need help with a Window, york 12k intermitent cooling

[Gundulf]

I bought this A/c 2 weeks ago, its used a york 12k btu
it worked just perfect for the first week then it stopped working as good in the next week. the unit will turn on, and cool for about 15-20 mins, then the compressor would stop cooling for another 15 min (in which the room temp would rise practically to the outside temp) then it would start cooling again for another 15 mins. Ive been searching all over the internet for possible problems, and loss of refrigerant seems to be a good posibility, i noticed that when the compressor turns off, theres a knock noise. its not too loud but its noticeable.
so can my problem be refrigerant?, since i would think refrigerant loss would make it stop cooling at all. it does cool well when the compressor is on.
is there a way to know other then gauging the pressure if i need more refrigerant, like i was reading touching the compressor after it kicks off if its hot that would mean (posibbly) loss of refrigerant?
should i look into other posibilities? thermostat is set to lowest, is it posibble to refrigerant to leak out untill certain amount is left? like a pressurised leak?
and lastly how could i visually detect a gass leak, if possible, does it always leaves marks, oil or something?
Thanks in advanced Gabriel.:confused:

[goshen]

HI u are probably in the right direction !but u should call a ref tech in to examine and fix
the way u discribe sounds like a comp overheating but could be also a simple thermostat problem 2
good luck

[Gary]

First check for airflow problems, both inside and outside, such as dirty coils, fan blades, motor, etc.

[rizwankhan]

hi -first you check your unit thermostat cut-in position because in 15min starting unit very delay,and check thermostat bulb (capilary tube) may be possible touch with evaporator,s fins and keep distance,in this situation the compressor oveloaded.

[Sledge]

Need to know a couple of things....does the unit freeze up at all. Is the unit dirty?

If it was short of *****, it would not cool for 15 minutes, without some kind of freezing.

What it sounds like to me is that the coils are dirty, the unit is overheating and the compressor is shutting down on internal temperature protection.

Unplug the unit and take it outside, remove the covers, cover the fan motor with a plastic bag and wash the coils with a garden hose...do not use a pressure washer as they deliver less water at higher pressure...which can bend the fins on the coil. You want a high volume of water at moderate pressure to clean the coils. let it dry out before re-assembly and powering.

[Gundulf]

Ty guys for your responses,:rolleyes: I'm sorry i didn't replied earlier, i though every post on the thread would notify me by email. But oh well!
The unit is very clean(the one reason i bought it used) it seem to have done little work(i presume a month or so). It will run sometimes for 15 min or even 20 minutes then it will turn compressor off. It will at some point in the night shut off itself(the ice protection) if i where to set it to less than 22 degrees C. It did shut off today (around 4 pm) and was set to 22 which surprised me. I've noticed for it to trigger the freeze protection needs to cool down a few times the room maybe 4 times before it would actually "freeze" it does however not happen every day. I have noticed that after i turn the unit on, it will cool for the 15 min or 20 period then, it will go off, maybe 10 mins, then cool for 15 min again and then go off for 30 mins. which does gets the room heated up. I can also "force" the compressor on by turning the ac off and on immediately. i usually do this when the room temp gets back to the 30C. i got another unit same model york ycusc12-2r that would run for maybe 10 seconds and then would shutoff with the error message for evaporator sensor error. maybe i can fix that other with this one? lol, but anyways is it then a low ***** problem?, thankyou all for your help im very thankfull.;)