AC Unit fault

[Tomion]

Carver 444 2001.

My Marine Air AC is doing some weird things. An AC repair guy replace the control unit and it seemed to work ok for a week or so. When I select it on it shows correct cabin temp and the AC unit cones on and starts cooling, then after a minute or so the indictated cabin temp starts fluctuating to lower numbers like 80 then 76, 73, 67. I selected 65 degrees to keep the unit runing as the indicated temp fluctuates up and down but eventually I get a HPF fault.

I swapped the the control unit with my aft AC but get the same fault. I presume it is a problem with the AC computer but afraid to bring AC guy out again without some input as when I tell him what it is doing and provided a video of it he seemed perplexed.

[tomschauer]

HPF is "high pressure fault" which means you are not moving enough water through the unit to keep the condenser cool.
The "moving temp display could be an issue with the controller on the wall or the control board at the a/c unit.

[Tomion]

Thanks. I am moving lots of water thru the unit and swapped the control head with aft AC Must be the control board. Is it difficult to change contol board yourself?

[tomschauer]

If you have two units and one pump are you sure you are moving enough water through the problem unit?
If you have decent access, it is not difficult to replace the control board. Make sure power is off to both units and the pump, as the pump power goes through relays of both units.

[g36]

Hpf pressure fault can also be insufficient air moving across the coil causing hi head pressures

[tomschauer]

G36, in the cooling mode, lack of air flow will give a low pressure fault and cause the coil to freeze (low head pressure). It will not give a high pressure fault.

[Tomion]

Thanks for everyones input. Definitely moving water fully. Good airflow too. Unit is definitely not freezing over as it goes to HPF in a minute or so. The very odd ocurance is the ccabin temp indication initially come up accutately say in the 80 range. The AC comes on and starts blowing cold. In a minute or two the indicated cabin temp starts to tic down showing the exact same nimbers each time 76-73-67. If I set desired temp to 65 the unit continues to run fir several minuts and then I get the HPF. I presume the Cabin temp sensor is in the control head which has been swapped with aft AC unit with no change. Would like to order a new control circuit board and feplVe myself to save a few hundred bucks but still uncertain its the problem.

[waybomb]

I believe you'll find the air temp sensor in the return air duct about a foot away from the fan.
Do you have a cooking thermometer you can stick in the supply outlet? How cold is it?
Can you gain access to see the evaporator coil? Is it shiny clean?

[bud37]

Tomion, have you tried the makers tech support.....there may be a few quick tests to establish what is broken, switch, control board etc....they may be able to walk you thru it.

[tomschauer]

The space temp sensor is most likely in the wall mounted control. An optional remote sensor is available for most units but not often used.
It kind of sounds like it tries to go into a calibration mode. I would check the plug in connection in the back of the wall unit and at the a/c unit itself to make sure it isn't that simple.
If the other unit is the same brand and vintage you could try swapping the control boards and see if the problem moves to the other unit.