How can i tell if my thermostat is bad?

HOW CAN I TELL IF MY THERMOSTAT IS BAD. THE REASON WHY I ASK IS THAT THE COMPRESSOR JUST KEEPS RUNNING. AND THE MACHINE ONLY GETS COLD FOR THE FIRST 45 MIN THAT I PLUG IT IN. AFTER THAT IT SEEMS THAT IT DOES NOT GET COLD AND THE COMPRESSOR JUST KEEPS RUNNING.
THATNKS FOR THE REPLIES IN ADVANCE.

I hate to tell you this but it's more likely the cooling system than the thermostat. If your cooling system was all working properly but the thermostat was stuck on, the machine would get very cold and probably stay that way. How cold does it get when it stays on? With my Cavalier 96 turned all the way down (1) it stays at ~37. The Vendo 23 I had would freeze the cokes at about halfway up. If it's not getting in this range (where it would kick out the thermostat) there's something wrong. If it is getting past freezing then maybe it is the thermostat.
  
As for what could be wrong with the cooling system there are many possibilities. Do both fans work? Do both push enough air to feel it flow? Is the compressor quiet? The refrigerant level may be low or there are a variety of possible other ills.

Nate,
Both fans do work. Both do feel like they push enough air. The compressor is quiet. The coils on the evaporator get icey, but after an hour they start to melt away. Would it be that it needs more freon? I just rechagred it with 1 can of R12 wich is 349 grams if that means anything.

Based on your description it appears that something is changing in the sytem as it goes. If you have the capacity to make frost you wont lose it just by running, something has to "happen". In this case I suspect that you may have some water in the system. This would freeze at the evaporation point and block up the system. The only problem I see with that logic is a plugged up system would almost always make the compressor sound different. A low freon level could keep this from happening though.

I'm inclined to think you need a filter-dryer. I would run this by an HVAC / refrigeration guy first though. I design custom machinery and I'm not all that up to date on this stuff.

Nate

[globalcompressors]

Gabe,

  What was the pressure reading on your guages before you added freon? Has your machine just started this or did you buy the machine and this was the problem from the start? Were the coils frosty or covered in ice? What was the temp on your thermometer? Can you tell if the unit was previously serviced i.e. service valve present? If there was a service valve, there could be moisture in the system as Nate said. If no service valve is present, the unit is a sealed system from the factory and there won't be a moisture problem. Something else is causing it.

Eric

Eric,
When I bought the machine the previos owner added a service valve and tried to add a dryer filter, but did not soder it, so I sodered it and added a can of R12 freon that a freind had in his garage for years. After I added the can of R-12 freon the coils got frosty, but after a while it would start to melt away. As fas as the temp I don't know what it is. Thanks for your help.

[globalcompressors]

Gabe,

  Nate was heading for something about moisture. When you soldered the dryer on, did you pull a good, long vacuum on the unit? This is the reason for a vacuum, moisture can travel around the system and settle at the cap tube and will freeze up causing a blockage. The other thing I'm interested in is what kind of solder did you use? If you used soft solder such as acid core on a role, this could have done the same thing, blocking the cap tube. Either case, your unit won't function properly. I would recommend a good set of guages and a thermometer. You really need both to do a good job! A thermometer is a lot cheaper than a box of busted bottles that froze because the stat didn't shut the compressor off at temperature!  '>

Eric