isolate the problem by doing as I suggested and disconnect the yellow wire to see if the problem persists.

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There must be 12 vdc getting to the relay to supply 120 vac to the compressor. Trace where the 12 vdc is coming from. then follow it to where it stops in the system when the compressor is stopped. that should tell you what is bad.

Or the compressor is stuck on some how and it is shutting off due to a thermal limit switch on the compressor itself.

That is to things that may give us a better way to figure this thing out for you.

 

 

Safe Travels, Vern

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One other thing that mays give you a problem is there are some thermostats that operate on 7.5 volts. So if you find that voltage it might be correct. I just cannot remember what they look like but it does play with you thoughts.

 

 

Safe Travels, Vern

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I have never seen an RV AC unit with a pumpdown. This is some thing you see on larger direct expansion systems, not small cap tune feed systems.

 

We are seeing some issues with our RVP heat pump/AC and furnace. After a couple of discussions with RVP, they say it is an interference issue caused most likely by florescent lights near by. WE only have one florescent light in the trailer, but we do have a bunch of LED fixtures on dimmers.

 

Ours keeps trying to start the furnace when in heat pump mode even when the control scheme says no furnace. Upper unit control boards have bene replaced and until we can get to the factory, I have pulled the furnace fuse to keep it off. The digital T'stat locks up and will not allow any changes and we have to pull the control fuse and let it bleed off and restart the T'stat.

 

Some of the dimmers are pretty noisy. You can try to see if you can track down the noise using a cheap AM transistor radio. Turn it on and tune it to a frequency that is not on a station and to the lowered end of the frequency range. Check the lights and dimmers and also the electrical panels. If you get a lot of static, you have found a noise source.

 

If these AC control are now so sensitive to RF (Radio frequency) interference, the need to start supplying them with shielded control cables.

 

Ken

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Exactly Iceman....pumpdown systems are mainly used in systems with TX valves.

 

I studied the OPs pictures and downloaded an in depth installation manual for his unit. Apparently this AC can be used with a variety of control systems...some are fairly sophisticated. This subbase however points to a very simple analog thermostat so I'm thinking the problem most likely is in the relay portion of the system as it switches, like previously mentioned , the 120 volts that runs the compressor with a 12 v dc signal. Good point about radio frequencies...that is a possibility IMO.

 

What kind of bugs me on the picture with the tstat base is the green wire being used for the incoming power to the stat. I would love to be able to measure that with my multimeter because generally at least in home systems the green wire is used for running the fan and is connected to G. In my mind the Red should be power to RH and Rc and green to G. That said ....RV systems dont always follow what is done in the rest of the world. Need to really check this out with a multimeter.

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Thanks, I'll try unplugging the yellow wire. The thermostat is a programable home thermkstat, not the original analog one, so perhaps it is connected wrong. However, even when I totally disconnect the thermostat from the wall (which pulls out by pins and disconnects from all wires including the yellow) the behavior persists so I doubt that's the problem.

 

EDIT: I totally forgot that the wall base has a red jumper wire from RC to RH. This still makes a connection even with the thermostat pulled off.

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Given that the problem remains with the t-stat removed from the base, I'm betting on a staple pinching the wires. Next step would be to remove the wiring from the terminal strip on the A/C. If the problems disappear, the problem is wiring. If it continues, there's a short on one of the boards.

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RC is the incoming power for the tstat to switch the fan (G) and cooling ( C ) ........RH is incoming power for the tstat to switch the heat (W) . The jumper ties the two together because you have a single power supply to the stat so I wouldnt worry about that.

 

If it were a pinched or stapled wire the compressor would not behave normally when the OP has it set to cool and the compressor behaves as it should. It would also not have a consistant pattern of being off and then coming on and then shutting off again.

 

Something is causing that compressor to start and run for a bit and then shutting it off again. The only thing that is supposed to start the compressor is the relay in the relay box with a signal coming from that yellow wire from the stat.

 

The reason I asked about whether the OP has his fan running steady is because I have seen tstats backfeed power to C and bring on the cooling when only the fan is supposed to run. This is because those old analog stats switch from RC to both G and C to bring on the fan and the cooling at the same tme.

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Remove all wires from the thermostat. Find the power wire, then power the fan wire then the compressor wire. If the compressor still does not function properly it is in the wiring, board or thermal switch.

If still not correct I would think the relay on the board is the problem but since this is not the normal thermostat then it could be any part there of.

 

 

Safe Travels, Vern

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Also check the running amps on the compressor as I have seen them climb real slowly to over 20 amps before it get too hot and trip the thermal switch.

Normally you can hold your hand on the compressor for a shot time. But if it is really hot to the touch then it is most likely tripping out due to thermal switch.

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It just started doing it again. I tried unplugging the yellow wire and the jumper wire and it had no effect.

 

The pattern I'm noticing is that it does seem to occur more when I set the thermostat to 74 or lower. I usually leave it around 77 and it doesn't tend to happen at all. But when I bump it down to 74 or lower, it cools to that temperature,but starts to act up,usually when the thermostat is near or at the selected temperature.

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It's been doing it the last hour and I have been paying atention to it. I have noticed something really strange. The cycle starts every 8 minutes, and lasts a little over 90 seconds.

Does it seem to happen only or more frequently at night, or does time of day seem to have any effect? There used to be a thermostat that did have a problem with RF from some florescent light fixtures. Unfortunately, I can't recall just what the symptom was as I've not seen it discussed in some time.

 

I think that you are on the right track with trying to pin down any pattern or common things about when the problem takes place versus when it does not. I'd look for things which seem to be in use when the problem happens and try turning them on or off it you find one. Observe also the ambient temperatures and any other possible hint. Trouble shooting is often a case of good detective work, especially for an intermittent problem.

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