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That is the manual for the unit, see page 1-12 (page 12) for a diagram that shows the location of the 'sight glass'... use a flash light, if you can see a thin stream of fluid or bubbles the system is merely low on refrigerant.... IF the sight glass is *clear* then the system is either completely out of refrigerant, or completely full of refrigerant.
That aspect takes some study as you look at the sight glass, an empty sight glass will generally have a trace of oil or something you can see laying in or around its edges, and of the course the compressor will be running but tube number 18, the suction line will not be cold.
We can *start there... let me know what you find.
I can coach you through the entire manual if you like, but it will best if you read it for yourself, paying close attention the trouble shooting section, then ask me questions based on that.
Thanks, ***** ***** most likely empty if the compressor is running. If the compressor is not running a sight glass reading doesn't tell us anything. These systems are leak prone.
A refrigerant leak will have oil around it... a big leak will leave a drop of oil under the tube or fitting that is leaking.
Tell me if the compressor is running or not, you can feel the compressor cylinder heads if one side of them is hot, its running. Look for signs of oil, especially any dripping oil on any of the *refrigeration components. (not from the engine, those will always leak a little oil).
We can go from there.