Air Cond. short cycling

[Snotter]

I have an older Cruise Air 16000 reverse cycle marine air conditioner that is starting to short cycle. Specifically, when the compressor comes on, the temperature registered by the digital readout drops 2 degrees. When the compressor shuts off, it goes up, causing it to short cycle.

The unit cools good. However, I have to run the unit at a low temp setting to prevent it from shutting off.

I installed the unit about a year ago (bought it used off of ebay) and it worked fine for the year (I live aboard and I run the unit all year). When I installed it, I put the thermal coupler in the air vent that feeds the compartment that the unit is installed in. I don't believe it is influenced by temperatures in the compartment.

If I had to guess, I would say there is a voltage drop going on in the low-voltage section of the unit. It will be a royal pain to hunt down the connection. Can anyone shed light on the problem?

 

[DeniseO30]

First make sure of all the typical things.

is there water flowing?
is the strainer clear?
Do you have full voltage to the unit?
if you have an air filter is it clogged?

You can only ck the temp when it's running if you use a thermometer AT the cooling coil. in and out temps should be around 18- 20 degrees. the water coming out the discharge should only be about 10 degrees warmer the ingoing. all of the prior are reversed in heating mode.

Does the fan keep running and just the comp short cycles?
are there low and high pressure controls?

by pass the thermostat to stay on cool see it the unit runs continuous.

Do not put gauges on unit until your very sure the refrigerant charge is suspect.

There are more things to ck after all the above.

 

[BillyK]

you might have lost some refrigerant.. i believe that causes short cycles as well..

 

[Snotter]

Plenty of water is flowing. The water goes through a 1-1/4 inch sea cock to a cal pump 1200. I can watch the temperature go from 74 deg to 77 deg in a couple of seconds when the compressor stops, which is why I thought it would be a voltage drop in the low-voltage side. (Isn't a thermal couple a temperature sensitive variable resistance device?) I don't believe the 110VAC is low, but when I open the compartment up I will check that. It is just too hot outside to bring the unit down to do that until I can work on it at night. I'm no expert, but, I wouldn't think that a loss of coolant would cause the digital thermometer to register a temperature swing. Thank god that the A/C runs fine as long as it doesn't cycle because it is hot in Alabama.

 

[kloudie1]

You are probably on to something, Snotter.. The temperature reference that the thermocouple or RTD (whichever it might be) works off a voltage reference that has probably gone south.. If you can get a circuit diagram of the control board you could find an electronics dude to look at it and the board and see what has gone wrong.. The Cruise Air (now Dometic ) folks should be able to help troubleshoot the board

 

[Tim R.]

Check the easy/cheap stuff first.

I can program the differential into my thermostat for my Mermaid unit. Has yours changed recently? Changed the batteries?

It could also be the location of your thermostat. Your AC should not be blowing directly on the thermostat or it will short cycle just as you have described.

 

[Witzend]

Is there a coolant pressure sensor on the low side of the compressor? This is normally designed in the system to protect the compressor from burnout if you have no coolant w/oil present. What happens is that since the pressure level is low when the compressor turns on is pulls down the low side even lower opening up the switch a shutting off the compressor, once the pressure equalizes, the switch closes and it starts up again (short cycles). This may be what is happening. An easy test is to short the switch out and see it it keeps running. If it does than you are low on coolant.

 

[Benny17441]

In the winter time perhaps I would be fishing for a solution, but at this time of the year in the Gulf I would have a marine refrigeration pro fixing it.

 

[Snotter]

I am not in the Gulf, I'm in northern Alabama. There is only one guy remotely in the area who will work on marine ACs. However, he requires the unit to be pulled out and brought to him. I'm on my own.

Witzend, the unit cycles because the thermometer is telling it to.

Kloudie1, I didn't know Dometic could help me. This weekend I will open up the compartment and do some probing with a digital meter. I will also look for ID numbers and talk with Dometic.

 

[kloudie1]

AC Short Cycle

If it runs fine when set at a really low temp, then you are right about the control system.. The normal AC folks will not be much use because the marine controls are very different from home thermostats.(Some of hthe new high eff. units have similar controls, though) . I had to replace the electronic controls on my old Marine Air, Cabin Mate .. I used new controls from Micro Air Corp, of Allentown, New Jersey.. Your problem may only be in the digital head itself where the thermocouple is usually located around the display...
As Tim notes, I too can program the span (widen the gap) between set temp and "turn on" temp by holding two buttons and scrolling down through the adjustable parameters.. Your manual should have that, and by changing the gap, you may be able to get to cold weather season and have the time to repair it..

 

[Snotter]

kloudie1, come to think of it, I do remember there being a manual with the unit. I'll start hunting for it to see if I can also change the gap parameter. If I can't find a manual, I am hoping there will be one on the internet somewhere.